2 Life in hot deserts and subdeserts.
1.1 Living with almost no water
The main stress factors in arid and semiarid environments are water shortage and high temperatures. Water is essential for plant metabolism, especially for photosynthesis and nutrient transport. Lack of water thus prevents primary production, which relies on available water, the mineral nutrients present in the soil, favorable temperatures, and sufficient sunshine.
Scarce, intermittent surface water resources
Surface waters in deserts and subdeserts are characterized by their irregularity and their intermittent nature. Periods of 10-20 years without any rain at all have been recorded in the Chilean-Peruvian Desert and in the eastern Sahara, in Libya, Egypt, and Sudan. Temporary watercourses (wadis, arroyos, creeks, etc.) only flow for a few hours after the rains, and surges occur less than once each year on average in the case of basins located totally in desert areas.
Some hydraulic systems in deserts and subdeserts--the Nile River, for instance--are entirely derived from external sources (exogenous), while others are only partially so. Rivers with partly or completely external sources include the Niger, Senegal, Orange, Limpopo, Shebeli, and Awash in Africa; the Colorado, the Great Salt Lake River, the Rio Grande, and the San Joaquin River in North America; the Salado, Colorado, Negro, Chubut, Deseado, Santa Cruz, and Gallegos rivers in South America; the Yellow (Huanghe), Amu Darya (or Oxus), Syr Dar'ya, Euphrates, and Tigris in Asia; and the Gascoyne, Darling, and Murray in Australia. Some desert river systems are permanent in their headstreams but intermittent in their lower courses, among them the Oued Draa, Oued Rheris, Oued Ziz, Oued Guir, Oued Zousfana, and Oued Saoura in the Saharan regions of the Maghreb. Exogenous rivers with sources in high mountains--sometimes above 16,400 ft (5,000 m)--are characterized by their regular and highly reliable volume of flow from year to year. In hot dry years, water shortage is compensated by greater melting of glaciers, while in cold rainy years, excess is compensated by greater accumulation of ice; the result is that the flow of these rivers is almost constant from one summer to the next.
Endogenous river systems in subdeserts may bear water one to five times a year. Rain of 10-15 mm is enough to cause local surges in small catchment basins. Catastrophic surges occur periodically. In September 1969, the volume of flow of the Zeroud and Marguellil wadis on the Al-Qayrawan (Kairouan) plain in central Tunisia increased in a few hours from zero to 18 million [m.sup.3]/s (equivalent to peak surges on the Rhone at Valence, France). Even in the central Sahara, the Mya, and Igharghar wadis have devastatingly large surges once or twice a century.
On the scale of a large hydraulic system, the mean annual runoff (R) can be assessed using the equation R = 100 P3/3, where P is the mean annual rainfall in meters. The value of R is about 2-5% of rainfall in subdeserts and 0.2-2% in deserts, but varies greatly depending on the nature of the rainfall and the site. These values may seem low, but as the areas concerned are enormous, these low percentages mean that considerable quantities of water may be involved. Considering that the hot deserts and subdeserts cover an area of 23 million [km.sup.2] with an annual average rainfall of 8 in (200 mm) and a runoff value of 2%, the quantity of water involved represents an average flow of 2,920 [m.sup.3]/s. In other words, the total yearly runoff is enough to meet the irrigation needs of 17 million ha (1 ha=2.5 acres) of arid land at a rate of 5,000 [m.sup.3]/ha per year, or the equivalent of a mean annual rainfall of 20 in (500 mm) over an area of 170,000 [km.sup.2].
The decisive importance of the water table
The large aquifers of the deserts and subdeserts are a major, and even essential, part of the local water resources. Their abundance or scarcity depends on many factors, mainly on the nature and thickness of the sedimentary cover. In the areas where the pavement has little or no
sedimentary cover, hydrogeological resources are limited or virtually absent, as are hydrocarbon resources. This is the case of a large part of the Sahel and southern Africa, areas where ancient continental shields have been intensely metamorphosed with a poorly developed sedimentary cover that may be locally absent.
In the northern Sahara and on the steppes that prolong it to the north lies a very important aquifer, the Continental Intercalary, in Jurassic and Cretaceous rocks, mainly from the continental Lower Cretaceous (the Wealden Series) more or less contemporary with the Aptian Stage and Albian Stage marine formations (100-120 million years old). This aquifer covers an area of 800,000 [km.sup.2] in Algeria and Tunisia. Its volume of flow is 9.5 [m.sup.3]/s, 2.7 [m.sup.3]/s as natural discharge (springs, sabkhas, drainage galleries, or foggara), while 6.8 [m.sup.3]/s is extracted from the 110 artesian or semiartesian wells drilled since the 1930s. Another important aquifer, the Continental Terminal, is sandwiched between rocks of the Mio-Pliocene period. The total discharge of this aquifer is about 21.5 [m.sup.3]/s, of which 13.5 is natural discharge into the great salt lakes of the region (Chott Melrhir, Chott el Gharsa, Chott Djerid, and others); the other 8 [m.sup.3]/s is extracted from the two thousand or so artesian wells. The estimated input is about 18 [m.sup.3]/s derived from runoff, mainly in the Saharan Atlas. The Nubian sandstone aquifer in the eastern Sahara occupies an area of 1.8 million [km.sup.2] and has a flow of 4.3 [m.sup.3]/s, mainly as springs that supply oases (Siwa, Bahariya, Dakhkla [Villa Cisneros], Kharga, Farafra, and Al-Kufrah); it lies between continental rock formations dating from the Jurassic to the Mio-Pliocene. This aquifer, in addition to creating oases where springs emerge, supplies 70 more-or-less artesian wells, mainly in the Al-Kufrah artesian basin in Libya, where an artificial river now under construction (as of late 1999) is intended to transport water to the Gulf of Sidra coastline in an enclosed channel 13 ft (4 m) in diameter. All these are partially or totally fossil aquifers. The waters of the Continental Intercalary aquifer are 4,000-8,000 years old (the time of the Saharan Pluvial) and those of the Nubian Sandstone aquifer of Al-Kufrah, probably derived from the Paleo-Nile, are 20,000-30,000 years old.
The aquifers of the Near East lie 1) on Cretaceous to Cenozoic calcareous and dolomitic rocks that have undergone karstic erosion and 2) under Quaternary alluviums. Total flow from karstic springs in Turkey, Syria, Lebanon, and the Gulf States is about 100 [m.sup.3]/s. A flow of 1 [m.sup.3]/s permits intensive irrigation of 3,000-6,000 ha, depending on the season and the crop cultivated.
Thus, the deep water resources in northern Africa and the Near East represent about 800 [m.sup.3]/s, that is to say, the quantity necessary to irrigate 2.4-4.8 million ha. Hydrogeological resources represent more than 50% of total water resources in the region and much more in countries like Algeria, Tunisia, Libya, Israel, Saudi Arabia, and Iran, where they may represent 50-90% of all water resources, and sometimes almost 100% (Sahara, Niger, Sinai, Arabia).
Strategies for efficient catchment and use of available water
The quantity of available water in the soil is the difference between the total amount of water present and that which is held too tightly by the soil for it to be taken up by the roots (which exert a pressure of 15 bar). The quantity of available water depends essentially on the climate, especially on the total annual rainfall and its distribution, or, more precisely, the ratio of the rainfall to the potential evapotranspiration (the climatic supply and demand). Water availability also depends on soil characteristics (depth, texture, location within the topography, presence or absence of a water table accessible to the roots). The quantity of water retained, and thus the water really available to the plants, may vary greatly, depending on the soil texture: 0.75 l/[m.sup.2] per cm soil depth (= 0.75 mm/cm) in coarse sand; 1.7 mm/cm in mud: 1.2 mm/cm in clay (see also vol. 1, pp. 336-338).
The amount of water necessary for biomass production depends on the climatic supply and demand, as well as a genetic factor that is characteristic for each species of plant and/or cultivar. Experiments have shown the differences in the transpiration coefficient--how much water is needed to produce a given quantity of biomass in a given climate--between the main cultivated species. It is known that alfalfa needs 1,057 qt (1,000 l) of water on average to produce 2.2 lb (1 kg) of dry matter in controlled conditions, but the same amount of wheat only requires 740 qt (700 l) of water, barley only 581 qt (550 l), and pearl millet only 317 qt (300 l; see also vol. 1, p. 191).
Outside the laboratory, much more water is needed due to the large losses through evaporation, runoff, and drainage. These figures may be up to 10 times greater than experimental values in controlled conditions, though well-managed irrigated land approaches the laboratory values for the coefficients of transpiration.
The water-use efficiency of productivity index is the quantity of dry material produced per unit water lost by the plants through evapotranspiration. It is expressed in mg dry weight per g water, or g of dry matter per kg water. For a large area, productivity can also be measured in terms of the dry weight produced per mm of rainfall (known as the rainfall-use efficiency coefficient). For example, the world's subdeserts have a rainwater-use efficiency coefficient for epigeal production of about 4 kg dry matter/ha per year per mm rainfall.
The rainfall-use efficiency coefficient may also vary greatly, between about 1-10 kg dry weight/ha per year per mm of rain. Surprisingly, this variability depends less on the type of climate and the average rainfall than on other factors such as the nature of the soil (its permeability, location within the relief, depth, and fertility) or the dynamic status of the ecosystem (degraded or in equilibrium). Thus, for example, rainfall-use efficiency coefficients of 5-6 kg/ha per year per mm of rain have been measured in some types of vegetation in hyperarid areas such as in protected sandy areas in Kuwait.
The degradation of an ecosystem leads to soil compaction and reduction of biological activity, resulting in a decline in the rate of turnover of the biogeochemical elements, and in fertility, production, and biomass. This downward spiral leads to a reduction in the rainfall-use efficiency coefficient, which may decline from 4-6 kg dry weight/ha per year per mm rainfall in a relatively well-conserved ecosystem to just 1 kg in the same ecosystem after degradation; values as low as 0.1-0.5 kg dry matter/ha per year per mm rainfall have been measured in ecosystems that have been turned into deserts. The most productive ecosystems in the arid areas may have rainfall-use efficiency values as high as 10 kg dry weight/ha per year per mm rainfall. The biological limit is obtained in artificially irrigated and abundantly fertilized CAM or C4 plants (cactus, agaves, saltbushes, sugarcane, elephant grass, guinea grass, etc.) is about 30-50 kg dry weight/ha per year per mm rainfall. Such high values are not reached in arid areas in natural conditions.
As degradation gets worse, the rainfall-usage efficiency coefficient declines and the variability of production also increases. This variability may be seen in the ratio of the coefficient of variation of annual production and the coefficient of variation of annual rainfall. Dynamic balanced ecosystems have a value of 1.1-1.3, but in degraded ecosystems it increases to 2-3, and in desertified ecosystems it is around 5. The variability in annual primary production is 10-20% greater than the variability of annual rainfall in healthy ecosystems but it may be five times greater than the variability of the rainfall in ecosystems that have been converted into deserts.
The influence of the soil on water shortage
The nature of the soil exercises a determining influence on the productivity and primary production in arid areas. The greater the permeability of the soil and the more favorable the position within the topographic relief, the greater the amount of water filtering into the soil during the brief and scarce episodes of rainfall. And, obviously, the deeper the soil, the greater the volume of water stored in the soil profile. The amount of rain falling in a given landscape is received and redistributed very unequally between the different soil compartments and types. This redistribution is governed by factors such as the relief and slope, the permeability of the upper soil horizons, the state of the soil surface, and the soil depth.
The topography and slope influence the runoff. An area at the base of a slope or in the center of a depression may receive 10 times more water than one at the top of a steep slope. Soil permeability depends on the texture and structure of the surface horizons and their content of organic matter and greatly influences both the infiltration of water and its opposite, runoff. The state of the soil surface, mainly the presence of slightly or highly impermeable films or crusts, also influences whether rainfall filters into the soil or is lost as runoff.
Raindrops falling onto the soil, for example, often form a muddy slurry that may greatly reduce soil permeability and, in turn, water balance, plant cover, and the soil's productivity. Crusts or biological carpets of cyanobacteria, green algae, and lichens may act like this slurry by making the soil almost impermeable and thus unproductive. Finally, the soil's depth limits its capacity to retain water, on which both water balance and productivity depend.
The most productive land in arid and desert areas is generally deep, sandy soil with a stable surface. Here, runoff is low or virtually nonexistent, as almost all the rain infiltrates the soil. As the force with which water is retained on the surface of a soil particle is inversely proportional to its diameter, plants can absorb a greater proportion of the soil water from sandy soils than from mud or clay soils. The quantity of water retained by the soil at the permanent wilting point (the water present in the soil but not available to the plants) is approximately five times greater in mud than in coarse sand and ten times greater in clay. When rainfall is scarce and intermittent, much more of the total soil water is thus available to plants in sandy soils. Evaporation from the soil surface and the rise of moisture by capillary action are much lower in coarse soils (sands) than in fine ones (muds and clays).
Deep sandy soils may thus buffer the variability of rainfall in time and space, while fine skeletal soils increase this variability. Detailed studies of primary productivity in arid Tunisia and other subdeserts have shown that subdesert steppes on coarse sand soils produce an average of 4-6 kg dry weight/ha per year per mm rainfall as opposed to only 2-3 in muddy soils and 1-2 in skeletal soils; the rainwater usage efficiency coefficient of sandy soils is twice as large as on silty soils and three to four times greater than on skeletal soils.
1.2 The low availability of nutrients
The low fertility of desert soils is due to both chemical and physical factors. In the first place, nutrient availability is low because the soil contains few nutrients or because, if they are present, they are immobilized in forms that cannot be taken up by plants. On the other hand, water shortage, the low moisture retention capacity of the desert soils, and their high risk of erosion are the most important physical factors limiting their fertility.
The nutrients in short supply
Nitrogen is the nutrient most likely to be in short supply in desert soils because of their low organic matter content. Furthermore, most of the nitrogen is immobilized in organic forms that cannot be used by plants, while the inorganic forms of nitrogen--such as nitrate and ammonium ions--are scarce or absent. Electric discharges such as lightning help to fix atmospheric nitrogen in the soil, but in deserts they are infrequent and are of little importance.
Another source of nitrogen, the mineralization of organic compounds, depends on temperature and humidity that are not always optimal. Other inputs include that from nitrogen-fixing organisms living on the roots of leguminous plants, that are able to fix atmospheric nitrogen in forms plants can use. Sulfur bacteria of the genus Azotobacter and also cyanobacteria can fix nitrogen.
Phosphorus is the second nutrient most likely to be limiting in these soils. Relatively large amounts of mineral phosphorus are often present in desert soils, but in insoluble forms that plants cannot take up. Insoluble calcium phosphates are formed, especially on calcareous soils, and these immobilize the phosphorus. This is why phosphorus availability is greatest in moderately acid soils that lack calcium carbonate. Most of the phosphorus available to plants in the soils of arid regions is thus derived from the phosphorus released in the mineralization of organic matter.
The next nutrient most likely to be deficient is iron. Like phosphorus, iron is often present in inorganic forms that cannot be assimilated by plants, a situation that is accentuated in the presence of calcium carbonate, since iron's solubility is lowest at a pH of 7-9. Iron deficiencies in calcareous soils cause yellowing of the youngest leaves while the nerves remain green; this process is known as chlorosis.
Soil fertility and biological productivity
Soil fertility is a complex concept, as it depends greatly, but not exclusively, on the soil's nutrient content. In general, soils contain variable but sufficient quantities of all the nutrients needed for plant growth and development: 1) the formative elements (nitrogen, phosphorus, potassium), 2) those necessary for metabolic processes (magnesium, calcium, sodium, iron), and 3) the trace elements (copper, cobalt, boron, iodine, sulfur, manganese, molybdenum, zinc, fluorine, and others). Soil fertility is obviously related to nutrient levels, but it also depends on the roots' ability to explore (and exploit) a relatively large volume of soil. For this reason, soil fertility is also related to its texture, structure, and compaction. Some deep sandy soils that on analysis may seem chemically deficient and infertile are in fact fertile because in these circumstances the roots can explore a large area. Thus, for example, the olive groves of Sfax in Tunisia, cultivated at a density of 16 trees/ha (1 ha=2.5 acres) on a deep sandy soil with 8 in (200 mm) annual rainfall, explore a volume of approximately 1,250 [m.sup.3] per tree. Analysis shows the sand's nutrient content is very low, but the trees compensate for this by exploiting a very large volume of soil.
Soil fertility also depends to a large extent on its internal biological activity, which is in turn conditioned by the presence and regular input of new organic matter (leaf litter) and the soil's level of humidity, porosity, degree of ventilation, and temperature. Overall biological activity can be measured by the amount of carbon dioxide emitted and, more analytically, by drawing up an inventory and count of 1) the mesofauna (nematodes, annelid worms, microarthropods, arachnids, apterygote and pterygote insects, myriapods, isopod and decapod crustaceans, rotifers, tardigrades, molluscs), 2) the microbiota (protoctists and the microorganisms that perform the essential processes of denitrification, ammonification, and nitrification; the other bacteria; and microsymbionts like Rhizobium, Klebsiella, Spirillum, or Azospirillum), and 3) the mycota (fungi and lichens).
Precarious nutrient recycling
Biological activity--a measure of how dynamic and healthy an ecosystem is--influences nutrient recycling. As a first approximation, biological activity in arid zones can be considered proportional to the amount of organic matter incorporated in the soil, which is not necessarily true in all environments (in peat bogs and swamps, for example, the opposite is true). How much organic matter is incorporated depends on the biomass, on the epigeal and hypogeal production, and also on the degree of ground cover by plants and leaf litter.
There are relatively few data on nutrient recycling in arid areas. The ash content of the nonhalophilic vegetation of the subdesert steppes of northern Africa is approximately 1215% of its dry weight, while the mineral index (the percentage of nutrients), is 1.8-5% of dry weight (32% nitrogen, 30% calcium, 15% magnesium, 13% potassium, 7% phosphorus, and 3% sodium). The remaining 10% of the dry weight corresponds to elements like silicon, sulfur, chlorine, iron, aluminum, and manganese. In wastelands with many crassulacean halophytes, the values for the mass of mineral nutrients and ash may be two or three times greater. A portion of the elements needed for life, probably around 50%, are recycled in the dung and urine of animals, especially ruminants.
In the Sahel subdeserts, the mass of mineral nutrients is about 1.2% of the epigeal plant biomass (almost 25 kg/ha for a biomass of 2,000 kg dry weight/ha) and of the hypogeal biomass. This mass of biogeochemical minerals is composed of 32% nitrogen, 31.2% potassium, 26% calcium, 9.1% magnesium, and 1.6% phosphorus. This clearly shows the phosphorus deficiency characteristic of the vegetation of relatively acid, leached, tropical soils.
In tropical biomes, the trees--especially mimosoid legumes such as Acacia, Albizia, and Prosopis and caesalpinoid legumes such as Cassia--play a major role in the recycling of the biogeochemical elements. This function has been studied in detail in the kad (Acacia [=Faidherbia] albida) in Senegal, in the jhand (Prosopis cineraria [=P. spicigera]) in India, in the honey mesquite (Prosopis glandulosa) in the United States, and in the espino or caven (Acacia caven) in Chile. Under these trees and immediately around them, the soil is greatly enriched in organic matter and in phosphorus, nitrogen, potassium, calcium and trace elements (zinc, copper, manganese, iron); in fact, these nutrients may be 20100% more abundant (sometimes even more) than in a control plot. This is essentially due to the incorporation into the soil of leaves and other organs shed by the trees. The yield of millet (Pennisetum) under kad in Senegal and under jad in Rajasthan is 200250% that observed in the same conditions in soils lacking these trees. The increments in the different nutrients observed under kad in Senegal show increases of 30-600% in nitrogen, 90-269% in carbon, 100% in calcium, 134% in phosphorus, and 70% in magnesium. At a density of 50 trees/ha, these trees provide 911 lb (413 kg) of calcium oxide, 93 lb (42 kg) of potassium oxide, 132 lb (60 kg) of phosphate, 661 lb (300 kg) of nitrogen, and 33 lb (15 kg) of magnesium. At the same time, the soil's cation exchange capacity increases by 100%, its organic matter by 160%, its water storage capacity by 40%, its ability to retain water by 43%, and the proportion of clay by 7%. The figures for nutrient enrichment under Prosopis cineraria in Rajasthan are almost identical.
2. Flora and plantlife
2.1 The origins and diversity of the hot desert and subdesert flora
Is there a flora specific to deserts and subdeserts at a global level? And are there any taxonomic groups of plants limited to the desert environment? In practice, no. The elements making up the desert and subdesert flora are certainly specialized, but the origins of their floras lie in those of the large floristic regions in which they are located.
The multiple origins of the arid flora
The Mediterranean flora is the basis of the plant populations of the Mediterranean deserts and subdeserts (the northern Sahara, the subdesert steppes of northern Africa, the steppes and scrublands of the Mediterranean region). The result is a flora with predominantly Mediterranean affinities, both at the level of the family (Caryophyl-laceae, Cistaceae, Brassicaceae, Geraniaceae, Lamia-ceae, Umbelliferae, Ranunculaceae, Scrophulariaceae) and at the level of the genus; they have even given rise to some species that are proper to the desert.
The Old World tropical deserts are colonized by a Paleotropical flora, characterized by 1) certain families (Acanthaceae, Aizoaceae, Apocynaceae, Asclepi-adaceae, Bignoniaceae, Bombacaceae, Capparidaceae, Celastraceae, Leguminosae-Caesalpinoidae, Poaceae tribe Chlorideae, Combretaceae, Commelinaceae, Loganiaceae, Malpighiaceae, Moraceae, Moringaceae, Nyctaginaceae, Olacaceae, Oleaceae, Pedaliaceae, Rubiaceae, Salvadoraceae, Sapindaceae, Simaroubaceae, Sterculiaceae, Zygophyllaceae), 2) distinct subfamilies (for example, the grasses of the Andropogonoideae and Panicoideae subfamilies) of families with wider or even cosmopolitan distribution (in the cases cited, the Poaceae), and 3) genera and species more or less confined to the intertropical areas of the Old World. One special case is the rand flora, the xerophytic flora of the deserts and subdeserts of southern Africa, containing 1,140 genera (420 of them endemic) and 12,700 species (10,500 of them endemic, including 1,900 endemic species of Aizoaceae, 1,800 of Asteraceae, 1,100 of Leguminosae, 800 of Ericaceae, 800 of Liliaceae, 800 of Iridaceae, and 500 of Asclepiadaceae).
The hot deserts and subdeserts of the New World have a Neotropical flora, characterized by several families and genera that are exclusive or almost exclusive to the New World such as the Agavaceae, Bromeliaceae, Cactaceae, Fouquieriaceae, Garryaceae, Hydrophyl-laceae, Krameriaceae, Loasaceae, Passifloraceae, Taxodiaceae, and Turneraceae.
The salt-tolerant (halophilous) flora is an exception to the generalization that the world's different desert floras are from different origins in different areas. The world's halophytes belong to just a few taxonomic groups, despite their great variation. There are about 6,000 species of terrestrial halophytes and about 2,000 coastal ones (mangroves) and genuinely marine ones (marine hydrophytes). About 25% of the terrestrial halophytes are chenopods, 10% are Poaceae, 4% are Asteraceae, 4% are Plumbaginaceae, and the rest belong to many different families, including the Aizoaceae, Cyperaceae, Leguminosae, Tamaricaceae, and Zygophyllaceae.
There is still no overall analysis of the flora of the world's hot deserts and subdeserts. The only detailed inventories are for individual countries, and sometimes by regions, but these inventories are generally drawn up on the basis of administrative divisions rather than through the use of climatic or ecological criteria. Any global assessment must, therefore, be based on precise, well-documented examples.
A good start is to compare the floras of the subdesert steppes of North Africa and those of the Karoo in southern Africa. A recent and very detailed analysis showed that the steppes of the arid zones of northern Africa (630,000 [km.sup.2]), between the Atlantic and the Red Sea, contain about 2,600 species. The floristic richness of this zone is thus 41 species per 10,000 [km.sup.2]. The roughly comparable Karoo in South Africa (500,000 [km.sup.2]) has nearly 7,000 species, or 140 species per 10,000 [km.sup.2]. The total flora of North Africa is estimated to be about 8,000 species, while that of southern Africa is about 24,500 species; the arid extratropical areas of Africa therefore represent 33% of the total flora of northern Africa and 29% of the southern African flora. These figures emphasize the fact, already well known to botanists and phytogeographers, that the South African flora is extraordinarily rich, with 180 species per 10,000 [km.sup.2] as opposed to only 10 species per 10,000 [km.sup.2] in North Africa. It should, however, be remembered that the figure for Northern Africa is greatly reduced by the large proportion of area occupied by the Sahara Desert. Excluding the Saharan zone, the North African flora has 65 species per 10,000 [km.sup.2], making it only 36% as floristically rich as South Africa.
The set of plant species typical of the world's hot deserts and subdeserts can be estimated to number about 50,000 species, about 15% of the world's flowering plants (given that estimates suggest roughly 250,000-300,000 species) distributed over an area of almost 23% of the world's land surface. This figure may seem high, but it has been shown that, for example, the arid zones of the Mediterranean Basin, which have been thoroughly studied, contain more than 23% of the total number of species of the Mediterranean flora (about 22,000 species). The dry zones of Africa have almost 18,000 species out of a total flora for the continent of 68,500 species--about 26%.
2.2 The biological types and the plant growth-forms
Independently of the ecosystem and the climatic regime, all sorts of plants have developed morphological, anatomical, and physiological adaptations to optimize their ability to survive. Convergent adaptations have arisen in unrelated species belonging to different families in response to similar selection pressures, and nowhere is this as clear as in the desert and subdesert environments.
The biological types
All plants need water to survive, and this is why desert plants adapt to obtain the maximum benefit from the water available and to reduce water losses by evapotranspiration. Some plants adopt passive solutions to cope with aridity: they avoid the drought by remaining more or less inactive during the hot dry season. Other plants adopt active strategies, resisting the drought and remaining active all year-round. The most frequent biological types in deserts and subdeserts (therophytes, many geophytes, hemicryptophytes, and some chamaephytes) are species with passive strategies; only chamaephytes and phanerophytes have to adopt active strategies to deal with the arid climate.
Plants are most vulnerable to drought when they are germinating and establishing themselves. Some ephemeral plants, or ephemerophytes, are able to complete their life cycle in the few weeks of the rainy season; consequently, their life cycle has to be extremely short. Then there are Nyctaginaceae (Boerhavia) and species of Tribulus (Zygophyl-laceae), for example, which produce their seeds within only 8-15 days of germination. There are some perennial herbaceous plants that, especially in conditions of stress, behave as ephemerals or at most produce seeds every few years. The trees and shrubs, however, take many years to reach reproductive maturity and are thus more exposed to grazing and shortage of rain before they attain maturity and can reproduce. Not even sufficient rainfall ensures success, for example, when attempts are made to introduce economically useful woody species into a new area. In the Sahel zone of the southern Sahara, for example, the high saturation deficit and low relative humidity prevent cultivation--except in the western coastal area where conditions are slightly more favorable--of many plants that might be useful, among them the prickly pear (Opuntia), saltbushes (Atriplex), and the Australian wattles (Acacia, subtribe Phyllodinae).
The forms of germination and dispersal
Annual life cycles are one way of ensuring the genetic future of a species (dispersing and surviving in the form of seeds). The strategies developed by desert plants to ensure the survival and dispersal of their seeds include endogenous rhythms in the seeds that control the proportion of seeds germinating at any given moment so that germination occurs when the temperature regime and photoperiod are at their most favorable. Seeds are often shed at the beginning or middle of the dry season, though some species retain them on the plant until conditions are right for germination.
The large grass family (Poaceae) is an evolutionary success, with species in all the world's climatic regions and often with a major, if not dominant, role in the landscape. In a semiarid environment, the annual grasses such as Sporobolus festivus in the Sahel are the first plants to appear when the rainy season starts. The short-lived S. festivus completes its life cycle in a few weeks before it is smothered by the growth of the slower-starting perennial grasses. Annual herbaceous plants are usually small opportunist species that can occupy a suitable niche while the growing conditions are good. They can survive the long dry season and many years of prolonged drought as seeds, then germinate when the rains arrive and complete their entire life cycle before conditions deteriorate. Although their way of life is very well adapted to the desert environment, these plants often show little or no adaptation to dry conditions.
Many desert plants produce seeds that require some degree of moisture before they germinate. In Mesembryanthemum nodiflorum (Aizoaceae), the seeds at the end of the capsule are shed first; as the plant is wetted, the central and then the basal seeds are released; the terminal seeds germinate after about four years, while the others need to ripen for several years before germinating. In the Namib, almost all the seeds of Delosperma pergamentaceum (Aizoaceae) germinate at the same time. This is a high-risk strategy, because not enough rain may fall and the seeds may not germinate successfully. But as it is a slow-growing and very long-lived succulent plant, only a few seeds need to germinate successfully to ensure the population's survival. The single-seeded fruit (cypselas) of the annual Asteriscus pygmaeus (Asteraceae) remain on the dead plant for several years within an involucre of hard bracts (a whorl of modified leaves) below the inflorescence, and they are only released from the dry capitulum when it rains. Other Asteraceae, including species of Crepis, Geropogon, and Picris, bear cypselas of different shapes (heterocarpy); the central ones have a normally developed pappus, while the peripheral ones have only a rudimentary one. The peripheral cypselas will not be dispersed until the beginning of the windy rainy season, and then they will not be dispersed far from the mother plant.
Amphicarpy is the ability to produce inflorescences with two types of fruits: a normal aerial fruit and a smaller underground one. The aerial fruits produce seeds adapted to long-distance dispersal, while the seeds in the underground fruits are dispersed only within the same site. Examples of amphicarpous plants include Emex spinosa (Polygonaceae), an annual Saharo-Arabian and Mediterranean herbaceous plant, and the annual Gymnarrhena micrantha (Asteraceae). Emex asperatus and E. cylindricus are two amphicarpic Australian annuals that, together with the pan-tropical E. desvauxii, produce cleistogamous flowers (inconspicuous flowers that never open) that are well protected within a sheath and are dispersed when the stem segments break off at the basal node. When it rains sufficiently, the annual legumes Ononis sicula and Trigonella arabica produce seeds with very impermeable coverings; the seeds that mature on the same mother plant when rainfall is scarce have even less permeable coverings. Something similar happens in many other Fabaceae of the genera Astragalus, Medicago, and Trigonella, whose fruits contain many seeds, only one of which germinates each year.
The presence of germination inhibitors in the propagules (shoots) and the leaf litter is one way of delaying germination until there has been enough rain to leach the allelopathic (growth-inhibiting) substances from the soil, thus ensuring there is sufficient moisture. In Israel, Pectis papposa (Asteraceae) needs at least 10 mm of rainfall to germinate. Germination inhibitors have been found in the valves of the fruit of Zygophyllum dumosum (Zygophyllaceae), the bracts of saltbushes (Atriplex), in the leaves of the floral envelope or perigonium of the sorrel Rumex roseus (Polygonaceae), and in the seed case of Colutea istria (Leguminosae). The nordihydroguaiaretic acid present in the leaves of the creosote bush Larrea tridentata (Zygophyllaceae) may be the agent responsible for the allelopathic action it exerts on the plants that try to establish themselves near it (including its own seed-lings), one way of eliminating a threat to its survival.
The different life-forms
Geophytes are plants with underground perennating buds that lose their aerial parts during the dry season so that they survive on the food supply accumulated in their reserve organs. A life cycle like this allows them to produce their roots, leaves, and flowers during the short growing season, when the temperature and humidity conditions are excellent; for the rest of the year, they merely survive the extreme heat and aridity. If rains are scarce, they may remain dormant all year-round or for more than one year. Surprisingly, some geophytes can grow and flower before the rains start. The species of Allium, Asphodelus, Bellevalia, Ornithogalum, and Tulipa (all Liliaceae) develop their leaves and flowers simultaneously, while others, such as Scilla (Liliaceae) and Urginea (Liliaceae) produce their leaves and flowers in different seasons. Perennial storage organs may include tunicate bulbs (such as those of Hesperocallis undulata, Liliaceae), corms (many members of the iris family, Iridaceae, including the genera Crocus and Gladiolus), rhizomes (such as marram grass Ammophila arenaria) and tubers (such as Pyrenacantha malvifolia, Icacinaceae).
Tunicate bulbs are highly modified shoots that form a fleshy underground perennating organ, consisting of many leaf bases arranged around a central apical bud, the future flower, surrounded by immature leaves and with some rudimentary adventitious rootlets at the base. Tunicate bulbs are common in the Liliaceae, Amaryllidaceae, and other related families of monocotyledons.
Corms are short, thickened underground perennating organs that develop leaves and flowers from one or more axillary buds; the plant begins to grow at the expense of the food reserves stored in the corm. The brown scale leaves that surround the corm in members of the Iridaceae are the remains of the leaf bases from the previous season.
Rhizomes occur in dicotyledons as well as monocotyledons. They are horizontal underground stems, normally branched, that produce roots at the nodes and serve to propagate the plant vegetatively. Rhizomes may be long and ropelike or thick and fleshy, as in the cattails (Typha, Typhaceae). The grass Sporobolus rigens, which grows on the mobile dunes of the Argentinian monte, has rhizomes more than 49 ft (15 m) long that grow 10-14 in (25-35 cm) underground, while its roots may be 16 in (40 cm) long and reach a depth of 28-31 in (70-80 cm). The tips of the rhizomes have resistant buds that drill into the soil. These rhizomes may grow 20 ft (6 m) in a year and produce aerial shoots every 7-8 in (18-20 cm). Having the ability to sprout after burial by the sand makes S. rigens one of the most useful plants for controlling mobile dunes.
Tubers are also produced by many species, dicotyledons as well as monocotyledons, in the hot deserts and subdeserts. Stem tubers are derived from the stem, and root tubers are derived from the root. Stem tubers bear buds (eyes), distinguishing them from root tubers, which do not. Typical examples of stem tubers include Pyrenacantha malvifolia (Icacinaceae) and Dioscorea elephantipes (Dioscoreaceae), whose thick edible tubers are derived from the first internode of the stem. Root tubers lack buds and are simple reserve organs that accumulate starch, water, or both. The mosmote or pochote (Ceiba acuminata, Bombacaceae) from the Sonoran Desert and C. parviflora from the semiarid Puebla and Oaxaca regions of Mexico, for example, both produce starchy root tubers; despite their low food value, production on a large scale has made them one of the basic local foodstuffs.
Finally, it is worth mentioning the curious growth-form of many species of Eucalyptus of the Australian subdeserts. The seedlings produce small protuberances where the two cotyledons are borne, which then grow downward around the stem (hypocotyl) until they form a sort of knot or woody tuber. The tuber produces aerial branches that grow to form a thin, umbrella-like, concave or flattened crown. Some similar growth forms, although with short-lived shoots or ones that shrivel every year, have evolved independently in about 30 families of plants in tropical and subtropical Africa; these are a response to unfavorable soil conditions, not primarily to fire.
2.3 Strategies to optimize use of water resources
In general, low rainfall, high temperatures, and the resulting high potential evapotranspiration force desert plants to use water economically in order to survive. Optimization in resource use affects both water capture and the adaptation of anatomical and physiological systems to make the best use of it.
Water uptake from the soil and water table
Phraetophytes, trees and shrubs with roots that can reach the water table (the tamarugo [Prosopis tamarugo], for instance), exploit the subsoil water reserves. During the construction of the Suez Canal, roots of afadar (Acacia tortilis) were found at depths of 98 ft (30 m), and it is known that the taproots of some American species of Prosopis can reach a depth of 262 ft (80 m). In the Marrakesh region of Morocco, 1 cm thick roots of Ziziphus lotus (Rham-naceae) were found at a depth of 197 ft (60 m) when digging a well.
Some phraetophytes grow into trees and tall shrubs, but other woody plants of the arid regions tend to have massive underground organs and annual, or very short-lived, surface shoots. This is true of the shrub Andira laurifolia var. laurifolia (Legumi-nosae) of the caatinga of northeastern Brazil. This plant produces shoots about 12 in (30 cm) tall with short leaves, allowing it to reduce transpiration to the minimum, and it grows stem storage organs in the top 7 ft (2 m) of soil, while the roots absorb water from the water table at depths of 59 ft (18 m). Another spectacular example is kad (Acacia [=Faidherbia] albida), which exploits all the aquifers it finds in the top 262 ft (80 m) of soil. The seedlings of phraetophytes have to grow their taproots deep into the soil as quickly as possible, because they will die unless they reach the moist layers of the subsoil before the dry season. In fact, very few seedlings make it, even in favorable years. In some deserts, such as the Sahara and the Arabian Desert, underground water is so deep that their roots cannot reach it. In others, such as the Atacama Desert (formed by the uprising of the bottom of the sea), even though there is a high water table fed by melting snow in the Andes it is so highly saline that plants cannot grow there.
But not all desert plants are phraetophytes. The alternative strategies
for nonphraetophytic perennial plants in deserts and subdeserts include 1) having a surface root system covering a large area, in order to make use of light rains, or 2) combining surface roots with a moderately long taproot branched at depth to absorb the water from any heavy rainfall that filters deep into the soil.
Surface rooting strategies are typical of many annual and ephemeral plants, while deeper roots are typical of many trees, shrubs, and herbaceous perennials, though many shrubby succulents, especially cacti, have a shallow root system. For example, the saguaro (Carnegiea gigantea) is a columnar cactus reaching a height of 39 ft (12 m), though its roots are often less than 3 ft (1 m) deep. Its many shallow lateral roots may be more than 98 ft (30 m) long, so it can take up the slightest rainfall-derived moisture from a large area.
The plants that have been able to root, regardless of whether they are trees, shrubs, or herbs, will only be able to survive if the water catchment area covered by their root system provides enough moisture for them to grow. This limits the density of the vegetation; trees and shrubs with a root system that spreads in a radius of 98 ft (30 m) need a space of at least 197 ft (60 m) between individuals, though plants with different root systems or with seasonally different water requirements can grow in the same space. Lateral root systems may be effective in bare soils, as, for example, in the thickets of mulga (Acacia aneura) in Australia. Regardless of whether there is a tree or shrub layer, the presence or absence of perennial, annual, and ephemeral plants is a reflection of the quantity and distribution of rainfall and of the competition between roots in space and time.
Their growth-form means many woody plants obtain additional moisture by condensing atmospheric water vapor within their crown. Rainwater or heavy condensation on the crowns can be channeled toward the soil, down the leaves, twigs, and branches, and then the stem. Once it has entered the soil, the water is absorbed by the roots and goes back to the canopy.
Grasses and other herbaceous plants may be relatively abundant in the microenvironment at the base of trees and shrubs, partly due to the shade, which creates a slightly cooler environment, and partly because humidity is slightly higher, due to the flow of water down the stem for absorption by the plant's roots. This example also illustrates how herbaceous plants with a shallow root system can compete successfully for water with deeper-rooted trees and shrubs.
Plants that resist the wind act, for this very reason, as points where sand accumulates. As a result, many plants have to adapt their growth pattern to the rising soil level in order to avoid being buried. For example, after a moderate accumulation of sand, the mother plants of the field southernwood (Artemisia campestris), the Rhanterium suaveolens (Asteraceae), and the saltwort Salsola vermiculata, split into several functionally independent plants. Other species such as markh (Leptadenia pyrotechnica, Asclepiadaceae) develop adventitious roots on their stems as they are buried, so they can respond to the changes in humidity and the shifting shape of the dunes. Rhanterium suaveolens does something similar, and its adventitious roots may eventually turn into the main roots of functionally independent plants.
The compact rhizomatomous perennial grass Stipagrostis pungens produces adventitious roots at the bases of the stolons that are going to be buried by the sand, allowing it to grow through any accumulation of soil. Where this species is dominant, after Rhanterium has disappeared, piles of sand up to 7 ft (2 m) deep accumulate. The roots of S. pungens are entirely covered by a sand sheath. The piliferous layer acts as a mucilage-secreting gland, helping the root tip to force its way through the sand. The grains of sand, moistened by the mucilage, adhere to each other and to the root to form a sand sheath. The cells of the piliferous layer grow into root hairs that do not secrete mucilage but absorb water and minerals. The sand sheath not only protects the root and the root hairs from the heat and drought but also increases the root's absorption capacity.
A recent study carried out in the United States detected the presence of a nitrogenase associated with the sand sheath of some American grasses, including Aristida purpurea, Oryzopsis hymenoides, and Stipa comata, although it has not yet been shown that these species can fix nitrogen. It is known that sand sheaths also form on many other desert grasses such as esparto grass (Lygeum spartum), Panicum turgidum, and Stipagrostis [=Aristida] obtusa. In the annual grasses--downy brome grass (Bromus tectorum), wall barley (Hordeum murinum), and Schismus barbatus [=S. calycinus]--the soil sheath is retained by the absorbent root hairs rather than a sticky secretion. Whether sand sheaths are formed in other families has yet to be investigated, but it has definitely been observed in the Sahel among species of Tephrosia (Fabaceae).
Leaf architecture and water economy
One very logical adaptation to reduce water losses is to shed the leaves, the organs responsible for most water loss. Leaves are normally shed when the water taken up by the roots is not enough to replace that lost by evapotranspiration during the day. The leaves are usually shed at the beginning of the dry season but may be shed during any period of prolonged drought. Shedding the leaves every year at the end of the growing season is typical of deciduous species, but evergreen species that normally keep their leaves all year-round may also shed their leaves under conditions of severe water stress. In the absence of leaves, part of the cortex can perform photosynthesis, as happens in the chenopods Haloxylon salicornicum (see pp. 326-329) and Anabasis articulata.
An even more drastic way of reducing water loss is by the physiological isolation and death of a leaf-bearing branch or group of branches. This is done by the saltbush Atriplex torreyi, a perennial chenopod shrub whose trunk is deeply grooved due to the separation of the cortical rays and the unequal growth of the cambium. This unusual stem structure means it can physiologically isolate some branches and concentrate water in just one or a few branches. Likewise, in the Negev, the trunks of wormwood (Artemisia herba-alba) have bands of suberized (infiltrated with corky tissue) cells that divide the plants into separate units, each with its own root system. The units that cannot supply enough water in an especially dry season will die. The same adaptation is found in the Asteraceae Achillea fragrantissima and in harmal, Peganum harmala (Zygophyllaceae).
Water loss by evapotranspiration through the leaves can also be limited by thickening the cuticle, and this is often accompanied by thickening of the walls of the epidermal and even subepidermal cells. Secretion of wax onto the leaf surface may be large enough to be commercially important, as happens with the carnauba wax palm (Copernicia prunifera).
Reducing the area of leaves also reduces water loss. This reduction is taken to extremes in microphyllous plants, as in the ephedras (Ephedra spp.) and several halophilous chenopods such as the glassworts (Arthrocnemum, Salicornia) and saltworts (Salsola), in which the leaves have been reduced to scales and photosynthesis is performed by the stems. A less drastic step is leaf dimorphism, in which the leaves borne in the dry season are much smaller than those produced in the wet season. In the Australian wattles (Acacia, subtribe Phyllodinae), the adults do not produce leaves, and photosynthesis is performed in the phyllodes, which are in fact flattened, somewhat rubbery, leaf petioles.
Producing a covering of short or long hairs (pubescence, pilosity) greatly reduces the wind speed over the leaf surface and, thus, losses by evapotranspiration. In some cases, however, eliminating the hairs diminishes transpiration. For example, the trichomes of Crassula easily absorb the surface drops of water, and thus it is not surprising that eliminating the hairs may reduce the transpiring surface.
Yet the hairs have other functions such as dispersing the sunlight reflected from the ground, thus cooling the leaf, and also protecting the plant from insects and other animals. The leaves and stems of many halophilous chenopods are covered in vesicular hairs that are filled with a salty solution in the young green leaves. During the dry season these vesicular hairs dry out and the salts they contain then precipitate, increasing reflection and reducing the sunshine reaching the leaf.
Finally, the stomata located on the lower face of the leaf are less exposed to the action of the wind, so that their water loss by evapotranspiration is lowered. Rolling up or folding the leaves further protects the stomata. Many herbaceous desert plants roll their leaves in conditions of water stress and bear only sunken stomata on the underside.
The role of spininess and succulence in water control
Leaf spines are modified leaves (such as the spines of cacti) or parts of a leaf (for example, stipules in the legumes Acacia and Prosopis). These modifications reduce the leaf surface and, thus, losses by transpiration. In cacti, the points of the spines act as nuclei for the condensation of water droplets, which then drip down to the soil, where the roots can use this moisture. The juvenile forms of the saguaro (Carnegiea gigantea) are protected from being eaten by small animals by their very long, sharp downward-pointing spines; as the plant grows, the spines straighten up and face outwards.
Spines and thorns are a good defense against herbivores, though in the seedlings they are usually soft or absent. Paradoxically, the seedlings of Acacia and Prosopis, are vulnerable to herbivores during the growing period when they are still vulnerable to the lack of water and must optimize photosynthesis in their limited foliage while they concentrate on developing their roots.
Succulence is an adaptation shown by many xeromorphic plants that allows them to store water and to reduce their losses by evapotranspiration. The tissues of the swollen stems or leaves consist largely of a translucent reserve parenchyma that accumulates water. Water loss is reduced because the lower surface/volume ratio means that less of the leaf or stem is exposed. The living stones of the Namib--Lithops (Aizoaceae) and Fenestraria (Aizoaceae)--reduce water loss even further by bearing much of the leaf underground, leaving only a window at the end exposed to light (see vol. 1, photo 119, p. 183).
Thickened stems specialized for storage of water are common in the Agavaceae, the aloes, Asclepi-adaceae, some members of the Aizoaceae (Lithops, Nananthus), Apocynaceae (Adenium), and Euphor-biaceae, as well as in the halophilous chenopods of salt deserts. Succulence has developed in parallel in a series of unrelated families, whose external similarities need not reflect any similarity in their internal structures. For example, the columnar cacti of the New World are very similar to the columnar euphorbias of the Old World (see drawing 58, p. 104); the American family Fouquieriaceae resembles the Didieraceae in Madagascar, and the Agavaceae in the New World resembles the Aloinae subtribe of the Liliaceae in the Old World.
Plants growing in saline soils in the arid and semiarid regions (where evaporation is greater than rainfall) have to deal with salinity as well as aridity. How salt absorption is regulated is still not completely understood, but, in most cases, it is based on highly developed osmotic adaptations to maintain the pressure needed to absorb water.
The halophilous plants that grow in these soils accumulate large amounts of sodium and chlorine ions, allowing them to maintain low internal potentials and avoid a water deficit. More salt is transported to the shoots than is required to maintain them turgid. The salt glands and scale-like hairs on the leaves of many halophilic plants actively secrete sodium and chlorine ions, thus eliminating their excess salts. This also occurs in grasses of the genera Aeluropus and Distichlis, Frankenia spp. (Frankeniaceae), Glaux spp. (Primulaceae), Limonium and Limoniastrum (both Plumbaginaceae), tamarisks (Tamarix, Tamaricaceae), and many chenopods. In a variety of chenopods such as the genera Arthrocnemum, Atriplex, Cornulaca, Haloxylon, Salicornia, and Suaeda, succulence also seems to be a way of reducing salt concentration by diluting it in a large volume of water.
2.4 The diverse vegetation of the hot deserts and subdeserts
The main characteristic of desert vegetation is that it is discontinuous in space and time. Vegetation in deserts is limited to depressions in the relief and the edges of the hydrographic network. The relief between the valley remains bare, except for ephemeral annual vegetation following the infrequent and sparse rainfall. Subdeserts, however, have a perennial vegetation that is relatively widespread and is to a greater or lesser extent regularly distributed within the landscape along geographical gradients. The vegetation's physiognomy and nature vary greatly with the climate and soil and with human pressure and management.
Perennial desert plants
The amount of perennial vegetation in deserts varies, depending on the quantity and frequency of runoff--both autochthonous (local) or allochthonous (from relatively distant hills or mountains)--and also depending on the frequency of surges. Desert vegetation may consist of scrub and shrubs, with or without trees; when there are no trees, it may resemble a wasteland (like the Artemisia steppes, the chenopod steppes, or the Rhanterium steppes, all in northern Africa and the Near East); a thyme scrub or a shrubby desert may also form (northern Africa, Near East, Sind, Thar, southern Africa, the American deserts, and Australia); even steppes or wooded savannahs may occur (northern Africa, Near East, Sahel, eastern Africa, and the American deserts). The trees present are acacias (Acacia ehrenbergiana, A. tortilis, A. raddiana in the Sahara and the eastern deserts; A. gerrardii in the Sinai and the Negev; A. nilotica in the Sind; A. karroo, A. giraffae, A. haematoxylon, A. erioloba, A. mellifera subsp. detinens, A. reficiens, A. [= Faidherbia] albida, as well as Dichrostachys cinererea, in southern Africa; Acacia tortilis, A. nilotica, A. bussei, A. mellifera, A. asak, A. nubica, and A. reficiens in East Africa).
The southern African deserts are home to some remarkable trees, especially the arborescent aloes (Aloe arborescens, A. dichotoma, A. pillansii, see photo 37, p. 72) and the pachypodiums such as Pachypodium succulentum and P. namaquanum, the latter showing surprising convergences with the remarkable boojum tree (Fouquieria [= Idria] columnaris) from the Vizcaino Desert of Baja California. There are also many convergences in apperance between some cacti in the Americas and the cactiform euphorbias in Africa. The Namib is also characterized by the presence of many succulent species, mainly members of the Aizoaceae with highly ornamental flowers, and also by a whole set of other even more surprising species. Among these odd species are the welwitschia (Welwitschia mirabilis, see box), a strange gymnosperm with indefinitely growing leaves that is a genuine living fossil, and the shrub Myrothamnus flabellifolius, the only mainland African species of its genus and of he family Myrothamnaceae--it contains only one other species, from the arid regions of Madagascar--which estivates (becomes dormant in the summer) by drying its leaves out, folding them like a fan, and turning black and brittle. (It appears totally dead, but the leaves revive when it rains.)
The Namib Desert's succulent plants include: pachycaul trees that accumulate water in their trunks, among them the Moringa ovalifolia (Moringaceae) and Commiphora saxicola (Burseraceae); the arborescent crassulacean Cotyledon paniculata; the cactiform euphorbias Euphorbia virosa, E. damarana, E. gariepina, and others; Cyphostemna uter and C. currorii (Vitaceae); Adenia pechuelii (Passifloraceae); Sterculia quinquiloba (Sterculiceae); and Ceraria namaquensis (Portulacaceae).
The hot deserts in North America are distinguished by their: 1) arborescent yuccas, known in the United States as the Joshua tree (Yucca brevifolia), and other species of yucca (Y. valida, Y. schidigera, Y. schottii); 2) the giant columnar cacti (Pachycereus pringlei, P. pecten-aboriginum, Lamaireocereus thurberi, and the saguaro, Carnegiea gigantea); 3) shrubs with a treelike growth form known as palos (Cercidium floridum--the palo verde, C. microphylla--the palo azul, C. sonorae--the palo santo, Olneya tesota--palo hierro); 3) pachycaul trees of the genera Pachycormus (Anacardiaceae) or Bursera (Burseraceae); 4) acacias such as catclaw (Acacia greggii), palo blanco (A. willardiana), the sweet acacia or huisache (A. farnesiana), and the Jerusalem thorn (Parkinsonia aculeata); and 5) the mesquites or algorrobos (Prosopis glandulosa, P. laevigata, P. torreyana, P. velutina).
The subdesert vegetation
The subdesert vegetation on the temperate (Mediter-ranean) margins of deserts is very different from that on their tropical margins. Mediterranean-type subdeserts consist largely of steppe formations such as wastes and scrub. These steppes may be primary (natural) or secondary (resulting from the degradation of forests by wildfires, grazing, deforestation, overgrazing, and clearing). Much of the scrub of northern Africa and the wastes and scrubs of the Iberian Peninsula above the 8 in (200 mm) annual rainfall isohyet are secondary in nature, derived from forests of Aleppo pine (Pinus halepensis), junipers (Juniperus phoenicea, J. thurifera), or arar (Tetraclinis articulata). In the Near East, the steppes above the 8 in (200 mm) annual rainfall isohyet are the result of degradation of open forests of white pine or Pistacia atlantica, with several other species of Pistacia, almonds (Prunus dulcis), and, locally, oaks (Quercus).
The vegetation of intertropical subdeserts is generally a grassy savannah with scattered trees, shrubs, and annual or perennial herbaceous plants. Shrublands are rare or absent. The Sahel savannahs, for example, consist of small, spread out, spiny trees (Acacia, Commiphora, Balanites, Ziziphus) and a more or less continuous ground cover dominated by annual grasses. This structure is shown by the savannahs in the arid areas of eastern and southern Africa and India, though the herbaceous layer consists of perennial grasses, not annuals as in the Sahel. This is partly due to the distribution of rainfall, concentrated in the three summer months in the Sahel but spread over four to eight months in eastern Africa, the Sahel, and the Kalahari, with two rainy seasons in the first case and only one in the second.
Like its deserts, the landscapes of the New World's hot subdeserts are characterized by a flora dominated by typically Neotropical taxonomic groups: mesquites or algorrobos (Prosopis), cacti, agaves, yuccas, and others like creosote bushes (Larrea). In South America, this space is occupied by monte, caatinga, and sertao.
The arid zones of Australia are typically dominated by trees and shrubs of the genera Acacia, Cassia, Eucalyptus, and Melaleuca. The most important species include the mulga (Acacia aneura) and the clump-forming spiny perennial spinifex grass (Triodia). Australia's subtropical arid and Mediterranean zones are distinguished from the tropical savannahs by the dominance of halophilous shrubby chenopods (Atriplex, Maireana), which provide much-appreciated grazing in relatively saline areas.
Sporadic flowering: the case of the Atacama Desert
The very special dynamics of the ecosystems of the Chilean-Peruvian desert region are associated with the recurrent but irregular disturbances of sea currents and the atmospheric circulation known as the ENSO (El Nino-Southern Oscillation). These disturbances make it easier for the moist air masses from the Pacific Ocean to penetrate inland. As a result, the arid areas receive, in a short period of time, a quantity of rainfall that may be greater than an average year's rainfall--maybe even many times greater. The favorable temperatures (59-63[degrees]F [15-17[degrees]C]), the accumulated moisture in the sandy desert soils, and the abundant seed bank mean that the Atacama Desert is carpeted with blooms during its growing season (August-November), with the spectacular and almost simultaneous flowering of a large number of plants with conspicuous blooms, a process known locally as the flowering of the desert. The entire region is covered by a thick multicolored cover of geophytes and annuals, and the shrub layer also sprouts. The dominant plants have flowers ranging in color from bluish to whitish, red to pink, fuchsia to lilac, and yellow to orange.
This flowering is most spectacular in the coastal lomas communities, dominated by perennial and annual herbaceous plants belonging to the genera Allionia (Nyctaginaceae), Argylia (Bignoniaceae), Caesalpinia (Leguminosae), Calandrinia and Portu-laca (both Portulacaceae), Cristaria (Malvaceae), Eragrostis (Poaceae), Leptoglossis (Solanaceae), and Nolana (Nolanaceae).
Farther south, where average annual rainfall is between 3 in (80 mm) 20[degrees]S and 1 in (30 mm) 27[degrees]S, this flowering is clearest in the coastal lomas and on the sandy soils of the plains and ranges of hills in the inland transitional desert. The dominant association varies, depending on the habitat (rocky or sandy) and the sector (coastal or on the inland plain). On the inland llanos (which should not be confused with the grassy savannahs of rainier regions of tropical South America that are also known as llanos), the most striking combination of colors are the formations consisting of the malvilla (Cristaria patens, Malvaceae), an annual with pink flowers, the rock purslane (Calandrinia longiscapa, Portulacaceae), whose flowers are purple, and the yellow-flowered flor del jote (Argylia radiata, Bignoniaceae). A large area of the central plain is also covered by the association of pata de guanaco, the whitish-flowered Nolana paradoxa (Nolanaceae), and the yellow-flowered Oenothera coquimbensis (Onagraceae), and an association made up of two malvillas (Cristaria cyanea, with lilac flowers, and C. patens, with deep fuchsia flowers).
The coastal area is dominated in some areas by quisco copiapoa (Cristaria dealbata), which has yellow flowers and an odd silver-gray stem; the other areas are dominated by an association of amancai (Balbisia penduncularis, Geraniaceae), churqui or churco (Oxalis gigantea, Oxalidaceae), and alcaparra (Cassia [=Senna] cumingii var. coquimbensis), all three of them shrubs with yellow flowers, accompanied by cacti of the genera Eulychnia, Eriosyce, Neoporteria, Opuntia, and Echinopsis. By the sea, where soils are mainly sandy, there are many bulbous species of the Liliaceae such as ananucas (Rhodophiala) with red, yellow or white flowers, huillis or cebollinas (Leucocoryne) with whitish or bluish flowers, or azulillos (Pasithea) with blue flowers. These are often accompanied by evening primroses (Oenothera), now widespread in gardens throughout the tropics and the Mediterranean areas.
The flowering of the desert is most spectacular at 27-29[degrees]S, where floristic diversity is high. The grasslands have countless annual species belonging to many families. The lily family is represented by several ananucas such as the yellow-flowered Rhodophiala and the white-flowered R. laeta, together with the red- and yellow-flowered R. phycelloides; there is also the mariposa de los molles (Alstroemeria pelegrina), the lirio del campo (A. magnifica), the lirio amarillo (A. kingii), and the lirio rosado (A. leporina), as well as different huillis or cebollinas and some azulillos.
Apart from the petaliferous monocots, including members of the Liliaceae, there are some Aristolochiaceae such as the birthwort (Aristolochia chilensis), the bindweed (Convolvulus chilensis, Convolvulaceae), the suspiro del campo (Nolana paradoxa, Nolanaceae), malvillas (Cristaria aspera, C. patens), violets such as the violeta del campo (Viola asterias, Violaceae), the ice plant (Mesembryanthemum crystallinum, Aizoaceae), as well as the groundsel (Senecio brunonianus, Asteraceae), the poppy or cardo blanco (Argemone hunnemannii, Papaveraceae), butterfly flowers (Schizanthus candidus, Solanaceae), pata de guanaco (Calandrinia longiscapa, Portulacaceae), the dondiego de noche (Oenothera coquimbensis, Onagraceae), the heath-like Frankenia chilensis (Frankeniaceae), and spurges such as Euphorbia lactiflua (Euphorbiaceae).
The trees include members of the: Leguminosae such as chanar (Geoffroea decorticans) and the alcaparra (Cassia cumingii), the retamo (Caesalpinia angulicaulis), the algorobillo (C. [=Balsamocarpon] brevifolium) and espino (Acacia caven); Onagraceae, such as palo de yegua (Fuchsia lycioides); Geraniaceae, such as amancai (Balbisia peduncularis); Boraginaceae, such as palo negro (Heliotropi-um stenophyllum), and carbonillo (Cordia decantra); Asteraceae such as flor de minero (Centaurea chilensis and C. floccosa), coronilla del fraile (Encelia canescens) or chamiza blanca (Bahia ambrosoides); Oxalidaceae, such as the churco (Oxalis gigantea); Verbeneaceae, such as the verbena (Junellia selaginoides); Apocynaceae, such as the cuerno de cobra (Skytanthus acutus), Nolanaceae such as Nolana coelestis, and cacti of the genera listed above.
The blooming of the desert is short-lived. The ecosystem starts to return to normal after October-November, when the strong sunshine, higher temperatures, and desert winds begin to dry it out. By January-February, the landscape has returned to its typical aridity.
A reminder of a better past
The trees in the Sahara are now almost entirely confined to the oases. These oases were probably occupied by acacias, species of Capparis and Maerua (both Capparidaceae), and the doum palm (Hyphaene thebaica), but they have been replaced by the date palm (Phoenix dactylifera) and other cultivated plants. One tree that was notable because it did not grow in an oasis was, until its destruction, the Tenere tree, a specimen of afadar (Acacia tortilis subsp. raddiana), probably the remains of a former thicket. It survived in a site with an annual average rainfall of less than half an inch (10 mm) thanks to its ability to use the water table at a depth of nearly 131 ft (40 m). Considered taboo by the Tuareg caravans and protected from browsing and cutting for firewood, the Tenere tree survived until a military vehicle broke off part of its forked trunk. Unfortunately, in 1973 it was hit by a truck and did not survive.
Other relicts of the historic climate (moister than present-day) in the Sahara at the beginning of the Quaternary still survive in the valleys of the Saharan mountains. On the southern foothills of the Atlas (in Morocco), the Mouydir Range, Tassili N-'Ajjer and Ahaggar (in Algeria), and Mount Marrah (Sudan), there is a kind of olive (Olea europaea subsp. laperrinei), named after Colonel Laperrine, its discoverer. It does not grow in other mountainous areas in the Sahara such as the Tibesti Massif (in Chad). This olive formed part of a group of Mediterranean species that spread into the central Sahara in the early Quaternary and whose distribution has gradually declined as the climate has become more and more arid; it is now restricted to the mountains mentioned above. Except for its narrow linear-lanceolate leaves, the subspecies laperrinei closely resembles the Afro-Asiatic olive (O. europaea subsp. cuspidata), the presumed ancestor of the cultivated olive (O. europaea subsp. europaea).
Another species that is a relict from the Pleistocene spread of the Mediterranean flora into the Sahara is an endemic cypress called the Tassili cypress, Duprez's cypress, or tarout (Cupressus sempervirens subsp. dupreziana), that has also been described as a separate species (C. dupreziana). It differs from the common Italian cypress (C. sempervirens subsp. sempervirens) in its characteristic flattened shoots and small leaves. The volatile oils of the leaves of the subspecies dupreziana and sempervirens are almost identical, confirming their close taxonomic relationship. The surviving specimens are confined to a few rocky valleys in Tassili N-'Ajjer in the region near the Ahaggar Massif. Knotty and twisted trees more than 1,000 years old may reach a height of 66 ft (20 m) with a trunk 10 ft (3 m) in diameter. The cypress's wood was formerly used by Tuaregs, and attempts are now being made to introduce the species into other regions, because it can grow in arid areas and may help to improve the drought resistance of related species.
Thickets of tamarugo (Prosopis tamarugo) grow where there are depressions in the thick (4-24 in [10-60 cm]) saline crusts in the Pampa del Tamarugal, in the north of the Atacama Desert. Though the water table is at a depth of 5-131 ft (1.5-40 m), it is still not clear how these trees obtain water. It was thought that the leaves absorbed water vapor at night through the stomata and excreted the excess through the lateral roots, about 3 ft (1 m) below the soil surface. Physiologically, this is quite impossible, and furthermore the leaves have no adaptations to catch water. The tamarugo is in fact a phraetophyte that absorbs water through its roots. The dense network of lateral roots are presumably former root systems left stranded when the water table was lowered; this happened when the underground water flowing from the Andes were diverted to Chile's mining and coastal cities. Despite its remarkable ability to resist drought, the tamarugo has not been successful outside its original habitat.
3. The fauna and animal populations
3.1 Strategies for survival in arid conditions
The increasing aridity of the tropical and subtropical regions now occupied by hot deserts and subdeserts seems to have started in the Miocene period (24-5 million years ago). By then, Australia was isolated; this alone is enough to explain that country's unusual fauna, regardless of its present bioclimatic conditions. North and South America were also isolated and were separated from each other until the Pleistocene, when the formation of the Isthmus of Panama allowed animals to travel between them. The Miocene also saw the collision of the African and Eurasian plates, causing the disappearance of the ancient Sea of Tethys and joining the subtropical areas of what is now the Old World. This is why these areas have a relatively uniform fauna. It also explains the similarity of the faunas of the deserts and subdeserts of North and South America (despite their undeniable differences) and provides a reason for the fauna of the Australian deserts being totally different from that of all the others. These shifts, together with the gradual adaptation of less mobile animals to life in the desert, has resulted in the different faunas now found in the world's various deserts and subdeserts.
Desert animals all face a common environmental problem: the shortage of water made worse by the high temperatures that cause an increase in evaporation. Animals lose most water through sweating, respiration, and excretion. In order to survive on their low water intake, they have to reduce losses and conserve water as much as possible. This is achieved by many coordinated morphological, physiological, and ethological mechanisms.
Different forms of inactivity and dormancy
Insects, like other small animals, often evade the hot, dry desert conditions by completing their life cycles in the rainy season, before conditions become stressful. Insects survive by entering a state of suspended animation known as diapause, during which metabolism is slowed down to almost nothing, greatly reducing water loss by respiration. The eggs of many insects and of some crustaceans such as tadpole shrimp (Triops) survive the heat and drought of the desert summer in diapause. These eggs can resist both low and very high temperatures and are resistant to loss of water vapor by transpiration.
Many large desert animals also avoid the heat and drought by estivating in a quiescent state. They do not, however, enter dormancy, as hormones are not involved and the animals return to activity as soon as conditions improve. For example, during estivation, the mouth of the shell of desert snails is closed by a thick epiphragm of calcified slime, which reduces water loss by evaporation so much that some desert snails can remain dormant for five years and revive when wetted. Their metabolic rate is reduced to allow them to survive these long periods of dormancy.
Other animals such as amphibians estivate in deeply buried, relatively waterproof cocoons of dried mud or dead skin. Their metabolic rate is reduced to almost zero, and they survive on the fat reserves in their abdominal cavity. American spadefoot toads (Scaphiopus), for example, spend most of the year in estivation up to 3 ft (1 m) below the soil surface. Every summer, they emerge on the first night of heavy rain (generally in early July in Arizona) and spawn in the puddles that form. The following period of favorable conditions lasts little more than eight weeks, when the tadpoles have to grow very fast and the adults must eat enough to lay down fat reserves for the next long period of estivation. Under natural conditions, they are stimulated to leave the soil by the sounds caused by thunder and heavy raindrops.
Unfortunately, they also respond to the vibrations produced by motorcycles and other mechanical vehicles crossing the desert, which may induce them to emerge before the start of the rainy season, when there is no food or water for them. This stresses them greatly; they do not have enough reserves to dig back into the soil, so they die. Entire populations of American spadefoot toads can be inadvertently exterminated in this way.
Avoidance and escape from heat and drought is achieved by morphological and physiological traits (to reduce both heat gain and water loss) and by behavioral strategies (such as hiding in burrows and shelters during the hottest hours of the day). Most desert lizards, for example, are diurnal (active during the day), but they maintain their body temperature remarkably by shuttling between the sun and the shade.
All aspects of ecophysiology are interrelated, but in hot deserts none are closer than thermal and water relations. Because dry air is highly desiccating at high temperatures, heat and drought are by far the most important features of the climate. Arthropods and reptiles both have relatively impermeable outer surfaces. Desert reptiles show a higher water loss through the skin than was formerly thought, but even so it is clearly much lower than in reptile species from moister environments.
In most cases, little is known about the factors responsible, but the presence of scales seems not to be significant. The rate of water loss in a scaleless mutation of the North American bull snake or gopher snake (Pituophis melanoleucus) was about the same as those of a normal individual, even though the scaleless individual lacked the outer superficial dermal layer and had a much thinner keratin layer than normal.
Once a layer of moist sand has accumulated over the nostrils, it may reduce respiratory water loss by moistening the inhaled air and trapping some of the water in the exhaled air. This strategy, which is used by many burrowing lizards such as the sand-diving lizard Aporosaura anchietae, prevents sand grains from entering their nostrils by means of special nasal valves. Tortoises hold their breath when they are desiccated. Jerboas, kangaroo rats, and other small rodents have narrow nasal passages with a large surface area that are cooled by evaporation when air is inhaled; the water is recondensed during exhalation. The saw-scaled viper (Echis carinatus), the horned viper (Cerastes cerastes), and the Saharan common sand viper (C. vipera) emit a noise by rubbing together the keels of their lateral scales (known as stridulation), thus avoiding water loss through hissing.
Cutaneous water loss is far less important in larger vertebrates such as birds and mammals than in lizards and small snakes. This is because the surface area of a body only doubles if its volume is tripled-the larger the animal, the smaller its surface area/volume ratio. This simple relationship explains why desert sheep and goats have comparatively longer legs than breeds from cooler climates. Only comparatively large desert mammals have a sufficiently low surface area/volume ratio to allow them to cool their bodies by sweating. Because they have a greater reserve of water per unit surface area, large animals like the dromedary (Camelus dromedarius), the ass (Equus asinus), the addax (Addax nasomaculatus), the oryxes (Oryx), and the Dorcas gazelle (Gazella dorcas) can resist a given rate of water loss for a longer time than small mammals (such as jerboas and kangaroo rats) without dying.
Heat loss by sweating does not start in these larger vertebrates until the body temperature rises far above normal. Heat stored during the day is lost at night when the ambient temperature falls, thus preventing excessive water loss. Camels, for example, do not start to sweat until their body temperature has risen to 105.2[degrees]F (40.7[degrees]C). Furthermore, when the camel's temperature rises, the difference between it and that of the surrounding air decreases, so that less sweat is needed to prevent a further increase in body temperature.
The coarse hair of the camel's back acts as a barrier to the sunshine and slows down the conduction of heat from the environment. Body temperatures in the addax, oryx, Dorcas gazelle, and ostrich may also reach high values. The body temperature of a gazelle or antelope may reach 115[degrees]F (46[degrees]C), but the blood supply to the brain is cooled by heat exchange in the carotid sinus rete, a network of small blood vessels in the cavernous sinus. This sinus is full of venous blood draining from the nostrils, where it has been cooled by evaporation from the moist mucous membranes.
Birds cannot sweat, but they lose heat and moisture 1) by panting and 2) by flying, when the air is forced through their lungs into the air sacs. As a result, a relatively large amount of water is lost by evaporation. Small birds generally lose more water than small mammals in arid environments. Birds need to drink to survive, but this is made much easier by their mobility. Larger birds of prey may even obtain enough water for their needs from the body fluids of their prey.
Physiological optimization of water intake
One common physiological adaptation among desert herbivores and carnivores is the ability to live on the water obtained with their food. Insects tend to conserve the high water content of plants they feed on, and their bodies may contain far more water than that contained in their diet. Gazelles and antelopes browse at night on leaves that are actively transpiring and contain maximum amounts of water. By sheltering in their relatively cool, moist burrows during the daytime, kangaroo rats and jerboas can maintain their water balance on a diet of dry seeds without ever needing to drink. Yet, if they had to breathe the dry air outside their burrow during the daytime, the rate of evaporation from their lungs would exceed the rate of formation of metabolic water.
Other desert mammals may also obtain enough water from their food; among these mammals are the sand rat (Psammomys obesus), which feeds on succulent plants, the North American pack-rats (Neotoma), ground squirrels (Spermophilus [= Citellus]), which feed on the juicy fruits of the prickly pear (Opuntia), and the American grasshopper mice (Onchomys), whose insectivorous diet covers all their water requirements. The crest-tailed marsupial mouse, or mulgara (Dasycercus cristicauda), also feeds basically on insects and excretes only a small volume of very concentrated urine. Large desert mammals also reduce their water losses by producing extremely concentrated urine and bone-dry dung and by selecting the plants whose leaves contain the most moisture.
The excretory products of insects (uric acid) and arachnids (guanine) are highly insoluble. As a result, these creatures can eliminate their nitrogenous wastes from the body as a dry solid, meaning no water is lost in the process. The urinary wastes of reptiles and birds are eliminated as a pulpy or semisolid mass containing high levels of uric acid and very little water. The excretion of uric acid is quite advantageous: urine can be produced that is more saline than blood plasma; consequently, this excretion system is associated with the production of terrestrial eggs enclosed within relatively impermeable membranes. If a bird or reptile embryo released its nitrogenous wastes as ammonia, as fish do, it would poison itself. The urea produced by mammals is soluble, and it might become concentrated enough to alter the internal osmotic balance of the egg, which would also cause the embryo to die. Uric acid, however, can be left inside the eggshell when the young insect, reptile, or bird hatches. Uric acid excretion developed by the embryos has been retained by adult animals; it is especially useful in regions where water is scarce. Thanks to these adaptations, desert insects, spiders, and scorpions are extremely economical in their use of water.
The protective cuticle and anatomy of arthropods
With few exceptions, those desert arthropods that do not have short life cycles with diapause spend the hot, dry, daytime hours under rocks or stones or in deep burrows that they dig in the sand. Because they are small, their surface area/volume ratio is extremely high, and to survive in the desert (or in any other terrestrial environment that is not very humid) they must possess a body covering resistant to water loss. In deserts, even woodlice (isopod crustaceans, most of which live in moister environments) have developed cuticles that are surprisingly resistant to water loss. Thanks to its ethological adaptations, the woodlouse Hemilepistus reaumuri is one of the most resistant herbivores and detritivores of the microfauna of many arid areas of North Africa and the Middle East.
Arachnids and insects have cuticles that are even more resistant to water loss than desert woodlice. The outer layer of the integument, the epicuticle, contains a monomolecular layer of lipid (the wax layer) that is responsible for almost all the integument's resistance to water loss. The temperature at which the molecules of the wax layer allow water to pass is much higher in desert insects and arachnids than in comparable species from temperate environments.
An arthropod completely covered in an almost impermeable epicuticle wax layer would have the great advantage of losing very little water through transpiration, but it would be unable to breathe because the oxygen molecule is larger than the water molecule. Thus, they have had to evolve respiratory mechanisms that allow gas exchange without excessive water loss. In myriapods and insects, oxygen diffuses through the spiracles and along the tracheae and tracheoles to the metabolically active tissues. In most arachnids, the main respiratory organs, called book lungs, contain many blood-filled plates (lamellae) that communicate with the external atmosphere through small pores. Scorpions have only book lungs, while spiders have both book lungs and tracheae, although the tracheae are not very important in their respiration. Tracheae have evolved secondarily in the wind scorpions (order Solifugae), and their presence is related to an extremely active mode of life. Normally, spiracles and book lungs are closed by special muscles that relax only when the carbon dioxide content of the blood reaches or exceeds about 5%. Thus, the water loss that inevitably accompanies respiration is kept to a minimum.
Many beetles and other desert insects obtain moisture by chewing hygroscopic (moisture-sensitive) plant material that has absorbed atmospheric moisture during the night. Sea mists and fogs are a source of moisture at night in some desert regions, especially the Namib and Atacama deserts. Some wingless insects such as ticks, bristletails, fleas, and some cockroaches and insect larvae can absorb water vapor directly from unsaturated air by an as-yet-unknown mechanism. The rectum seems to be the site of water uptake, except in ticks and the North American desert cockroach Arenivaga investigator, in which the mouthparts are involved. This has not been shown among adult desert darkling beetles (family Tenebrionidae), possibly because they have developed such low rates of water loss that it is unnecessary.
Like desert beetles, scorpions are extremely resistant to water loss and have even higher lethal temperatures. Desert beetles shelter from the heat and their enemies under rocks, stones, and detritus. They do not dig into hard compact soil, but some dune species such as Lepidochora discoidalis from the Namib are saucer-shaped and can burrow rapidly into loose sand. Scorpions from the families Diplocentri-dae, Vaejovidae, Chactidae, and Scorpionidae have unusually thick and heavy prehensile claws (chelae) that they use to dig their burrows. (The claws act as supports while the animal digs with its legs.)
Most scorpions of the family Buthidae, which have slender claws, do not burrow but inhabit scrapes under stones and surface litter. Exceptions include Leiurus quinquestriatus, which occurs throughout the Western Sahara and the Middle East, and Parabuthus hunteri from the Sudan.
Desert scorpions show four clearly different lifestyles, each with its own morphological adaptations. Lithophiles, or rock-dwellers, are adapted to life in cracks and crevices in rocks and have a flat, elongated body with curved pedipalpal claws to provide a strong grip on steep rocky slopes. Psammophiles have long pincers to dig in the loose sand. Fossorial or burrowing scorpions have large, strong, crablike claws with great crushing power and spend almost all their life in their burrow, moving actively from the entrance to catch their food. Errant scorpions also search for food and possess a long, slender body and pedipalps.
Many deserts are inhabited by large tarantulas (mygalomorph spiders, suborder Mygalomorphae) that live in burrows. Like scorpions, they do not normally emerge in the daylight hours, when they would be vulnerable to their predators. Other spider families that are well represented in arid environments include the jumping spiders (Salticidae), wolf spiders (Lyco-sidae), and crab spiders (Thomisidae). In the North American deserts, tarantulas are found in burrows or rabbit holes, while trapdoor spiders and burrowing tarantula spiders are important elements of the Australian desert fauna. It seems likely that the dominance of the mygalomorph spiders in Australia may be due to the absence of competition from wind scorpions (Solifugae), which do not occur there. In North America, wind scorpions (Solifugae) are also much less dominant than in the Great Palaearctic desert, where mygalomorph spiders are not significant.
Desert spiders can be divided broadly into two ecological groups: 1) the large, long-lived spiders, including the digging mygalomorphs and some of the larger burrowing wolf spiders, and 2) the small, comparatively short-lived hunting spiders of the families Gnaphosidae, Salticidae, and Thomisidae. The main adaptations of spiders to the desert are based on burrowing habits and metabolic compensation for the high temperatures.
3.2 The struggle between predators and prey
The flat eroded relief and the sparseness or absence of vegetation on deserts can make them dangerous places for the creatures living there; in this terrestrial environment, inhabitants are most exposed to their predatory enemies. At the same time, however, these predators often find it difficult to stalk their prey without being detected in such an open landscape. Predators have to capture and kill their prey, and they do this in different ways, using a large variety of weapons such as the scorpion's sting, the spider's web, the sidewinder's camouflage and venom, and the falcon's stoop. Most desert predators hunt without a fixed pattern, moving apparently at random until they detect their prey, though some search in especially favorable sites. Desert carnivores eat almost anything and may develop special methods to deal with difficult prey. For example, mongooses throw eggs or centipedes against rocks to break them, while southern grasshopper mice (Onychomys torridus) bite the tail off a scorpion before eating it. In general, carnivores prey on herbivores or other similar-sized or smaller carnivores. Lions and leopards are able to kill animals much larger than themselves, but their prey rarely exceeds twice their own weight. The limited food supply of desert animals cannot always support packs of wolves or wild dogs--animals that can kill prey much larger than themselves.
The forms of mobility of reptiles and their tactical value
Desert reptiles show a variety of locomotion patterns to obtain food and find shelter. Adaptations for burrowing in the sand are not especially important, though so-called sand-swimming desert lizards and snakes often have a pointed or shovel-shaped nose (or rostrum), and the nostrils may be directed upward rather than forward. The eyes, nostrils, and mouth can be closed by valves to prevent sand particles from entering when they dive into the sand. Their bodies are covered with small smooth scales, and the legs of desert lizards may be reduced or even lost so that they move only by wriggling. The worm lizards (or amphisbaenians, the family Amphisbaenidae) are also legless burrowers with very smooth scales. Lizards have evolved fringes of elongated projecting scales on their toes at least 26 times and in seven different families. The morphology of these fringes varies according to the type of substrate they live on. Species that run on windblown sand usually have triangular, projecting, and conical fringes, while riparian species that run on water tend to have fringes whose shape varies from narrow to wide rectangles.
Snakes can move in several different ways, but the most common is serpentine locomotion. In this method, the propulsive force is derived from the thrust of the curves of the body against projections rising from the ground level (stones, plant stems, and other irregularities).
Another method of locomotion is concertina movement, again pressing the coils of the body against the irregularities in the bark. Sidewinding means that short-bodied snakes such as the Egyptian asp (Cerastes cerastes), the Namib desert viper (Bitis peringueyi), and the American desert rattlesnake, or sidewinder (Crotalus cerastes) can move fast over smooth sandy surfaces while preventing most of their body from coming into contact with the hot soil surface (see figure 67). Finally, the pythons (Python) and the large vipers move by rectilinear locomotion, creeping forward with the body extended in an almost straight line. The broad ventral scales are raised, drawn forward, and placed on the ground, and then the rest of the snake's body is dragged forward.
The difficulties facing predators
Some predators hunt by actively searching for their prey. Even more lie in wait for their prey, sometimes camouflaged, and ambush them. Others such as vultures never capture live prey; they have adopted a scavenging way of life. Combinations of predatory techniques are often used.
Natural selection generated by competition between predators (for food) and their prey (to avoid being eaten) leads to a sort of arms race between predator and prey. But even the most powerful predators, among them lions and leopards, are, at some stage of their life, potential prey for some other predator. As a result, their adaptations represent a compromise between effectiveness in hunting and in escaping capture. For example, mantids are fierce predators of other insects, but they are the prey of insectivorous birds.
As noted above, many predators hunt without a fixed pattern, moving apparently at random until they see, smell, hear, or touch their prey or detect it in some other way. Even large speculatory hunters often increase their chance of finding prey by seeking it in especially promising places such as at water holes or in patches of succulent vegetation. Birds of prey tend to hunt in specific areas frequented by potential prey. Active searching for prey is present in all the predator taxa occurring in the desert ecobiome and is dependent on locomotion.
Predatory animals of arid lands include arthropods, especially arachnids and insects, reptiles, birds, and mammals. Of less importance are the myriapods that are only found on the edge of the desert and the few desert-dwelling amphibians. Amphibians are, in general, not very well adapted as predators. Frogs and toads adopt a sit-and-wait strategy, but salamanders actively search for their prey. (The prey is caught by protruding the tongue.) The tiger salamander (Ambystoma tigrinum) of the North American deserts captures insects by striking them with the rear half of its tongue, covered in a glandular secretion to which they stick. Frogs and toads, on the other hand, enfold their prey with the tip of their tongue, which is then rapidly pulled back into the mouth.
Sit-and-wait predators include the sidewinder rattlesnake (Crotalus cerastes). When lying partly buried in the sand, this rattlesnake is almost invisible to the kangaroo rats and the other small animals on which it preys. At the same time, despite its dangerous venom, it is vulnerable to its own specific predators such as the roadrunner (Geococcyx californianus). Likewise, African desert vipers (Cerastes), puff adders (Bitis), and cobras (Naja) are the prey of the secretary bird (Sagittarius serpentarius) and of mongooses. Natural selection for predators (increased efficiency through speed or silence, aggressive crypsis, etc.) is less marked than the adaptations of prey animals, but it is nevertheless present. Wild animals are simultaneously adapted to many factors in their surroundings, making precise analysis rather complicated.
Most desert reptiles are carnivorous. They search for and eat a wide range of preyvertebrates or invertebrates, depending on their availability and size. Many small lizards feed almost exclusively on insects, while larger species eat smaller lizards. Monitor lizards (Varanidae) devour all the eggs they find, swallowing them whole and crushing them with contractions of the muscles of the gullet. The Nile monitor (Varanus niloticus) is commonly known in Sudan as the enemy of the crocodile because of its marked preference for crocodile eggs. Some lizards such as the African rainbow lizard (Agama agama) are fairly omnivorous and eat flower petals, grass, dead leaves, pieces of peanuts, and other vegetable matter, as well as ants and termites. As they grow larger, they become increasingly carnivorous; adults have been observed feeding cannibalistically on their own offspring. The spiny-tailed lizards (Uromastyx spp.) are also omnivorous, but they become increasingly vegetarian as they grow older. The Australian moloch lizard or thorny devil (Moloch horridus) and the North American horned lizard, (Phrynosoma platyrhinos) are both specialized predators of ants. Slow-moving animals like these would be highly vulnerable if they had to rely on running away to escape from their own enemies.
It is often assumed that for predators to exploit prey efficiently they must allow the prey population to maintain a level that maximizes the number of prey that can be taken without decreasing the population. In the case of the horned lizard and the harvester ants on which they feed, a foraging strategy allows maximization of the availability of prey on a scale of weeks or months rather than of hours or days. The number of horned lizards is regulated by the availability and productivity of their prey, which is extremely high.
Whiptail lizards (Cnemidophorus spp., also known as racerunners) find their prey by probing their long snouts under twigs and taking food objects from the substrate, by chasing prey that they detect visually, and by climbing vegetation to flush out prey. By combining strategies like this, the whiptail lizards can consume a wider range of animals. They reject the many potential prey with aposematic coloration (warning coloration), demonstrating that they recognize a range of colors. Vision is vital to day-active lizards foraging for prey, although they may also be guided by odor cues.
While small lizards generally feed only on insects, the larger species may eat smaller lizards. Likewise, young snakes and smaller species of snake prey on insects and arachnids, in contrast with the larger species that prey mainly or almost exclusively on vertebrates. Small desert snakes tend to be generalized predators. Shovel-nosed snakes (Chionactis occipitalis), from the deserts of North America, are nocturnal, like most other desert inhabitants, foraging in the open for insects and their pupae, spiders, scorpions, and centipedes. In contrast, the Namib viper (Bitis peringueyi) hides under the sand and emerges to take its prey (lizards like Aporosaura anchietae) during the daytime. Unlike most other snakes, its eyes are located on the top of its head, so that it can remain almost entirely buried in the sand and still have a full field of vision. A sit-and-wait predator, it lies with most of its body and head buried in the relatively cool sand while it waits to ambush lizards and other small vertebrates, including birds.
Concealment and warning coloration-the art of deception
The evolutionary effect of prey selection by predators has led to a considerable number of marked adaptive responses. Some desert animals have cryptic coloration, meaning that they match the color of their arid sandy background, or they are black and conspicuous. Considerable evidence suggests that cryptic coloration has evolved as a defense against predators, while conspicuous aposematic coloration is a warning to potential predators that the animal is distasteful or has formidable defensive weapons. Neither cryptic coloration nor aposematic coloration is restricted to desert animals, but as the desert is generally such an open environment, both are taken to the limits of evolutionary efficiency.
In deserts, vulnerable species are usually cryptic and inconspicuous. Not only are their backs similar in color to the landscape of the desert they live in, but their ventral surfaces are pale, thus eliminating the effect of shadow. Crickets and grasshoppers, which have a compact solid body, often resemble stones, while mantids, which have a long thin body, more often resemble twigs. A fine example of this type of protective resemblance is provided by the round-tailed horned lizard (Phrynosoma modestum) of the American deserts. From afar, this small lizard matches its background, but close up it could be mistaken for a stone. Many small desert insects imitate ants, bees, or wasps that are protected by their poisonous stings. Desert Hymenopterans are normally uniformly black. This makes them conspicuous, warning potential enemies not to attack them and thus avoiding accidental wounds by potential predators. In temperate regions and the moist tropics, where vegetation is abundant, bees and wasps are normally black and yellow, a combination that is most conspicuous in this environment. However, in the desert and against a sandy background, plain black is even more conspicuous, especially for very small animals like insects.
Aposematic black coloration does not only give warning of a poisonous sting; it may also indicate an obnoxious taste or smell. Skunks have anal glands that secrete such a foulsmelling liquid that they have almost no enemies, and they proclaim this with their conspicuous black and white coloration. Darkling beetles (Tenebrionidae) taste so foul that few animals apart from whip scorpions (Solifugae) will eat them; and, as might be expected, they are uniformly black. As in scarab beetles (Scarabaeidae), the black warning coloration of darkling beetles is associated with a very hard integument and an unpleasant smell and taste. Scorpions almost never eat darkling beetles unless they have not eaten for months, but some whip scorpions eat them readily. When threatened, the American darkling beetles of the genus Eleodes, pinacate bugs, lower their head and spray their enemy with a smelly solution containing benzoquinones. This irritates the skin and has been shown to repel a wide variety of predators very effectively. Yet, no defense is perfect: southern grasshopper mice have learned to subdue these beetles by pushing the tip of the abdomen downward, so that the repugnant chemical compounds are discharged harmlessly into the sand.
An animal can make itself conspicuous by warning sounds as well as by aposematic coloration. Porcupines are strictly nocturnal, but their white hairs, when raised in their characteristic fan, make them conspicuous in the dark. Furthermore, when hard-pressed by an enemy, porcupines make as much noise as they can, shaking their tail quills (modified to form a rattle), making guttural grunts, and emitting an intense stink. If all these warning signals do not make their enemy desist, the porcupine launches itself backwards and hits its aggressor with the bunch of quills on its tail. These quills are barbed, easily detached, and may severely wound a predator unable to remove them.
Several snakes make a hiss as a warning signal, as do the only poisonous saurians, the Gila monster (Heloderma suspectum) and the Mexican Gila monster or beaded lizard (H. horridum). Desert snakes are invariably cryptic for defensive and offensive purposes, but the Gila monsters are black and yellow. They pass unnoticed in the patchy shade of the desert vegetation, but their colors are clearly aposematic out in the open. Even if an animal is extremely well protected by venoms, toxins, and other chemical and mechanical mechanisms, it is always better not to be attacked than to be involved in a struggle in which, even if victorious, the animal may be wounded. This is the basic principle of aposematic coloration. Venomous snakes that do not lack enemies only produce warning sounds (hisses, tail rattling) when they are threatened. Apart from their poisons, desert snakes are small, delicate animals in comparison with the poisonous saurians, which are stronger, heavier, and much bigger animals.
3.3 The rhythm of animal life
Most desert animals survive by avoiding their habitat's adverse conditions as much as possible. Many insects have short life cycles and are only active after rain, spending the rest of the year in a drought-resistant resting stage, normally as eggs. Often, longer-lived animals also show seasonal rhythms of abundance and hide away during the long unfavorable period of the year. Many species that are active all year-round are usually nocturnal and hide in burrows and retreats during the daytime.
Animals active by day or by night
Some species that are active all year round are diurnal (active during the day). Many of them become crepuscular--active mainly at dawn and dusk during the hot season. These include several species of beetle, as well as most lizards and birds. In the Namib Desert, two species of darkling beetle (Tenebrionidae)--the white Onymacris langi and the black species Physosterna globosa--respond to high soil surface temperatures by employing a bimodal pattern of activity that avoids the intense midday heat. O. langi becomes active later in the morning than P. globosa and returns to its shelter earlier in the afternoon. Possibly, its white color reflects more sunshine during the hottest period of the day. Many other species of darkling beetle follow a similar bimodal pattern of activity, while others such as the nocturnal Lepidochora species avoid the excessive daytime heat entirely. Even so, one of the species of this genus, L. discoidalis, emerges in the late afternoon, when the wind cools the dune surface enough for it to forage among wind-borne grass fragments. Some species have probably evolved daytime activity in response to competition with nocturnal forms.
Most desert reptiles are diurnal. They emerge at dawn and bask in the sun until their body temperature has reached an optimum, which they maintain by shuttling between the sun and shade. When they overheat, some climb up among the branches of the desert shrubs to cool themselves in the wind. Likewise, gemsbok (Oryx gazella) regularly climb up the tall dunes in Sossusvlei in Namibia to cool themselves in the breeze during the heat of the day. In the same region, the sand-diving lizard (Apo-rosaura anchietae) lives on the soft wind-blown sand near the crests of the dunes, where their preferred average temperature of 86-104[degrees]F (30-40[degrees]C) occurs for only a short time every day. They have little time to forage before the sand becomes too hot and they have to dig down to the cooler subsurface environment.
Desert birds can fly--an efficient means of locomotion that allows them to retreat easily from unfavorable conditions. They seek shelter in the shade of rocks, trees, shrubs, or anything that casts a shadow in the hottest hours, and feed in the early morning and the late afternoon. Vultures and birds of prey soar effortlessly at great height during the hottest hours of the day, while nightjars and owls are nocturnal like the smaller mammals (jerboas, gerbils, kangaroo rats, and others). Desert hares take shelter among the vegetation, radiating heat through their long ears, while ground squirrels shelter in the shade of their own tail.
Synchronization of favorable circumstances
To survive in the wild, all plants and animals must be adapted to and interact with the entire ecosystem of which they form a part. An animal's physiological and behavioral activities, controlled by its biological clocks and conditioned by environmental synchronizing factors, are thus adapted, on a temporal basis, to the ecosystem. Biological clocks with a period of approximately one year are known as circannual; those with the same periodicity as the Moon are circalunar; daily clocks are called circadian. For example, the reproduction of some desert birds is triggered by the combination of two factors: their innate seasonal reproductive rhythm and the stimulus of rain. If no rain falls, they are unable to lay eggs, as egglaying is inhibited. This also happens in the American spadefoot toads (Scaphiopus spp.), whose emergence from the deep burrow where they estivate is stimulated by their seasonal rhythm of reproduction and by the vibrations and the sounds of thunder and large raindrops falling on the ground.
A slightly different example is the maturation of the reproductive organs of the desert locust (Schis-tocerca gregaria), which is stimulated by terpenoids and other aromatic chemicals released by some desert shrubs when their leaf buds start to open shortly after the beginning of the rainy season. These compounds, responsible for the characteristic smell of frankincense and myrrh, have no doubt evolved as insect repellants, but in the case of the locust it is more appropriate to consider them as aphrodisiacs. Locusts have a life cycle of less than one year, and these compounds are very important in preparing them physiologically to make the best use of the rainfall that stimulates the growth of grasses and other annual plants. The plant's circannual clock indirectly regulates the locust's reproductive cycle in such a way that they are ready to lay their eggs when rain falls, just before the growth of the fresh green grass that the immature locusts feed on.
Circadian rhythms inform animals of the change from day to night or vice versa. Without them, nocturnal desert beetles and scorpions living in deep burrows would be unaware that night had fallen and that it was time to emerge. Biological clocks are seldom completely accurate, but they are synchronized daily by the onset of night or by the dawn. The clocks of nocturnal animals are usually synchronized by dusk and those of day-active animals by the dawn. Many scorpions such as Paruroctonus mesaensis are active and emerge from their burrows to hunt on the desert surface only once every four days, but laboratory experiments have shown they possess typical circadian rhythms. Presumably, these cause them to rise to the mouth of their burrow every night and synchronize their clocks by the environmental light intensity. The reverse phenomenon occurs in day-active lizards.
In addition to timing from seasonal reproductive cycles and daily rhythms of movement and rest, biological clocks are important for navigation by the Sun, the Moon, and the stars. The ability to steer by the position of the Sun and to analyze the pattern of polarized light in the sky enables many insects and other arthropods (such as the desert woodlouse Hemilepistus reaumuri) to guide their movements. Migrating birds crossing monotonous stretches of desert guide their flight path by observation of the Sun's position. As they follow the same course all day long, they must make allowance for the Sun's apparent movement across the sky between dawn and sunset. Similarly, night-flying birds use time-compensated lunar navigation or steer by the stars.
From an ecological point of view, internal timing mechanisms play an important role in the behavior and physiology of desert animals. Even so, in most cases, their behavior is determined by rhythmical environmental stimuli; internal or endogenous clocks only provide a time-dependent readiness to respond appropriately to these stimuli. Thus, they are able to react to a given stimulus at the right moment and are not easily misled by small environmental disturbances. So, however important their biological rhythms may be, it should be remembered that physiology and behavior are determined primarily by direct responses to environmental stimuli; these are modified or accentuated secondarily by biological clocks.
3.4 The fauna of the Saharo-Sindian deserts
The Sahara is immense. From the biogeographical point of view, it belongs to the Palaearctic faunal region (which consists basically of Europe and part of Asia), to the Ethiopian region (which is essentially African), and to the Saharo-Sindian region, which runs through the Libyan Desert, the Sinai, the Syrian Desert, Mesopotamia, and to the Arabian Desert, thus stretching from the Atlantic coastline of Africa to the Near East and India. This ancient desert's size has varied greatly over its history as a result of changes in aridity. Most of the animals of the Sahara, especially the vertebrates, show close affinities with the Palaearctic fauna and the Ethiopian fauna, but most of those living in the Saharo-Sindian deserts of southwestern Asia are closely related to Saharan species, so that their fauna can be discussed together with that of the Sahara.
Wildlife of the Sahara
In wetter periods in the past, some regions that are now deserts were green and crossed by rivers, even some large rivers. The Atlantic coastline of the Sahara in particular was a pathway for the invasion of the plains and mountains of northern Africa by the fauna of the tropical savannahs, which, in turn, were in contact with the fauna of southern Africa. Over time, these exchanges and movements of wildlife, both within and around the Sahara, meant some animals could evolve the adaptations needed to live in a hot desert, typified by the Sahara, and become truly Saharan. Despite wet periods that sometimes greatly reduced the Sahara Desert's size (but never caused it to disappear), its great age has allowed the gradual evolution of a series of endemic species--species belonging to genera or families originally from one of the neighboring biogeographical regions that are now highly adapted to all the problems raised by the desert environment.
Most of these adaptations are directly or indirectly linked to the limited amount of water available to plants and, thus, the limited food resources available to animals. Animals are well adapted to a shortage of water, using it as parsimoniously as possible, hiding underground or in sheltered places, where they almost always live in a moisture-saturated atmosphere and may enter lethargy. But they cannot live without eating: however frugal they may be, only a few can find enough food to live on, and they are almost always dispersed among the different environments offered by the immense desert. Many desert animals know how to travel long distances and then return to their starting point, or, when appropriate, they can emit signals that can be received by a distant potential mate, thus allowing them to meet and reproduce.
Predators are also adapted to frugality and have specializations to help them locate their prey, but they are at risk of starving to death if they increase in numbers beyond the level that can be sustained by the number of available prey. Predation has clearly played a major role by further limiting the number of individual prey, but without threatening the prey species' survival. There are also some species of animals, especially vertebrates, that are not true desert animals, but arrived in the desert in a wetter period in the past and were trapped by the returning drought.
Saharan landscapes are very poor in vegetation, and, even though some plants can grow and reproduce extraordinarily fast after the slightest rainfall, plant production is very low and cannot feed many primary consumers. Consequently, the biomass of primary consumers available to the secondary consumers is very low. The low, irregular rainfall makes the survival of aquatic animals--especially that of fishes and amphibians (see p. 153)--very problematic.
The important role of reptiles
Reptiles, except for geckoes and some very small nocturnal thread snakes (family Leptotyphlopidae), can withstand high temperatures. Carnivorous reptiles obtain the water they need from the bodies of their prey; herbivorous ones obtain it from their plant diet. The most desert-adapted species of reptile need high temperatures to feed and carry out their normal activities. Most require strong sunshine and almost always bask in the sun. In the Sahara, reptiles are by far the most important vertebrates. The large herbivorous tortoise Testudo sulcata is common in the southern savannahs and has even been found in Tenere. In the Sahara, if there is no fresh food, it spends the summer resting in a burrow that it digs. A carnivorous freshwater turtle, the Spanish terrapin (Mauremys caspica leprosa), whose range stretches as far north as the foothills of the Pyrenees, feeds on fish in the pools of water in the Sahara.
Among the lizard families best adapted to life in the desert are the agamid lizards (family Agamidae), which occur only in the Old World (Asia and Africa) and in Australia. The desert agama (Agama mutabilis) is active by day (favoring high places) and in the summer; it shelters at night under a stone. Bibron's agama (A. bibronii) prefers crests and rocks. Tourneville's agama (A. sarignii) climbs to the top of clumps of vegetation in dune areas, and its range reaches Palestine. The Sinai agama (A. sinaitica) occurs on both sides of the Red Sea. In the desert, the common agama (A. agama), from the more southerly savannah regions, lives in the shelter of screes. The dab (Uromastyx acanthinurus), a large herbivorous and insectivorous agamid lizard, can reach a weight of 16 oz (500 g). A closely related species, U. geyri , lives in the mountains of the central Sahara. Another species, the Egyptian dab (U. aegyptius), occurs in northern Egypt, Palestine, and the Syrian Desert.
The geckoes (family Gekkonidae) are represented by about 20 Saharan species. They are insectivorous and nocturnal and live in cracks in rocks or within houses, where they can run up and down the walls and ceilings. The Sahara is the home of two Mediterranean species--the white spotted gecko (Tarentola annularis), typical of trees and rocks, and the common wall gecko (Tarentola mauritanica), which is more anthropophilous--and of two species with tropical affinities--one from the Ahaggar, T. ephippiata, and the other from western Africa, T. neglecta. In the rocky regions of the central Sahara and Arabia, the loud cries of Steudner's tarente (Ptyodactylus steudneri) can sometimes be heard. The small fine-toed dragons such as Steudner's pygmy gecko (Tropiocolotes steudneri), the Tripoli pygmy gecko (T. tripolitanus), or the elegant gecko (Stenodactylus stenodactylus) are distributed as far as northern Arabia, where they shelter under stones or sand, leaving their tracks all around the entrance to their burrow. The small helmeted gecko (Geckonia chazaliae) of the Atlantic Sahara shelters under cactiform species of Euphorbia. The rough-skinned gecko (Cyrtopodium scaber) occurs from Egypt and the Arabian Peninsula to India.
The true lizards (family Lacertidae) are represented in the Sahara by species of the genera Acanthodactylus and Messalina. The leopard lizard (A. pardalis) lives in the northern Sahara, in the flatter, less sandy areas around small pools. The Bosc's sand-racer (A. boskianus), which is larger, and the mottled lizard (A. scutellatus) prefer to live on sand in localized areas that are distributed throughout the Sahara and to the eastern bank of the Red Sea. The Messalina lizards have shorter digits than those of the genus Acanthodactylus. The spotted messalina (Messalina guttulata) lives at the northern edge of the Sahara and through the Near East to the Sind Desert. The red-spotted messalina (M. rubropunctata) is larger and more common.
The distribution of the skinks (family Scincidae) covers most of the Old World. Some species show reduction of the front and rear limbs and a smooth, elongated, streamlined body. Sand skinks, or sandfish (Scincus scincus), swim like fish through the sand of the dunes in search of the larvae on which they feed. The cylindrical skink (Chalcides ocellatus) has smaller front legs than rear legs, and it occurs as far east as the Sind Desert. The Algerian skink (Eumeces schneideri) and the western striped mabuya (Mabuya vittata) live in the northern Sahara; the first reaches as far east as Afghanistan, while the second reaches the Near East. The monitor lizards (family Varanidae) are represented by the grey monitor, or desert monitor (Varanus griseus), which can reach a length of 5 ft (1.5 m) and whose distribution reaches east to India.
The snakes that live in the Sahara have different feeding strategies. The thread snakes (family Leptotyphlop-idae) are digging snakes that are very small and thin (as their name implies), and feed on small invertebrates. Leptotyphlops macrorhynchus, for example, is distributed from the Sahara to the Sind. The many aglyph colubrine snakes (family Colubrineae) lacking poisonous fangs catch their prey between their jaws and then swallow it. The viperine snake (Natrix maura) is common in southwestern Europe, also occurs in northern Africa, and even reaches the Sahara, where it can sometimes be seen in pools with many fish. Some colubrine snakes (the opisthoglyphs) have venomous teeth on their upper jaw that poison their prey, which they then swallow. The green or Montpelier ground snake (Malpolon monspessalanus) can reach a length of 8 ft (2.5 m) and almost never enters the desert, while M. moilensis is distributed throughout the Sahara and occurs in the Arabian Peninsula. The diadem snake (Sphalerosophis diadema cliffordi) is Saharo-Sindian, as is the red-backed snake (Coluber rhodorachis). The Algerian snake (C. florentulus) lives on the northern edge of the Sahara and in the Ahaggar. Some colubrine snakes (the proteroglyphs) poison their prey by biting them with the firm venomous fangs at the front of the upper jaw. This is also shown by the Egyptian cobra (Naja haje, family Elapidae), whose range includes the tropical savannahs, spreading down the Nile Valley, and reaching the northern edge of the Sahara and the Arabian Desert.
In the viper family (Viperidae), the poison fangs are large and articulated and ready to inject poison as soon as the snake opens its mouth and takes its prey. The greater horned viper (Cerastes cerastes) lives in all the Saharo-Sindian deserts. The Saharan lesser horned viper (C. vipera), smaller and lacking horns, lives only in the sands of the Sahara and Arabia. The blunt-nosed viper (Vipera lebetina) occurs mainly in the northern Sahara but also occurs in Turkey, Mesopotamia, and Iran. In Egypt, it is replaced by the viper of the pyramids (Echis carinatum pyramidum).
Nesting and transitory birds
Although many birds migrate across deserts, only a few nest in the desert strip running east from the Atlantic to India. As a result of competition for food between local and migratory species, nesting areas often appear to be disjunct.
The ruddy shelduck (Tadorna ferruginea) nests among rocks--near water--in the northwestern Sahara, Syria, Turkey, and from Iran to Afghanistan. The griffon vulture (Gyps fulvus) is not Saharan but has a similar eastern distribution. The buzzard (Buteo rufinus cirtensis) and the falcon (Falco biarmicus), which can both be considered Saharan, also have a similar easterly distribution. The hubara bustard (Chlamydotis undulata) nests in the northern Sahara, the Arabian Peninsula, Iran, Afghanistan, and, farther east, in all the eastern deserts. The distribution of the European stone curlew (Burhinus oedicnemus) goes from the central and northern Sahara to the Sind Desert, like that of the cream-colored courser (Cursorius cursor). The spotted sandgrouse (Pterocles senegallus) and the crowned sandgrouse (P. coronatus) are found in small groups everywhere from the Sahara to Baluchistan. The large desert horned owl (Bubo ascalaphus) and similar forms or local forms of the eagle owl (B. bubo) are scattered from the Sahara to the Near East.
The larks best adapted to life in the desert are the desert lark (Ammomanes deserti, family Alaudidae), which occurs from the western Sahara to Baluchistan, and the black-tailed desert lark (A. cincturus), found as far as the Sinai, eastern Iran, and Afghanistan. The hoopoe lark (Alaemon alaudipes, family Upupidae) has the same distribution, as does the pale crag martin (Hirundo obsoleta) and the brown-necked raven (Corvus ruficollis), whose distribution reaches northeast to the Sind Desert. Among the wheatears (thrush family, Turdidae), the desert wheatear (Oenanthe deserti) has a clearly Saharo-Sindian distribution; the white-crowned black wheatear (O. leucopyga) is a Saharan endemic that also nests from the Sinai to Arabia; Hume's wheatear (O. alboniger), which does not live in the Sahara, nests from eastern Iraq and Arabia and southern Iran east to Baluchistan and the Sind. The rufous bush robin (Cercotrichas galactotes) nests in the central Sahara and the entire Mediterranean region as far as Tajikistan, as does the fulvous babbler (Turdoides fulvus). The olivaceous warbler (Hippolais pallida) lives in tamarisks throughout the Saharo-Sindian region. The scrub warbler (Scotocerca inquieta) nests in the northern Sahara, as well as in Palestine, Jordan, Iran, and Afghanistan. The desert warbler (Sylvia nana) has a disjunct distribution occurring in the central-western Sahara and eastern Iran and western Afghanistan. The great gray shrike (Lanius excubitor) has been sighted nesting throughout the Sahara and in Palestine and Afghanistan. The trumpeter bullfinch (Rhodopechys [=Bucanites] githaginea), the house bunting (Emberiza striolata), and the desert sparrow (Passer simplex) are Saharo-Sindian, but the desert sparrow only lives in Asia in the sands of eastern Iran.
The mammals living in the Sahara are either Palaearctic or Ethiopian. Some mammals, however, are so specialized as a result of their long evolution in the Saharo-Sindian deserts that they have become true desert endemics, colonizing one desert space after another. They have thus had to adapt to collecting their food over a huge area, in competition with ants and seed-eating birds. This is true of most species of rodents, hedgehogs, and hares, as well as of their predators, whose number is, like everywhere else, proportional to the quantity of food available. Some of these animals, compared with their homologues in nondesert environments, may sometimes have such an extremely low population density that each individual may live far away from the nearest member of the same species.
Some studies of rodents have shown a correlation of three distinct factors--the pattern of distribution of individuals, the ability to return to a burrow from a long distance, and the ability to remember places visited for a long time--with hypertrophy of the tympanic bullae. Tympanic bullae are bony coverings of the middle ear, and it seems logical to suppose that their large size plays some role in hearing. To a limited extent, increasing the volume of the air-filled cavity allows better transmission of low-frequency sounds. In one way or another, mammals can make several types of noises that correspond to sounds of this type; furthermore, they can memorize the location of low-frequency sounds, as well as the sounds of the environment in which they live. Several Saharan rodents tap the ground with their rear legs in a rhythm typical of each species. These acoustic signals emitted by rodents in their burrows in certain periods of excitation, especially during the breeding season, are audible to human beings for hundreds of meters. Thus, desert mammals with hypertrophied tympanic bullae, as well as insectivores, rodents, carnivores, and even ungulates, are more sensitive to sounds, footsteps, or cries emitted by animals of their own species. The overgrowth of the tympanic bullae is therefore an important adaptation acquired over a long period of natural selection and is a response to the pressure of the need to breed, ensuring that meetings are not left to chance, despite the large distances separating them. It seems unlikely that predation has played a major role in natural selection for this character.
In the Sahara, the insectivores are represented only by the African desert hedgehog (Paraechinus aethiopicus), which is replaced in the Near East by the large-eared desert hedgehog (Hemiechinus auritus). The bat order, Chiroptera, contains only three truly Saharo-Sindian desert species, the mouse-tailed bat (Rhinopoma hardwickei), Hemprich's long-eared bat (Otonycteris hemprichi), and the trident leaf-nosed bat (Asellia tridens).
The most notable adaptations to the desert environment are shown by the rodents, which lead a very active social and sexual life. In the Saharo-Sindian deserts, just two families of rodents, the gerbils and (Gerbillidae) and the jerboas (Dipodidae), have adapted so effectively that between them they can exploit all the desert environments. In the Sahara, they are joined by the gundis (Ctenodactylidae). Yet these rodents, or at least the more specialized forms, can no longer recolonize the richer environments in which their ancestors lived, as they would have to compete with other species, which are, in turn, unable to survive desert conditions, especially the absence of water.
The gerbil family is represented in the Saharan-Sindian deserts by five genera and many species, all of them mainly nocturnal, except for the day-active fat sand rat (Psammomys obesus) of the Sahara and Arabia and the great gerbil (Rhombomys opimus) of the Sindian deserts (also present in the cold deserts of middle and central Asia). All the species move in small leaps except the fat-tailed gerbil (Pachyuromys duprasi) which walks and runs without jumping. The genus Gerbillus contains many species without hair on the sole of the feet, including the dwarf gerbil (G. nanus), found from the western Sahara to India, and the slightly large North African gerbil (G. campestris), which occurs north of the Sahara, mainly in the Atlantic parts of Morocco; east of the Nile Delta, it is replaced by G. dasyurus. The gerbils with hair on the soles of their feet include the dune-living small sand gerbil (G. gerbillus); the large sand gerbil lives in ordinary sandy areas throughout the Sahara and in Palestine. The genus Meriones, the tamarisk gerbils or jirds, is represented by two species, the Libyan jird (M. libycus) and the desert jird (M. crassus), found throughout the Sahara, Mesopotamia, Arabia, Iran, and Afghanistan. Under normal desert conditions, both species have a very low population density, about one individual per 10 ha (1 hectare=2.5 acres), and both have extremely large tympanic bullae. Both species dig complicated burrows where the nest, like those of most desert rodents, stays at the average annual temperature--73-77[degrees]F (23-25[degrees]C)--in an atmosphere saturated in humidity. Shaw's jird (M. shawi) lives on the northern edge of the Sahara, often reaching high population densities of 50-100 individuals/ha. It must have liquid water in its diet, and it can only compete with the Libyan jird on the edge of the desert. East of Palestine, this species is replaced by M. tristrami, whose tympanic bullae, like those of Shaw's jird, are not enlarged. The Persian jird (M. persicus) occurs farther east and is the most common in Iraq and Iran, but from Iran to India it is replaced by the Indian desert jird (M. hurrianae). The day-active sand rats (Psammomys obesus) live in concentrated populations of 50-100 individuals/ha; their tympanic bullae are not greatly enlarged. Their strictly vegetarian diet consists of leaves and stems they clip from the clumps of chenopods that grow on the saline soils. Every day they consume about 80% of their body weight of these plants, containing about two grams of oxalic acid and more than one gram of sodium; they excrete urine with a salt concentration four times greater than seawater.
In the Sahara, the jerboas (family Dipodidae) are represented by a single species--the desert jerboa Jaculus jaculus. It is even smaller than the North African jerboa (J. orientalis), but its tympanic bullae are greatly enlarged. Several species of the genera Jaculus and Allactaga live in the deserts of Asia as far east as the Sind. The gundis (Ctenodactylidae), an African family, are represented by the Saharan gundi (Ctenodactylus vali), the M'zab gundi (Massoutiera mzabi), and the Felou gundi (Felovia vae). The Saharan gundi has larger tympanic bullae than the Atlas gundi (Ctenodactylus gundi), whose distribution in northern Africa does not reach the desert.
The carnivores (order Carnivora) are represented by four families: the Canidae (dogs), Mustelidae (weasels), Hyaenidae (hyenas), and Felidae (cats). The dog family is represented in the Saharo-Sindian region by the golden jackal (Canis aureus) and Rueppell's fox or sand fox (Vulpes rueppellii). The fennec (Fennecus zerda) occurs in the Sahara and in Arabia. Blanford's fox, the hoaryfox (V. cana), occurs mainly from Jordan to Baluchistan. The common fox (Vulpes vulpes), whose distribution is Palaearctic, gets into the desert in the shelter of rivers or wadis, where it competes with Rueppell's fox, which is Saharo-Sindian. The specimens of the common fox living in the Sahara, and to an even greater extent Rueppell's fox and the fennec, have greatly enlarged tympanic bullae and exceptionally large outer ears (see photo 75, p. 137). The weasel family (Mustel-idae) is represented by just one species, the North African spotted weasel (Poecilictis libyca), whose tympanic bullae are greatly enlarged. The striped hyaena (Hyaena hyaena) is a Palaearctic and Saharo-Sindian species. The wildcat (Felis silvestris), one of the possible ancestors of the domestic cat, and the sand cat (F. margarita) are species that are both Palaearctic and Saharo-Sindian, as are the caracal (F. caracal) and the cheetah (Acinonyx jubatus), which are now scarce in the Sahara.
The hyrax order (Hyracoidea) contains a single family, the rock hyrax family (Procaviidae), represented by the large-toothed rock hyrax (Procavia capensis), which lives in the rocky areas of the southern Sahara, as well as in Palestine and Arabia. The hare order (Lagomorpha) is represented by a desert form of the Cape hare (Lepus capensis), found in other forms throughout Africa, as well as in Europe and Asia. Whyte's hare (L. whytei), which is found throughout sub-Saharan Africa, has been caught at a single site in the northwestern Sahara. The bovids (family Bovidae) of the even-toed ungulates (order Artiodactyla) are represented in the Sahara by the addax (Addax nasomaculatus), the dama gazelle (Gazella dama), the Dorcas gazelle (Gazella dorcas), the rhim or slender-horned gazelle (G. leptoceros), and the aoudad or Barbary sheep (Ammotragus lervia). In the Near East, the Persian gazelle, or djeiran (G. subgutturosa), is distributed from Iraq to Baluchistan. The Arabian oryx (Oryx leucoryx) also occurs in Iraq.
The horse (family Equidae, order Perissodactyla) is represented by the Asiatic wild asses (Equus hemionus) of Iraq and Arabia to eastern Iran. The onager (E. h. onager) is found in Iran, but the chigetai (E. h. hemionus), which used to occur in Syria and Mesopotamia, is now extinct there. The camel family is represented by the dromedary (Camelus dromedarius), which was originally domesticated in Arabia, and is replaced by the Bactrian camel (C. bactrianus) in the cold deserts of Asia.
3.5 The fauna of the Namib and Kalahari deserts
Despite their dune formations, it is wrong to try and define a vertebrate fauna typical of the Kalahari and Karoo. Most of the medium-sized or large animals present also live on the savannah, and in both areas they are adapted to life in conditions linked to seasonal rainfall.
Morning fogs form in the Namib Desert 100 days a year and do not disperse until well into the morning. It is for this reason that from the desert's northern edge to its southern tip, the dry salt lakes, the large stone-covered plains (flat apart from a few stony hills), and the dunes covering more than one-third of the desert's surface area, together with the sandy areas, all provide the setting for vegetation to develop and a multitude of invertebrates to live. Some small invertebrates even have special adaptations to obtain part of the condensation water.
The birds and reptiles
The small species of vertebrates in these deserts include some endemic species that are more typical of sandy environments. This is true of the legless blind skinks (Typhlosaurus) such as the Kalahari striped blind skink (T. lineatus), the Gariep blind skink (T. gariepensis), and Brain's blind skink (T. braini), as well as the Kalahari tree mabuya (Mabuya spilogaster) and the southern Namibian striped mabuya (M. striata sparsa), which lives in the dunes of the Namib. Other endemic reptiles include the western Namibian blue-tailed lizard (Cordylosaurus subtessellatus), the ocellated Namibian lizard (Meroles suborbitalis), the dune-digging lizard (Aporosaura anchietae), the crying dragon (Ptenopus arrulus, which is heard in Namibia on summer evenings), the Kalahari ground dragon (Colopus wahlbergii), the Namibian giant ground dragon (Chondrodactylus angulifer), the Namibian diurnal dragon (Rhoptropus afer), as well as the tiny dwarf snake (Dipsinia multimaculata), the Namibian sand snake (Psammophis leightoni), and the Namib desert viper (Bitis peringueyi). Many other much less specialized reptiles are also occasionally found in these deserts, among them the western striped mabuya (Mabuya occidentalis), the viper Bitis caudalis, which also lives in relatively moist areas, the Cape cobra (Naja nivea), the Cape side-necked turtle (Pelomedusa subrufa), which makes use of even the smallest pools in the Kalahari, and the leopard tortoise (Geochelone pardalis), which terminates its hibernation when the summer rains begin.
The birds include the pale-winged starling (Onychog-nathus nabouroup), the Kalahari robin (Cercotrichas paena), the ant-eating chat (Myrmecocichla formicivera), the swallow-tailed bee-eater (Merops hirundinaceus), the harlequin quail (Coturnix delegorguei), the pygmy falcon (Polihierax semitorquatus), the small white-faced owl (Otus leucotis), and many very widely distributed vultures and eagles.
The highly diverse fauna of large mammals
The large mammals are represented by herbivores like the gemsbok (Oryx gazella), the brindled gnu (Connochaetes taurinus), the red hartebeest (Alcelaphus buselaphus), and the common eland (Taurotragus oryx). These animals can always obtain enough water during the dry season from watermelons (Citrullus lanatus), gemsbok cucumbers or wild cucumbers (Acanthosicyos naudinianus), and African cucumbers (Cucumis africanus). The carnivores are represented by the lion (Panthera leo), the laughing hyena (Crocuta crocuta), the brown hyaena (Hyaena brunnea), the aardwolf (Proteles cristatus), the cheetah (Acinonyx jubatus), the black-backed jackal (Canis mesomelas), the South African silver fox (Vulpes chama), and the bat-eared fox (Otocyon megalotis). The smaller mammals present include the Cape hare (Lepus capensis), the mole rat (Cryptomys hottentotus, which eats gemsbok cucumbers), the South African ground squirrel (Xerus inauris), the South African hedgehog (Atelerix frontalis), the meerkat (Suricata suricatta), and many others that can survive in the harsh desert environment.
Among the rodents, some species of the mouse family (Muridae)--for instance, the South African strioed mouse (Lemniscomys rosalia) and the four-strioed grass mouse (Rhabdomys pumilio)--have managed to survive in the Kalahari. The gerbil family, which contains the species best adapted to desert life in the Sahara, is represented in the Kalahari by only a single species, Gerbillu-rus paeba (they have hairy-soled feet and relatively small tympanic bullae), and by species that normally live in the savannahs of the region, including Tatera leucogaster and T. brantsi. Gerbillurus paeba also occurs in the Namib, along with the long-tailed gerbil (G. tytonis), which has larger tympanic bullae, the dune gerbil (G. vallinus), whose tympanic bullae are greatly enlarged and whose long tail ends in a tuft, and Setzer's gerbil (G. setzeri), which has enlarged tympanic bullae and a short tail. The rare short-tailed gerbil (Desmodillus auricularis) also occurs in the Namib; other species include the dassie rat or rock rat (Petromus typicus), which lives on rock slopes like a rock hyrax, the dwarf rock mouse (Petromyscus collinus), which moves among the stones like a gecko thanks to the adhesive pads on the sole of its feet and its palms, and the large-eared mouse (Mala-cothrix typica). The only mammal present that is not naturally homeothermic is Grant's small golden mole (Eremitalpa granti), which swims through the sand of the Namib dunes in search of its invertebrate prey like a fish in water. It burrows at a depth where the temperature of the sand in contact with its body allows it to maintain peak activity.
3.6 The fauna of the hot deserts of North America
The hot deserts of North America are not so arid as the Sahara, as annual average rainfall is 5 in (130 mm), and there is vegetation almost everywhere. Their fauna has little in common with the fauna of the Old World deserts, though they share many species, both Holarctic and Neotropical, with the neighboring biomes and with the desert and subdesert regions of South America. For many vertebrates, the great diversity of habitats in the desert provides suitable living conditions, at least for part of their life, even if they are not specifically adapted to the desert biome. Even so, a few species are adapted to the dry conditions and scattered food resources typical of deserts.
The diversity of snakes and lizards
Reptiles are abundant. The tortoise order (Chelonia) includes the desert tortoise (Gopherus agassazii), which can reach a length of 15 in (37 cm), eats grass and cacti, and digs a hole to lay its eggs. Other important reptiles include the banded gecko (Coleonyx variegatus), which is nocturnal and can live in the driest deserts by sheltering underground or under stones, and the flat-toed gecko (Phyllodactylus xanti), which climbs unceasingly over the rocks.
The iguana family (Iguanidae) consists of the true iguanas and several species commonly known as lizards that are comparable to the Old World true lizards (Lacertidae). The desert iguana Dipsosaurus dorsalis lives in sandy areas, moving between the clumps of vegetation whose leaves and flowers it eats. The chuckwalla (Sauromalus obesus) lives among the rocks, sheltering in the cracks. The umas (Uma notata, U. inornata, U. scoparia), members of the iguana family with black-striped flanks and toes fringed with spiny scales, live on the sand dunes and can run upright on their rear legs. The leopard lizard (Crotaphytus wislezenii) is a large species with black spots; it lives on the arid and semiarid plains and can also run upright. Sceloporus magister, a spiny lizard of the American deserts, has sharp-tipped scales and feeds on insects and leaves on the hills of the arid and semiarid regions where it lives, though some other species of the same genus live in the forests of southern Mexico. But the most abundant lizard of the arid and subarid regions of western North America is the side-blotched lizard or ground uta (Uta stansburiana), a small iguanid marked with a black patch behind its rear limbs. The long-tailed lizard (Urosaurus graciosus) is another New World species; it has a tapering tail roughly twice as long as its body. The horned lizards are represented by one desert species--Phyrnosoma platyrhinos, a small ant-eating iguanid with a very short tail and spiny scales or warts all over its body (those on the back of its head are very large). The desert night lizard (Xantusia vigilis) shelters under fallen branches during the day throughout the hot season. The tegus and whiptail lizards (family Teiidae) are mainly South American, but they are represented in North America by eleven species of whiptail lizards in the United States, three of them desert-living species known as the racerunners (Cnemidopho-rus exsanguis, C. uniparens, C. tigris). The beaded lizards (family Helodermatidae) are represented by the Gila monster (Heloderma suspectum), the famous poisonous lizard that lives in all the arid and semiarid zones of North America, where it feeds on small mammals and even on other lizards.
There are also many snakes, ranging from small snakes with vestigial eyes (known as blind snakes, and belonging to the thread snake family, Leptotyphlopidae) to the rosy boa (Lichanura trivirgata). The small blind snakes such as Leptotyphlops dulcis and L. humilis are worm-like digging snakes and prefer the playas of sandy soil, where they burrow in the sand to catch the small invertebrates they eat. The rosy boa is a relatively small member of the boa family (Boidae). It rarely exceeds 3 ft (1 m) in length and prefers rocky places, where it feeds on small mammals and birds.
The colubrine snakes include nocturnal snakes, among them the leaf-nosed snakes (Phyllorhynchus brownii, P. decurttus) that live in sandy deserts and are also burrowers, as well as diurnal snakes such as the swift-moving Masticophis flagellum, a coachwhip snake, or Arizona elegans, the shiny snake. Both avoid the vegetation and hunt in the open countryside, while the first frequents all sorts of habitats and is not restricted to deserts.
Other snakes occasionally found in the desert include the Californian lyre snake (Trimorphodon lambda), Salvadora hexalepis, with its spotted snout, and Rhinocheilus lecontei, which has a long snout. The garter snakes (Thamnophis)--the black-necked garter snake (T. cyrtopsis) and the Mexican garter snake (T. eques)--frequent water holes. Many nocturnal digging snakes (Chionactis occipitalis, C. palorostris, Chilomeniscus cinctus), which use their blunt noses in the sand like a shovel, are almost exclusively desert species and feed on small invertebrates. The black-headed snakes (Tantilla), like the desert species (T. planiceps transmontans) or the Huachuca snake (T. wilcoxi), occur in small scattered populations.
The most important representative of the coral snakes (family Elapidae) in the North American hot deserts is the Arizona coral snake (Micruroides euryxanthus). The best-known poisonous snakes--the rattlesnakes--belong to the viper family (Viperidae). Rattlesnakes feed mainly on small mammals and birds and are best known for their rattle, which consists of horny, loosely connected segments at the end of the tail that they shake to make a warning sound. They are very widespread in the different biomes of North and South America, but only two species are almost entirely desert dwelling: the sidewinder or horned rattlesnake (Crotalus cerastes) and the Mojave rattlesnake (C. scutulatus).
The nesting birds and birds of passage
Many birds cross the North American deserts. The few sedentary birds include some that live mainly in the neighboring mountains, while the others are adapted to the dry conditions and scarcity of food that is typical of deserts. The largest bird in North America is the California condor (Gymnogyps californianus). It was common on the edges of the desert until 1870, but only a few dozen individuals now remain, most of them in zoos (see also vol. 9, p. 247). The American kestrel (Falco sparverius) is common in the plains and deserts of California, where it nests in holes in trees or in cracks in the rocks. The white-winged dove (Zenaida asiatica) can be seen in oases, where it nests in colonies and visits the cacti when their fruits are ripe. The roadrunner (Geococcyx californianus) is found in arid areas with scattered clumps of low shrubs; it can run nearly 25 mph (40 km/h) and feeds on large ground insects, small snakes, and lizards. The pale great horned owl (Bubo virginianus pallescens) frequents desert sites where there are large yuccas but generally nests among rocks. The small elf owl (Micrathene whitneyi) lives in the Colorado Desert, nesting where holes have been made in the giant cacti by the woodpeckers. The burrowing owl (Speotyto cunicularia) lives in the abandoned burrows of small desert mammals. The lesser nighthawk (Chordeiles acutipennis) nests on the gravel-covered soil of the desert. The white-throated desert poorwill (Phalaenoptilus nuttallii) enters a state of lethargy in the winter, when there are no insects. One of the smallest hummingbirds, Calypte costae, lives in the hot deserts of North America and feeds on the flowers of several species of yucca, cactus, and sage. The yellow-shafted flicker (Colaptes auratus) nests in the west and north of the Mojave and Colorado deserts, where it drills holes in the giant cacti to make its nest, as does the Gila woodpecker (Melanerpes uropygialis).
In the Mojave Desert, there are horned larks (Ere-mophila alpestris) that are recognizable by the black feathers on their heads, which resemble ears. The cactus wren (Campylorhynchos brunneicapillus) can shelter in the spiny scrub where it nests. The Californian thrasher (Toxostoma redivivum) is a very common bird that searches on the ground for the insects, spiders, and the fruit it eats, and builds its nest among the scrub or among the branches of a mesquite. The black-tailed gnatcatcher (Polioptila melanura) is sedentary in these desert regions, where it nests in large clumps of vegetation, and it resembles a small warbler. The Sonora loggerhead shrike (Lanius ludovicianus sonorensis) hunts its prey on the desert plateaus. The black-throated gray warbler (Dendroica nigrescens) lives in rolling landscapes. Scott's oriole (Icterus parisorum) lives among yuccas and cacti but feeds on insects that it catches on the ground outside the desert. The American black-throated sparrow (Amphispiza bilineata) lives in the sunniest areas of the desert, where the vegetation is scarce.
Some species of mammal can be considered endemic to the hot deserts of North America. These include the desert shrew (Notiosorex crawfordi) in southern California and northern Mexico. The kit fox (Vulpes macrotis), easily recognized by its large ears, black-tipped tail, and ability to run fast, can be seen in the daytime in the Mojave and Colorado deserts.
The most numerous mammals are the rodents, especially the kangaroo rats (Dipodomys), the pocket mice (Chaetodipus), and the antelope ground squirrels (Ammospermophilus), named because the tail is often carried up or over the back, like the American pronghorn antelope (Antilocapra americana). The Mojave and Colorado deserts are home to the antelope ground squirrel (Ammospermophilus leucurus) and the roundtail ground squirrel (Spermophilus tereticaudatus), while the Mojave ground squirrel (S. mohavensis) is a smaller species that can be found (when not in a state of lethargy) in the Mojave Desert.
The rodents best adapted to fast movement in the American deserts are the kangaroo rats, which move in jumps using their rear legs. (Their legs are as long as those of the jerboas of the Sahara.) The most common rodents are the desert kangaroo rat (Dipodomys deserti) and Merriam's kangaroo rat (D. merriami). The pocket mice are characterized by their ability to carry large amounts of food in the pouches in their cheeks; the desert pocket mouse (Chaetodipus penicillatus), Bailey's pocket mouse (C. baileyi), and the spiny pocket mouse (C. spinatus) are restricted to the south of the desert, while the long-tailed pocket mouse (C. formosus) has the broadest desert distribution. The American desert pack rat (Neotoma lepida) generally lives among the cacti that supply its water needs and under which it digs its burrow. The southern grasshopper mouse (Onychomys torridus) lives in the sandy areas of the Mojave and Sonoran deserts.
3.7 The fauna of the hot deserts of South America
The fauna of the deserts of South America is totally Neotropical and thus very different from that of all the other hot deserts, except those in North America, which contain Neotropical as well as Palaearctic species. Birds and reptiles are the most abundant vertebrates, while arthropods are the most abundant invertebrates.
The reptiles of the Atacama
The most typical reptile of the coastal desert of Peru and Chile is the iguana Tropidurus peruvianus, a stout diurnal lizard that occurs along the Ecuadorian and Peruvian coastline, where there are numerous subspecies, the smallest of which, T. p. atacamensis, lives in the Atacama. Its diet is virtually restricted to what the tide washes up, such as seaweed, small crustaceans, and the remains of molluscs. At the southern tip of the coastal desert is another lizard, Callopistes maculatus atacamensis (family Teiidae), which coexists with T. p. atacamensis and feeds in the same way. The insectivorous iguana Liolaemus nigromaculatus bisignatus eats small amphipods as well as insects and lives on dunes covered by the ice plant Mesembryanthemum crystallinum. A flat-toed gecko, Phyllodactylus gerrophygus, feeds at night on insects. The small scattered patches of tillandsia (Tillandsia humilis and other species of the same genus) are almost the only vegetation of the Tarapaca Desert, one of the regions with the harshest climate (see figure 40, p. 77). These patches are home to Phrynosaura reichei, an insectivorous and oviparous iguanid that is extremely rare. Tropidurus tarapacensis, another iguanid, is more abundant, or even dominant, in this desert. It is vegetarian and omnivorous and, like many other iguanid lizards, can run upright on its rear legs. Phyllodactylus gerrophygus, the small flat-toed gecko, also lives in the same area. The valleys are likewise occupied by coastal forms such as the above-mentioned Tropidurus peruvianus and Callopistes maculatus, accompanied farther to the north by Liolaemus nigromaculatus and the flat-toed gecko most characteristic of Peru, Phyllodactylus inequalis. In addition, the valleys are home to some snakes, especially Dromicus angustilineaus, which is reminiscent of the Aesculapian snake (Elaphe longissima) from Europe, and Tachymenis peruviana, which resembles a European terrestrial smooth snake (Coronella). In the oases, there is a small tree-dwelling iguana, Tropidura theresoides, and the flat-toed gecko Phyllodactylus gerrophygus. In the supralittoral region of the Atacama is another species of lizard, Garthia gandichaudi.
The birds and mammals of the coastal desert
The most characteristic birds of the coastal desert are large carrion-eating vultures (family Cathartidae) such as the black vulture, also known as the black buzzard or carrion crow (Coragyps tratus); the turkey vulture or turkey buzzard (Cathartes aura), known locally as the jote, which takes eggs and chicks from the colonies of seabirds on the guano islands near the coast; and the Andean condor (Vultur gryphus), which feeds on the corpses washed up by the sea. The other abundant birds of prey include Harris's hawk (Parabuteo unicinctus), the white-tailed kite (Elanus leucurus), the black-chested buzzard eagle (Geranoaetus melanoleucus), the hawks (Buteo), the tiugue or chimango (Milvago chimango), and the American kestrel (Falco sparverius). In the valleys, there are vermilion tyrant flycatchers (Pyrocephalus rubinus), the groove-billed ani (Crotophaga sulcirostris), the red-breasted meadowlark (Sturnella [=Pezites] bellicosa), the white-winged dove (Zenaida asiatica), and the Chilean woodstar (Eulidia [=Myrtis] yarrellii). The open country is home to the American seedsnipes or perdicitas (Thinocorus), the Chilean tinamou or perdiz (Nothoprocta perdicaria), the tapaculo (Scelorchilus albicollis), the tenca or Chilean mockingbird (Mimus thenca), the ground tyrants or dormilonas (Muscisaxico-la), the nightjars or gallinas ciegas (of the family Caprimulgidae), bandurrillas (Upucerthia), and mineros (Geositta). The sparse wet areas are home to herons (Ardeidae), lapwings or plovers (Charadriidae), and canasteros (Thripophaga). It is also home to different species of swifts (family Apodidae), swallows (Hirundinidae), wrens (Troglodytidae), flycatchers (Muscicapidae), known locally as zorzales, finches (family Emberizidae, known as chirihues, chincoles, and semilleros; Fringillidae, the finches, linnets, and siskins known as jilgueros, diucas and yales), and the New World oriole (Icteridae) known as loicas and tordos.
Mammals are extremely scarce in this area. The colonies of vampire bats (Desmodus) feed on domesticated animals, while the insectivorous bats feed on the insects in the valleys. Sea lions occur along the coast in favorable sites. Rare small rodents such as Darwin's mouse (Phyllotis darwini) and several South American field mice of the genus Akodon live on the coastal vegetation.
The most noteworthy arthropods
The most important arthropods are the insects and arachnids. The most abundant and diverse families of insects are the Proscopiidae (elongated sticklike grasshoppers), the stick and leaf insects of the order Phasmatodea, the Acrididae (short-horned grasshoppers), and the Tettigoni-idae (katydids and long-horned grasshoppers of the order Orthoptera suborder Ensifera). The beetles (order Coleoptera) are represented by the darkling beetles (family Tenebrionidae), especially the genus Gyriosomus, and are abundant everywhere; other abundant groups include the ground beetles (family Carabidae), weevils (Curculionidae), blister beetles (Meloidae), rove beetles (Staphylinidae), and soft-winged flower beetles (Melyri-dae). The flies (order Diptera) are represented by the leaf-mining flies (Agromyzidae), parasitic flies (Tachinidae), and army worms (Sciaridae). The ants, bees, and wasps (order Hymenoptera) are represented by the ants (Formicidae), velvet ants (Mutillidae), and parasitic braconid wasps (Braconidae). The butterflies and moths (order Lepidoptera) are represented by the diurnal skippers (Hesperidae), the noctuid moths (Noctuidae), and some small lepidopterans that are remarkably abundant. The most abundant arachnids are the windscorpions (order Solifugae) and the red mites (family Erythraeidae). Occasionally, these red mites may be so numerous that the normally yellowish brown soil is given a reddish tinge.
3.8 The fauna of the Australian deserts
The immense desert covering much of the center of the Australian landmass is interrupted occasionally by deep valleys with streams and permanent lakes at the bottom. These waters shelter an entire fauna of invertebrates and fish that feed on them. The temperatures may exceed 104[degrees]F (40[degrees]C), and over large areas rainfall is sporadic and lower than 6 in (150 mm) per year, though sometimes torrential rains turn parts of the desert into immense temporary lakes.
A hard grass, porcupine grass (Triodia pungens), covers one-third of this desert area and shelters a whole fauna of small invertebrates, especially ants and small sapsucking insects. The mulga (Acacia aneura) growing in some places provides the more than 2,000 species of ants typical of these desert regions with a kind of honey. Eucalyptus clumps growing in wetter sites also provide food resources. After the rains, ephemeral plants such as the desert peas (Clianthus) produce a large number of highly nutritious seeds. There are, however, no fruits comparable to the watermelons of the Kalahari or the colocynths (bitter cucumbers) of the Sahara, which are both nutritious and full of water.
The remarkable reptiles and amphibians
After rainfall, numerous frogs that have sheltered underground during the long periods of drought appear. They secrete a layer of mucus around themselves and can remain dormant for many months or even years.
The reptiles' remarkable adaptations show how old Australia's deserts are. The lizards show great diversity, and some occupy ecological niches that are occupied by other vertebrate groups on other continents. The scaly-foot lizards (Pygopodidae), an almost exclusively Australian family with a few scarce representatives on the neighboring islands, replace the snakes, while the monitor lizards (Varanidae) occupy the space of the mammalian carnivores, and some skinks (Scincidae) and small Australian geckoes replace the small insectivorous mammals.
Spinifex clumps have played a major role in the diversification of the lizards, protecting them from predators and acting as a reserve for the invertebrates on which some prey. The skink Ctenotus piankai, the snake lizard Delma fraseri, and the wood gecko Diplodactylus elderi spend most of their time within spinifex clumps. Others such as the geckoes Ctenotus grandis and C. quattuordecimcostatus stay around the clumps. Still others--especially the agamid bearded lizard Amphibolurus isolepis and the skink Ctenotus calurus--live in the open spaces but are always prepared to return to the shelter of a clump of spinifex.
In contrast with the reptiles of the Kalahari or the North American deserts, the reptile species of the Australian desert (for example, geckoes and snake lizards) are generally nocturnal and thus avoid the heat of the day. Some species of Amphibolurus and Ctenotus, however, are active in the heat of the day, like most lizards in the Sahara.
The endemic birds
Australia has almost 650 species of endemic bird, together with 34 introduced or migrant species. Some of these birds nest throughout Australia, including the deserts, though some species are limited to the areas north of 20[degrees]S, others are limited to areas south of 30[degrees]S, and there are others that nest between these latitudes. The number of species adapted to breeding in the deserts, and that nest there, is below 90, and only some of them have a distribution restricted to the desert.
Other birds have very wide distributions, far exceeding the limits of the Australian landmass. Among these are the peregrine falcon (Falco peregrinus), the common barn owl (Tyto alba), and Richard's pipit (Anthus novaeseelandiae). The Australian dusky grasswren (Amytornis purnelli, Maluridae), the slate-backed thornbill (Acanthiza robustirostris, Acanthizidae), and the grey-headed honeyeater (Licheneostomus keartlandi, Meliphagi-dae) are the only Australian birds endemic to the central desert. These species belong to Australian families with only a very few species that have spread to New Zealand or other Pacific islands or even to Insulindia.
Apart from the above-mentioned birds of prey, there are a few more, all of them diurnal, that prey on the fauna of the deserts and subdeserts: the wedge-tailed eagle (Aquila audax), the little eagle (Hieraetus morphnoides), the Australian brown falcon (Falco berigora), and the Nankeen kestrel (F. cenchroides).
The small diamond dove (Geopelia cuneata) is endemic to western Australia and the central deserts. The crested pigeon (Geophaps [= Ocy-phaps] lophotes) is also endemic and lives in the west and the east. The spinifex pigeon (G. [= Petrophassa] plumifera) is endemic to the northwest but has been found nesting in the central desert. The parrots and relatives are numerous in Australia and mostly nest in areas with trees.
The red-tailed black cockatoo (Calyptorhynchus [= C. magnificus] banksii) may breed in the center of the continent, like the galah (Eulophus [=Cacatua] roseicapillus), a roseate cockatoo, and the pink or Leadbeater's cockatoo (Cacatua leadbeateri), the budgerigar (Melopsittacus undulatus), the Port Lincoln parrot (Platycercus [=Barnardius] zon-arius), which is totally absent from eastern Australia, or the mulga parrot (Psephotus varius), which has the same geographical distribution as the mulga (Acacia aneura), a species of acacia, although it does not depend on it for nesting or feeding.
Horsfield's bronze cuckoo (Chrysococcyx basalis) is the only Australian cuckoo that nests in the desert. The tawny frogmouth (Podargus strigoides, Podargidae), a large nightbird, only needs a hole in the ground as a nest for the female to lay its eggs, which the male incubates alone. The endemic red-backed kingfisher (Todirhamphus [=Halcyon] pyrrhopygia) is found everywhere, though it does not tolerate downpours well, as they damage its nest; it prefers less rainy environments, while the rainbow bee-eater (Merops ornatus) reproduces wherever it finds the insects it feeds on.
The white-backed swallow (Cheramoeca leucosternum) only nests in central and southern Australia, like the tree martin (Hirundo [=Cecrops] nigricans), whose range far exceeds Australia, while the fairy martin (H. [=Cecrops] ariel) is only absent from the Great Sandy Desert. The black-faced cuckoo-shrike (Coracina novaehollandiae) reproduces throughout Australia, except for the Great Sandy Desert--the same distribution as the white-winged triller (Lalage sueurii). The grey-crowned babbler (Pomatostomus temporalis) tends to nest in the north, even in the central desert, while the white-browed babbler (P. superciliosus) is typical of the south of Australia, like the rufous songlark (Cinclorhamphus mathewsi) and brown songlark (C. cruralis).
Australia's mammals are mainly marsupials. Though placental mammals arrived recently on a geological timescale, some have been present long enough to acquire effective adaptations. This is true of some rodents of the mouse family (Muridae), specifically the subfamily Hydrominae, which arrived in Australia only 5-10 million years ago. All nine species of Australian hopping mouse (Notomys) have elongated rear legs and a long tail with a tuft of hairs at the tip, but only one--the spinifex hopping mouse (N. alexis)--lives in Australia's central desert and is so well adapted that it does not need to drink water. In the same environment, there are two species of Australian mouse (Pseudomys): the Australian sand mouse (P. hermannsburgensis), which is social, and the brown Australian desert mouse (P. desertor), which, unlike the other 18 species of the same genus, is mainly herbivorous and solitary.
The case of the dromedaries, introduced by humans as beasts of burden, is very different from that of the mice. Once they were no longer useful for the purpose for which they were introduced, they were abandoned in large numbers and returned to the wild so successfully that there are now about 20,000 in the Australian desert, making this dromedary population the world's largest.
Four families of marsupial are represented in the Australian deserts: the rabbit bandicoot (Thylacomyi-dae), the dasyurid marsupial carnivores (Dasyuridae), the marsupial moles (Notoryctidae) and the kangaroos and wallabies (Macropodidae). Until 1930, there was also a representative of a fifth family, the bandicoots (family Peramelidae), the long-snouted desert bandicoot (Perameles eremiana), but it has not been sighted since then and is now thought to be extinct. The rabbit bandicoots are only represented by a single species (Macrotis lagotis), an animal weighing approximately 3 lb (1.5 kg); it digs a deep burrow to escape the heat and eats mainly termites and beetle larvae.
The dasyurid marsupial carnivores are represented in the desert by many species, all with pointed snouts and an insectivorous or carnivorous diet; they all nest in an underground burrow, but they move around on the surface. The kultarr (Antechinomys laniger) is a sort of solitary marsupial rat, with a tail longer than its body and very long rear limbs with four digits. It can scamper and has a sharply pointed snout, large ears and eyes, and a long woolly coat. The mulgara (Dasycercus cristicauda) lives in dunes and is larger, with a smoother and denser coat than the kultarr. It is a social animal, with short ears, feet with five digits, and a thin tail tipped with a tuft of black hairs. It consumes a quarter of its own weight in insects every day and can thus live without ever drinking water. The kowari (Dasyuroides byrnei) is the largest of the marsupial rats of the Australian deserts, weighing 2-5 lb (70-150 g). Its short ears have a thick tuft of black hair at the tip. The kowari lives in stony deserts and zones where vegetation is sparse. It is solitary, nocturnal, and may shelter in a burrow it has dug for itself or in one dug by a bilby; when it is cold, it becomes lethargic. The much smaller ninguais (Ninguai ridei and N. timealeyi) are tiny marsupial mice weighing only a fraction of an ounce (2-12 g). Like the other desert dasyurid marsupials, they have a pointed snout, rounded ears, long thin tails, and feet 2 mm wide. Ninguais are nocturnal and solitary. The wongai ninguai (Ninguai ridei), the more typically desert-living of the two, has greatly enlarged tympanic bullae like the desert rodents of the Sahara and may enter dormancy. Another insectivorous marsupial mouse, the fat-tailed antechinus (Pseudantechinus macdonnellensis), may thicken the base of its tail if its diet allows for the accumulation of fat reserves; the same goes for some dunnarts (Sminthopsis) such as Ooldea's dunnart (S. ooldea), discovered in 1970, the fat-tailed dunnart (S. crassicaudata), the stripe-faced dunnart (S. macroura), and probably also the hairy-footed dunnart (S. hirtipes). Hairy feet are very unusual in marsupial mice, and this is presumably linked to their movement over sand.
The family Notoryctidae is only represented by the blind marsupial mole (Notoryctes typhlops), which is relatively abundant in all the sandy areas of the Australian deserts and occupies the same ecological niche as Grant's golden mole (Eremitalpa granti) in southern Africa. The kangaroos and wallabies (family Macropodidae) are also represented in Australia's deserts, mainly by the common wallaroo, or euro (Macropus robustus), which is common throughout Australia, and the red kangaroo (M. rufus), some populations of which adapt very well to aridity.
4. Life in and around bodies of water
4.1 Water bodies in hot deserts
The definitive feature of deserts is that there are very few watercourses. Yet this does not mean that the water present in deserts occurs in quantities that are too small to be biologically significant. Nor does it mean that water bodies in deserts lack biodiversity and are of no scientific interest. All deserts, however arid they may be, receive rainfall from time to time. After rainfall, which is sometimes very intense, there may be abundant water on the desert surface, and it may persist for long periods of time, a year or more, allowing the growth of a wide variety of characteristic animals and plants. The water may flow along wandering (arheic) drainage lines or follow a well-defined drainage system (often forming part of endoreic systems). It sometimes accumulates in temporary pools or lakes that may become saline or remain fresh.
The variety and scarceness of bodies of water
Surprisingly, a great variety of permanent bodies of water exist in the desert, though they are not abundant. In sites where the water table reaches the surface, it gives rise to springs that may form pools, lakes, or rivers, and in some sites ground subsidence can open a so-called window on to the water table. Some small bodies of freshwater, not directly connected to the water table and in sites protected from evaporation, may persist from one rainfall episode to the next. There is also permanent water where rivers coming from wetter areas enter the desert. Underground waters can be found in the form of underground rivers and lakes. Nowadays, there are also bodies of freshwater formed artificially by pumping underground water to the surface, where it is stored before use in irrigation or for other purposes.
Leaving the external (allogenic) rivers aside, five features characterize bodies of water in the desert: 1) As an obvious consequence of the low amount of rainfall in deserts, the absolute volume and number of bodies of water is low. 2) Water bodies in deserts are highly unpredictable in space and time; the possibility of finding water in a given site at a given moment is, in general, correlated with the aridity, so that as the aridity increases, the unpredictability also increases. 3) Most bodies of water in the desert and some large permanent bodies show great physical, chemical, and biological variability. 4) The bodies of water are highly isolated from each other and are much more isolated than in other biomes; species richness often seems to be correlated with isolation and unpredictability, diminishing as they increase. 5) Relatively little is known about desert waters. This is due to their unpredictability, their isolation, and the fact that they are often inaccessible, but it does not mean they lack scientific interest. They are in fact of great scientific interest because their biota shows biological adaptations to the isolation and unpredictability of these bodies of water and to the other stresses experienced.
Episodic bodies of freshwater
The term episodic is appropriate for all the bodies of water in the desert that are only present for a limited time after rainfall, and it implies that they are unpredictable. The term ephemeral, though often used to describe temporary bodies of water in the desert, is less useful, as it does not indicate that the waters are unpredictable as well as temporary. Freshwater and salty water differ in their total ionic concentrations; freshwater is conventionally defined as water with a total concentration of ions of less than 3 g/l, while salty waters contain more than 3 g/l.
Episodic freshwaters are perhaps the most typical type of body of water in hot deserts. Standing water masses after a rain are invariably shallow and last for a few days or a few months, depending on the volume of water received, the morphology of the basin, the water lost by evaporation, and the permeability of the soil. Wind action is a key physical-chemical factor, meaning that turbidity is often high (and variable), there is great exchange between the water and the sediments, the water temperatures closely follow that of the air (with little or no thermal stratification), oxygen concentration is close to saturation, and the supply of nutrients borne by the wind (anemotrophy) and from the clay of the sediments (argillotrophy) have a significant impact on the bodies of water.
Little is known about the nature of the episodic flowing freshwaters in deserts. Yet it would be surprising if they were completely without life (at least in the short-lived residual pools). In fact, torrents in arid savannahs with physical characteristics only slightly less adverse than those in episodic flowing waters in deserts have well-defined ecological characteristics and a characteristic biota (see vol. 3, p. 223). The torrents in the Sonoran Desert, for example, flow irregularly, and there is a great variety of insects in the superficial waters and in the interstices of the sediments; there is also significant in situ growth of algae. A line of trees is present on the edge (a feature of many episodic torrents and not just those in the Sonoran Desert) and may indicate wetter periods in the past.
Permanent bodies of freshwater
Permanent freshwater in deserts is frequently derived from the water table emerging at the surface in the form of seepage (often trapped naturally to form a wetland or pool) or short watercourses or ones that only surface in depressions. Their surface area, depth, morphology, salinity, and other physical and chemical characteristics vary with the type of water supply (in the case of torrents), the degree of cover, the level of evaporation, and the chemical composition of the incoming waters.
These little-known bodies of water are of great scientific interest but are now threatened by human activity; the underground water of the deserts is being pumped to the surface for agriculture, mining, and other uses, thus lowering the water table. It should be noted that these uses may create permanent bodies of surface freshwater if the water pumped to the surface is stored in small pools before use. Most surface waters in deserts are at risk from trampling by livestock or are now being destroyed by them.
The water table may also form permanent bodies of freshwater in underground lakes and torrents in areas dominated by calcareous rocks. These large volumes of water are inhabited by a fauna that, while not very diverse, is of great scientific interest and feeds on the organic matter arriving from the surface. There are no plants, algae, or cyanobacteria in these underground lakes.
The zones of permanent freshwater that are not connected to underground sources of water receive their water from rivers with sources in wetter areas; these rivers cross the desert and flow into an endorheic lake or into the sea. A prime example is the Nile, which has its source in the wet areas of eastern Africa but flows through the Sahara into the Mediterranean (see vol. 3, p. 216). Almost all permanent bodies of freshwater in the deserts receive water either from allogenic watercourses (with external sources) or from underground sources, although small pools may form in depressions and holes in impermeable rocks that are well protected from evaporation. The longer they persist, the greater the chance they have a diverse fauna.
Saline bodies of water are not characteristic of hot deserts, as this biome is, in general, too dry for them to develop. In any case, the less arid deserts contain depressions that mark the boundaries of the inland river basins and contain saline waters for some time after the episodic rainfall, as exemplified by Lake Eyre in Australia. This large depression (about 10,000 [km.sup.2]) in the central desert of Australia fills at irregular intervals with water when the rivers flowing into it receive large quantities of rainfall. In the second half of the twentieth century, it was filled with water from 1949-1952, 1974-1976, 1984-1985, and 1989-1990; it probably also filled in 1890 and may have been partly full on several occasions that were not recorded. Despite its episodic nature, these temporary saline lakes have a diverse aquatic flora and fauna apparently characterized, like that of episodic freshwaters, by 1) a good ability to disperse and 2) low levels of regional endemism.
The deserts also contain a small number of sites where there are permanent bodies of saltwater. These bodies are derived from underground springs that emerge at the surface and are then subject to intense evaporation and become increasingly saline, as in the case of the small hypersaline torrents of the Namib Desert. Other larger examples include the Ounianga Serir, a lake occupying an area of 4.2 [km.sup.2] in Chad, and the lakes on the Jrebel Mount Marra (Sudan) in the Sahara.
4.2 Pools, riverbanks, and oases
There are two situations in which water in deserts is relatively abundant: in the large rivers of external origins crossing the deserts and in the oases. The rivers are atypical, as they are in transit and do not form part of the reality of the desert biome. It should be noted that some of the world's largest rivers cross desert areas-among them the Indus, Nile, and Niger-and that their presence is inseparable from the use that humans have made of these areas; directly or indirectly, then, they have profoundly influenced the area's development. Oases are an autochthonous phenomenon, now very often linked to the action of humans in pumping up relatively deep underground water.
Life in episodic and permanent bodies of water
Major adaptations are required to survive in the episodic bodies of freshwater in the hot deserts, including adaptation to the high water temperatures (which may exceed 104[degrees]F [40[degrees]C]), the strong light intensity, the isolation, the environmental instability, and, of course, the desiccation. Adaptations to isolation and desiccation seem to be highly correlated, so that desiccation-resistant stages of the life cycle (normally the egg or cyst) are usually also the dispersal stages. The strength of this linkage is still not fully known, but-aside from those species that can enter dormancy in a dehydrated form and resist desiccation for an apparently unlimited period of time without giving any sign of life-it seems logical to suppose that resistance to desiccation is limited and that dispersal mechanisms are well developed and considerably efficient. This coincides with current knowledge about the degree of endemism of the biota of the desert waters: unlike the temporary waters in less arid areas such as the savannahs, there are few or no endemic species in the waters of the deserts, and the organisms found there have wide distributions. One characteristic example is the tadpole shrimps (the notostracan crustaceans of the genus Triops, see figure 86, p. 153) in Australia. Throughout Australia's deserts, wherever temporary bodies of freshwater persist for a certain length of time, just a single species of tadpole shrimp (T. australiensis) occurs.
The macro-invertebrates considered most characteristic of the episodic freshwaters of hot deserts are branchiopod crustaceans, or the gill-footed shrimps (Notostraca, the tadpole shrimps; Conchostraca, the clam shrimps; Anostraca, the fairy shrimps). Even so, there are other forms, especially crustaceans of other subclasses, including copepod crustaceans, the ostracod mussel shrimps, and insects (especially dragonflies, beetles, flies, and true bugs). No well-defined sequence of colonization has yet been identified, but it seems that predatory forms arrive last. Some animals without any desiccationresistant stage in their life cycle-mainly frogs and fish-may also live in episodic waters in deserts, appearing shortly after the formation of these water masses. Frogs survive the long periods of drought as adults in deep burrows surrounded by an impermeable covering, and they show a wide range of physiological adaptations to ensure their survival. They can become active and leave their burrows shortly after the onset of the rains. Some fish survive in a similar manner, but most studies on the fish of episodic waters refer to specimens that have been borne by floodwaters from permanent bodies of water or occasionally by downpours and similar phenomena.
Little is known about the plants, algae, and bacteria of episodic water masses in hot deserts. There are no large water plants, and there is no evidence that algae or cyanobacteria show regional patterns of endemism; like the macro-invertebrates, they need to have great powers of resistance and a high ability to disperse in order to survive in the desert environment.
The biota of the permanent waters is diverse and often includes forms that lack a resistant stage in their life cycle, have little ability to disperse, and are regionally endemic. Many of the characteristic forms of the episodic waters of the same deserts are absent, especially the tadpole shrimps, fairy shrimps, and clam shrimps. Fish and frogs are more often found in these permanent freshwaters that in episodic ones. The growth of the algae and cyanobacteria is limited by turbidity and nutrient availability but is often high. Water plants may be abundant or completely absent.
The fish and amphibians
In the temporary pools at the end of the intermittent streams of wadis in the Sahara Desert, carps (family Cyprinidae) of Palaearctic origin are present: among them are the Biskra barb (Barbus biscarensis), which can reach a length of 8 in (20 cm); the Antoniri barb (B. antinori), which lives near oases and may reach 12 in (30 cm); and the desert barb (B. deserti), which can be found in isolated pools from Mauritania to Libya and Chad. The air-breathing catfish (family Clariidae) are of tropical origin and may be considered remnants of when the Sahara was much wetter. They still survive in temporary pools where they feed on other fish, and when their pool dries out they can travel overland to another one. The Imirhou catfish (Clarias gariepinus), which lives in the Imirhou wadi at the base of the northern foothills of the Tassili N-'Ajjer Massif, can reach a length of more than 3 ft (1 m); the Senegal catfish (C. anguillaris) is slightly smaller. The mosquito fish, or gambusia (Gambusia affinis), a small fish of the toothcarp family (Cyprinodontidae), was introduced from the United States to eat mosquito larvae; they are sometimes found in large numbers. The cichlid fish (family Cichlidae), whose distribution reaches southern Africa, also live in freshwater pools in the Sahara. They include the acar (Hemichromis bimaculatus), the males of which are reddish, Desfontaines' sargo (Astatoti-lapia desfontainesi), and Zill's tilapia (Tilapia zillii).
The conditions of life in the desert seem more unfavorable for amphibians than fish, as amphibians need to keep their skin permanently moist. Even so, the Saharo-Sindian deserts contain representatives of five families of amphibians, with some species endemic to northern Africa. All the species seek out a favorable microclimate such as a crack, a hole in a rock, or, if possible, a pool. The African clawed toads (family Pipidae) are represented by Xenopus muelleri, a species of tropical origin that only lives in permanent pools. Some toads (family Bufonidae) live in oases or in rugged sites. The small Brongersma toad (Bufo brongersmai), for example, lives in the northwest Sahara, while the Mauritanian toad (B. mauritanicus) occurs in mountainous areas; the green toad (B. viridis) and the panther toad (B. regularis) are very common in the oases of the Sahara and the Nile Valley, as well as in the west and south of the Arabian Peninsula. The discoglossid toads (family Discoglossidae) are represented by the painted toad (Discoglossus pictus), whose main area of distribution includes the Iberian Peninsula and northern Africa. The true frogs (family Ranidae) are represented by a Palaearctic species, the marsh frog (Rana ridibunda), which is active by day and by night, and by a tropical species, the African striped frog (Dicroglossus occipitalis), which is mainly nocturnal.
The amphibians with the most representatives in the deserts of North America are the toads (family Bufonidae). Woodhouse's toad (Bufo woodhousei) has a white line on its back and a prominent transversal crest on its head; the red-spotted toad (B. punctatus) is frequent in the oases and has rounded parotid glands; the prairie toad (B. cognatus), with brown markings, frequents the deserts as well as its typical prairie habitat; the green Sonora toad (B. retiformis) is marked with a black stripe on a gray-green background; and the North American green toad (B. debilis), with black patches, lives on the arid or semiarid plains of the western United States.
Oases--islands of water
Oases are naturally moist sites (despite being in the heart of the desert) that have been modified to make best use of the water present. It is easy to see why human beings have developed the most effective techniques of water usage in environments as inhospitable as the deserts. This way of using water has supported subsistence agriculture since time immemorial, favoring permanent settlements in the deserts and the transit routes that cross them. However, there is not usually a distinctive spontaneous fauna associated with oases, perhaps because they have been so altered by human beings or are totally artificial. In oases on wadis, people plant palms and sow their crops on the bed of a fossil river. In some cases-for example the Wadi Saoura between the Great Western Erg and the Guir Hamada-these oases run in a line along the dry river bed marking the route of the caravans of the former trans-Saharan route. The water is extracted by means of rockerarm wells from the water table, supplied by the erg that circulates along the underground riverbed flowing from the Saharan Atlas Mountains.
Erg oases occur in the middle of large expanses of sand and are simply small depressions where the water table is closer to the surface and where palms can be cultivated. They have to be maintained, since without human intervention the sand would invade the oasis and eventually bury it. To prevent this, the inhabitants build fences of palm leaves to retain the sand and thus favor the formation of artificial dunes, or afregs, that protect their settlement and crops. As the sand accumulates, more palm fronds are placed at the top so that the dune gets higher and higher. The crops in these oases are not irrigated, as the roots often reach the water table directly.
Oases in sabkha occur on the edges of these depressions, where the underground watercourses that supply the sabkha emerge. Water catchment is by gravity alone. Settlements are normally on the upper slopes, above the galleries that channel the water. The water is then directed to vegetable gardens and palm groves at a lower level by ingenious and complex channeling systems. Water supply in sebkha oases is not completely natural. The desert inhabitants learned long ago to locate the routes supplying water from higher areas and improved the way they were channeled by constructing water mines (see p. 206).
These underground tunnels, which can be seen in aerial photos, run over thousands of kilometers of desert preventing the dispersal of the runoff water. The flow of water reaching the village is continuous, and there are no reservoirs to store it. Nor is it possible for the inhabitants to take turns using the water, since the crops need water permanently, given the high potential evapotranspiration, the low water retention capacity of the sandy soils, and the high risk of salinization. Consequently, the water flow has to be divided using comb-shaped structures to split the flow so that it flows to every corner of the oasis along channels shaded by palms. Logically, one of the most important posts in these settlements is the water master, the authority who regulates the distribution of water, measures the volume of flow to see that it corresponds to the size of the plot to be irrigated, and resolves conflicts between families. This ensures the efficient use of an asset, water, which is scarce and valuable in the desert.
45 Succulence and spininess are common in the plants of hot deserts, as shown by this plant community in the Organ Pipe Cactus National Monument Biosphere Reserve (Arizona, United States). There are clumps of the yellow-flowered Encelia farinosa (Asteraceae), with specimens of the spiny ocotillo (Fouquieria splendens, see also photos 57 and 145) and the extremely prickly teddy bear cholla (Opuntia bigelovii), easily recognized by its abundant bright cream spines that embed themselves in passing animals. The stem pads break at the constrictions typical of prickly pears, and passing animals carry them away unintentionally, thus dispersing the plant vegetatively. The organ pipe cactus (Lemaireocereus [= Stenocereus] thurberi) that gives the park its name is very common and can be seen in the upper left-hand corner of the photo. In the background is Mount Ajo.
[Photo: Carr Clifton / Minden Pictures]
46 When wadis, like this one in Saudi Arabia, fill with water, the vegetation that grows could be considered lush for a desert. Any desert, however arid it is, receives some rain occasionally, and sometimes surface waters are very abundant. They may open new drainage channels or flow down the existing ones (such as wadis) and then may form temporary pools and lakes, some of which stay fresh, while others turn salty. In any case, they allow the development of the typical species of plants and animals.
[Photo: John R. Bracegirdle / Planet Earth Pictures]
47 Water resources in the Mediterranean coastal states of Africa and the Middle East in 1980. Water resources depend not only on the distribution and intensity of the rainfall but also on the potential evapotranspiration, the relief, and soil characteristics (depth, texture, permeability, slope), which determine the soil's ability to retain water, and, in turn, dictate the quantity of underground water reserves. In arid climates, water is the factor limiting biological growth, as the other resources are not usually in short supply.
[Source: data provided by the author]
48 The complex underground structure of many desert plants is exemplified by this cross section of a dune colonized by a specimen of Ziziphus lotus. It may grow into a tree 1013 ft (3-4 m) tall, but if it is browsed by goats or camels, it forms a low hemispherical mound that traps the sand borne by the wind. The plant produces horizontal underground branches from the base of the stem, and these then bear vertical branches that grow above the soil surface. The stem also grows slowly in the direction of the dominant wind; one of the lateral branches produces a swelling that will eventually become the new stem, and the old stem will then die. The horizontal roots may be very long, up to 33-49 ft (10-15 m), and they may grow far beyond the dune. Thus, they can absorb rainwater that falls on the flat area. Internally, dunes consist of alternating layers of sand (deposited by strong winds) and humus (which accumulates during calm periods). These dunes contain a rich fauna; many different animals, including reptiles, rodents, and small insects, dig all sorts of shelters and burrows in the area.
[Drawing: Jordi Corbera, based on Walter & Breckle, 1984]
49 Palms are the most representative plants of wet sites in arid regions. The palm family contains a total of about 200 genera with about 2,500 species. Their general appearance is unmistakable: a more or less lignified stem resembling a tree trunk, but often very slender and rarely showing branching, topped by a tuft of large palmate (fan palms) or pinnate (feather palms) leaves. Some species have a dichotomously branching stem, among them the doum palm (Hyphaene thebaica), which is widely distributed throughout the African continent. Palms grow in a wide variety of biomes from the rainforest, where they are very well represented (see vol. 2, photos 93 and 220), to the savannahs (see vol. 3, photos 75, 144, and 227) and the deserts. Their need for underground water, whether near the surface or at depth, means that in deserts they are restricted to oases and river valleys. Palms are very widely distributed, with representatives on all the continents, though they show greatest diversity in Asia. Nearly all the species are limited to a single region, and very few have a range that includes more than one continent. (The palmyra palms [genus Borassus], occur in Africa and Asia.) Other widespread species are cultivated in oases, the best known being the date palm (Phoenix dactylifera), originally from the delta of the Tigris and the Euphrates, where the first plantations were described about 6,000 years ago. The date palm is now widely cultivated in many arid tropical and subtropical regions for its fruit, dates, and for many other uses (see photos 118 and 119). Another wellknown palm is the Washington palm (Washingtonia filifera), which is widely cultivated as an ornamental in parks and gardens and along streets.
[Drawing: Jordi Corbera]
50 One of the most remarkable germination mechanisms adapted to desert conditions is found in Ble-pharis ciliaris (Acanthaceae), a herbaceous plant (annual or perennial) that grows throughout the Sahel and part of Arabia. The inflorescence's many bracts enclose the seed capsules, protecting them from the harsh climate and from predators while they remain closed. They can survive for several years and remain viable. Lasting moisture, whether due to rainfall or a surge, is essential for the bracts to separate and expose the capsule. (Dew is not enough.) The humidity then reduces the pressure of the internal walls of the capsule (the septa), causing the capsule to explode and scatter the seeds. (Capsules that have been kept dry for 40 years may still explode.) On wetting, the multicellular hairs adhering to the capsule move in such a way that orientates the seeds at an angle of 30-45[degrees] to the soil surface, with the tip of the radicle in contact with the ground. The cells at the tip of the hairs then rapidly release a mucilaginous layer that fixes the seed to the ground, helping successful germination. Dry seeds falling on wet ground normally only take 50 minutes to start germinating, and the radicle grows almost 0.5 cm in 24 hours. Only a few capsules from the mother plant release their seeds each time wetting occurs, so if the first attempt to establish is a failure, there will be additional opportunities.
[Photo: Hans Christian Heap / Planet Earth Pictures]
51 Tumbleweeds have a very unusual method of seed dispersal. The entire plant, or just the seed-producing parts, is broken off after fruiting by the strong winds and blown all over the desert, where there are almost no obstacles. (The photo shows a tumbleweed in Arizona that has run into barbed wire.) The seeds are shed and scattered widely as the weed is blown around. This dispersal system is used by herbaceous plants, normally annuals, belonging to several families (Chenopodiaceae, Amaranthaceae, Poaceae, Brassicaceae, Apiaceae). Tumbleweeds are most common in deserts and steppes, because strong, constant winds and open spaces are needed to ensure the success of this dispersal mechanism.
[Photo: Ramon Folch / ERF]
52 The different plant life-forms correspond to different survival strategies and reflect each plant species' defenses against low winter temperatures. These five types, established by Danish botanist C. Raunkiaer (1860-1938), are based on the height of the perennating buds above ground level. Phanerophytes, mainly trees and shrubs, have high perennating buds raised more than 10 in (25 cm) above the ground; they are totally exposed to the environment. Chamaephytes also include some lower shrubs, but they have aerial buds that are much closer to the soil. Hemicryptophytes are herbaceous plants with their perennating buds at ground level, while cryptophytes have their buds hidden underground (geophytes) or underwater (hydrophytes). Therophytes are annual plants that pass the unfavorable season as seeds. They are the most common life form in arid regions, dominating the areas with seasonal rains; chamaephytes dominate the drier deserts.
[Drawing: Jordi Corbera, bas-ed on several sources]
53 Stems adapted to store water are common among woody plants in arid areas, such as the pachypodium Pachypodium rosilatum gracilis (Apocynaceae), shown in this photo taken in the Isalo National Park (Madagascar). Its stem is soft and succulent and has a lot of water-storing parenchyma but very few woody components. Apart from the trees and shrubs of the genus Pachypodium (see also drawing 58), many arid plants of the arid regions of Madagascar have modified some of their basic structures to store water for use during the long dry periods.
[Photo: Nick Garbutt / Planet Earth Pictures]
54 The high osmotic pressure of the roots and shoots of xerophytic plants is a physiological adaptation that improves water absorption. Pressure increases as water availability decreases. Plants growing in saline environments (halophytes) have higher osmotic pressures than xerophytes living in nonsaline environments, as shown by comparing measurements taken in the Sahara Desert (see table of data). The osmotic pressure is not the same in all parts of the plant, as shown by cacti in North America. The cortex of the cacti has a higher osmotic pressure than the inner photosynthetic tissues, while the lowest pressure is in the reserve parenchyma. The same studies show that the osmotic pressure also varies in different parts of the plant, depending on their orientation. The highest values are in the part of the plant facing southwest, as the highest temperatures and driest air on sunny days in the Northern Hemisphere are on the southwest face exposed to the afternoon sun. There is also a clear relationship between the structure and the osmotic pressure, as shown by the isosmotic lines (measured in atmospheres) of a section of Ferocactus wislizenii (shown in the diagram of a cactus stem); the higher the osmotic pressure, the closer together the ridges and the better developed the vascular system.
[Drawing: Jordi Corbera, based on Walter & Breckle, 1984, and data supplied by the author]
55 Some desert plants store abundant water in their fruits, meaning they are a good source of water (and to a lesser extent, of nutrients) for the animals of this biome, including human beings. One example is Citrullus lanatus, a wild watermelon, shown in this photo taken in the Kala-hari Gemsbok National Park (South Africa). (See also photo 107.) The watermelon's fruit contains about 95% water.
[Photo: Nigel J. Dennis / NHPA]
56 Many species of cactus have ribbing like an accordion, allowing them to swell up and shrink depending on the amount of water they contain, as shown in this poster of the saguaro cactus (Carnegiea gigantea) of the Sonoran Desert in Arizona (United States). The saguaro swells up as it absorbs water. The plicate ridging allows it to expand around a woody skeleton, which consists of a cylinder of vascular bundles that anastomose halfway up the stem height and then separate again (see fig. 111, middle). Its slow growth and huge water storage capacity mean that this cactus can fruit every year, more or less regardless of the dry conditions. Water accounts for about 75-90% of the fresh tissue weight.
[Photo: Ramon Folch / ERF]
57 Spininess is an adaptation to water loss shown by several desert plants, including the ocotillo (Fouquieria splendens, Fouquieriaceae, on the left) and the Ferocactus (on the right). After rain, the cactuslike ocotillo usually bears many leaf shoots in the axil of its spines, as shown in the photo, which was taken in the Coachella Valley, California (United States). Its straight and virtually unbranched stems all arise directly from the rootstock, so they grow into a spiny and almost leafless screen 7-10 ft (2-3 m) tall (see photos 45 and 145). Ferocactus is a genus of barrel cacti from the deserts of southwestern North America, with photosynthetic stems bearing extremely long spines. (See photo, taken in Organ Pipe Cactus National Park in the Mount Ajo Range, Arizona, U.S.) These plants are also cultivated as ornamentals in parks and gardens in other parts of the world.
[Photos: Stephen Kraser-mann / NHPA and Ramon Folch / ERF]
58 Succulence, spininess, and the loss or extreme reduction of the leaf system characterize the cacti and cactiform plants so typical of hot deserts and subdeserts. True cacti belong to the cactus family (Cactaceae) and are from the Americas. Their succulent and normally photosynthetic stems have longitudinal grooves and ribbing, along which they bear areoles, corresponding to the leaf axils, from where the cacti produce their spines. All cacti share the same general features of being succulent, spiny, and basically leafless (drawings on left-hand page), but they vary greatly in morphology and size. Some cacti are hemispherical-Echinocactus from the deserts of North America and the north of South America, for instance-and can reach the size of a basketball. (The smaller drawing shows a single areole with spines.) Cacti vary in shape from the many pulvinate (cushion-forming) species of Copiapoa in the Atacama Desert (the drawing shows the cushion growth-form, a longitudinal section of a flower, and the taproot so typical of cacti) to monopodial arborescent forms such as the spectacular candelabro (Browningia candelabris) of the Andean slopes of the Tarapac Desert (Atacama), growing up to 16 ft (5 m) tall. There are also sympodial arborescent forms such as the organ pipe cactus (Lemaireocereus thurberi) of the Sonoran Desert, which is 7-10 ft (2-3 m) tall and not very spiny. Finally, some cacti are columnar, among them the pasacana of the Atacama (Trichocereus atacamensis, see a typical example in fig. 111), which is very spiny and hairy and grows up to 20-23 ft (6-7 m) in height. A comparable morphology has also been adopted by succulent members of the families Euphorbiaceae, Aizoaceae, Asclepiadaceae, Agavaceae, Apocynaceae, and others (see the drawings on right-hand page), represented in the same order by some African arborescent species of Euphorbia, the southern African species of stone plants (Lithops, which are the size and shape of a stone), the digitate Stapelia spp. from Africa, the agaves (Agave) of North America, or the bizarre species of Pachy-podium from Madagascar. (The drawing also shows a leaf with the stipules transformed into the characteristic bifid spines that may be more than 7 ft [2 m] long.)
[Drawing: Jordi Corbera]
59 In the protective shade of a palo azul (Cercidium microphyllum), a young specimen of saguaro (Car-negiea gigantea, Cactaceae) has managed to grow in the Organ Pipe Cactus National Monument in Arizona (United States). A saguaro produces millions of seeds over its life, but only a few will germinate and very few will grow into a plant that reaches maturity. The chance of survival is greatly increased if the small cactus grows among the branches of a tree or shrub that provides shade, protecting it from the winter cold and the summer heat. When the cactus is large, however, it will compete for water with its adoptive parent, which normally suffers the ill effects.
[Photo: Ramon Folch / ERF]
60 A diagrammatic section of the Mollendo lomas in the Peruvian coastal desert, showing the frequency of fogs, known as garuas, and the distribution of the vegetation along a slope descending northeast-southwest from a desert plateau to the Pacific coastline. At the top, where fogs are uncommon, the vegetation is dominated by bromeliads of the genus Tillandsia. A little below this, the fogs are more common and herbaceous vegetation can grow. Succu-lent species grow only in certain biotopes where the fog zone is not totally enclosed-for instance, in valleys where during the night dry winds blow down from the mountains and disperse the fog. Tender herbaceous plants cannot resist these winds periodically drying out the environment. However, this wind is just fine for the cacti, as it prevents infection of the seedlings by fungi and gets rid of competing species of grass. Near the coast, an open, herbaceous, highly xerophytic vegetation grows consisting of fog-loving species.
[Drawing: Jordi Corbera, based on Walter & Breckle, 1984]
61 The Atacama Desert in bloom after especially abundant rain is extremely beautiful, as shown in this photo of Cristaria (pink flowers) and Calendrinia (purple flowers). Flowering like this occurs every year during the growing season (August to November), whenever temperatures and soil water levels are suitable and there is a sufficient soil seed bank. The phenomenon is especially attractive on coastal hills and the sandy soils of the inland desert, hiding the dark soil with shades of bright pink, red, purple, lilac, yellow, orange, and white.
[Photo: Peter Francis / South American Pictures]
62 Desert burrowing amphi-bians such as Couch's toad (Acaphiophus couchii), shown in the photo half-buried in the sand of the Arizona Desert (United States), spend the dry periods between rains estivating. (Estivation is the equivalent in hot climates of hibernation in cold ones.) To survive the most adverse conditions, the Couch's toad digs deep burrows in the sand where it spends the hot season in dormancy, thus reducing its energy and water needs. When the first rains fall, it leaves its burrow to mate and spawn in the shallow pools that have formed. The fact that animals that are totally dependent on water (such as amphibians) can live in a desert is one of the more surprising aspects of the animal world and a good example of the great adaptive capacity of many species.
[Photo: John Cancalosi / Auscape International]
63 Deserts offer many ecological niches to small mammals that can resist the heat and are not fussy about what they eat. One small mammal common in the American deserts is the ground squirrel (Ammosper-mophilus [=Citellus] harrisii), a rodent that can often be seen on rocky slopes. The specimen in the photo is trying to get at a juicy cactus fruit. The white stripe on its flanks makes it easy to identify. This ground squirrel lives in shelters dug in the rocks, where most predators cannot reach, and it feeds mainly on cactus fruit and other plant organs that meet its need for water.
[Photo: Jean-Yves Grospas / Bios / Still Pictures]
64 Scorpions are especially abundant in deserts, where they can easily withstand the high temperatures and lack of water. They are nocturnal hunters that feed on insects and arachnids, though the larger species may catch lizards and small rodents. Scorpions are generally yellow, like the one in the upper photo, the dune scorpion (Opistophthalmus flavescens) from the Namib Desert, shown devouring a dune cricket (Comicus); most other scorpions are black, including the Urodacgus (lower photo) from the Victoria Desert in Western Australia. Some scorpions are highly poisonous, while others are not so poisonous, but no correlation has been observed between the color and how poisonous they are, so one cannot argue as previously believed that black is a warning color and that yellow is for camouflage. (In the desert even nocturnal animals need cryptic coloration, because the air is so clear that they can be seen by the light of the moon and stars.)
[Photos: Michael & Patricia Fogden and Reg Morrison / Auscape International]
65 Several species of desert plant have their own private army of ants, and the close association is beneficial for both. The ants obtain shelter and food (produced by the plant to attract them), and the plant reduces the losses it suffers from herbivores, as the ants capture other small insects that come to eat the plants. Sometimes a third player enters the story, a coccid (a soft scale insect). The ants take the eggs of the scale insect into their nests where they care for them until they hatch. They then let the larvae feed on the walls of the ants' nest (the plant); the larvae, in turn, secrete substances the ants eat. Longitudinally cutting open a nest of the spinifex ant (Iridomyrmex flavipes) reveals the ants and some of the larvae they are milking.
[Photo: Jan Aldenhoven / Aus-cape International]
66 Swimming on, or even in, sand is an ability shown by several animals that live on dunes, among them the sand fish (Scincus scincus), a small reptile that is common in the northwestern Sahara and can swim rapidly within sand for several meters. Over their evolution these and other digging desert reptiles have developed adaptations such as reduced legs (but not hips); small, flat, smooth scales; and a snout suited for tunneling.
[Photo: Cyril Ruoso / Bios / Still Pictures]
67 The tracks left in the sand by the Namib desert viper (Bitis peringueyi) are the result of its lateral locomotion, practiced only by short snakes. This type of locomotion is obviously advantageous when crossing sun-heated sand and is also much more effective on a mobile substrate than any other form of locomotion. The track left gives an incorrect impression of the direction in which the snake is moving. (The viper in the photo is traveling left to right, but the track suggests it is moving right to left.) Snakes that move by lateral displacements get a firm grip on the surface by holding their body at almost a right angle to the direction of movement. The tracks they leave on the sand consist of a series of parallel lines at an angle of 60[degrees]E to the direction of movement.
[Photo: Carol Hughes / Bruce Coleman Limited]
68 The cryptic coloration of this rock locust (Cryp-sicerus cubicus) from the Namib Desert in Namibia is so perfect that the animal is easily mistaken for the stones. Cryptic colors and morphology are very common among animals and allow them to avoid predators without having to flee and also to approach prey slowly without being seen. This represents major energy savings in both cases. Cryptic coloration is not exclusive to desert environments, but it is very frequent in them. The scarce vegetation in the desert biome makes it very common for animals to mimic stones or sand.
[Photo: Michael & Patricia Fogden]
69 The aposematic (warning) coloring of the poisonous Gila monster (Helo-derma suspectum), together with its grotesque and threatening appearance, warns inhabitants of the Sonoran Desert that it is dangerous. It used to be considered dangerous to humans, but the chance of a Gila monster biting a person is very remote. The Gila is really only interested in bird's eggs, chicks, and small rodents, the main components of its diet.
[Photo: John Cancalosi / Auscape International]
70 The desert animals with circadian rhythms include the sand-diving lizard (Aporosaura anchietae), a saurian of the inland Namib Desert (this specimen was photographed in Naukluft in Namibia) that lives on dry grass seeds. In the morning, when temperatures are about 30oC, the sand-diving lizard comes out from under the sand where it has spent the night and presses its belly against the sand to warm itself up. Once its body temperature is high enough, it leaves in search of food; when the temperature ap-proaches 104[degrees]F (40[degrees]C), it buries itself in the sand once more in search of a cooler environment. It emerges again in the afternoon and eventually takes shelter for the night. Like this lizard, most desert animals show behavioral adaptations to the high temperatures; they deal with drought by physiological adaptations.
[Photo: Michel Bureau / Bios / Still Pictures]
71 The area of distribution of the desert locust (Schis-tocerca gregaria) covers the arid regions of Africa and southwest Asia, but small swarms have also been recorded on the northern side of the Mediterranean and isolated specimens have even been found in the south of the British Isles. As long as they have enough food, desert locusts behave just like any other nonmigratory grasshopper; when food starts to become scarce, though, the developing individuals turn into a gregarious, long-winged brown form (the solitary form is green), and they join together in enormous migratory swarms that eat everything in their path.
[Drawing: IDEM, from several sources]
72 A male agama (Agama aculeata aculeata) basking in the sun in the Kalahari Gemsbok National Park. The agamas (Agama) are the most conspicuous reptiles in Africa. The male's bright colors are more intense in the breeding season, especially during their mating display and when they defend their mating territory. Many species of agama have heads colored electric blue, and when they wish to attract attention, they move it rapidly up and down. The juveniles and females, however, show cryptic coloration, normally with brown or gray patches, and some have a dorsal stripe that helps to blur the animal's outline to the eyes of a predator.
[Photo: Thomas Dressler / Planet Earth Pictures]
73 The Saharan lark (Am-momanes deserti), shown in this photo taken in Aqaba (Jordan), has found a good strategy to avoid the heat of the desert. It constructs its nest so that the sun's rays fall on it during the morning, but it remains in shade for the rest of the day. The nest is also orientated to receive the cool morning breezes, and thus the female can leave the eggs or chicks without the risk of them overheating. It should be pointed out that the most delicate phase of bird development begins when the eggs are laid and lasts until the chicks are fully fledged, and this period is especially critical in the harsh desert environment. Thus, the survival of any species of bird requires that it build a suitable nest to shelter the eggs and the chicks.
[Photo: Pekka Helo / Bruce Coleman Limited]
74 The common jerboa (Jaculus jaculus) lives in the desert and semidesert areas of Arabia and northern Africa from Mauritania to Morocco, and to Somalia and SW Iran. It is the smallest of the desert jerboas, measuring only about 4 in (10 cm) from its head to the tip of its tail and weighing only about 2 oz (55 g). Despite its small size, this endearing rodent can dig burrows more than 3 ft (1 m) deep, even in the most compact sands. Each burrow has several exits, one or two of them for emergency use, and at the bottom there is the dormitory with a bed of twigs and camel hair. The common gerbil normally moves by jumps up to several meters long using its long strong back legs. When it stops, it keeps its tail outstretched and arched with its end touching the ground, like the specimen in this photo taken in the Sahara Desert; it is thus supporting itself at three different points, leaving its hands free.
[Daniel Heuclin / NHPA]
75 At night Rueppell's fox (Vulpes rueppellii) emerges to hunt. It catches small mammals, reptiles, and insects, but its diet also includes some plants. Its hunting territory is normally shared by three or four couples and is very large. (Though not all studies agree, it would seem to be about 30-60 [km.sup.2].) During the day, Rueppell's fox withdraws to its den to rest. It is well hidden among the vegetation and rocks, but for greater ease of mind it changes its site every four or five days, making it harder to catch unawares. Still, this carnivore of the African and Arabian deserts can sometimes be caught by surprise, as in this photo, taken near Dubai in the United Arab Emirates.
[Photo: Christopher Maddock / Planet Earth Pictures]
76 The gemsbok (Oryx gazella) is one of the few large herbivores able to live in desert and subdesert environments, as shown by this photograph of a specimen in the dunes of the Namib Desert in Namibia. It forms herds of up to 60 individuals, with a very rigid hierarchical organization that is maintained by ritual jousts. Each herd has a territory that it defends from intruders, but in regions with abundant vegetation hundreds of animals may join together temporarily. To live in these arid environments, the gemsbok has to complement its diet of grass by browsing on many plants, ranging from acacias to wild watermelons, and all sorts of fruits, bulbs, and succulent tubers.
[Photo: Jim Brandenburg / Minden Pictures]
77 The desert gopher tortoise Gopherus agassizii is one of the many reptiles that lives in the deserts of the Americas. It can be found in oases, pools, at the bottom of canyons, and in slopes on sandy or stony soils, normally among vegetation consisting of spiny shrubs or cacti. G. agassizii builds an underground nest sometimes more than 33 ft (10 m) long but normally just large enough for it to hide itself completely. The specimen shown was photographed in the Arizona Desert, but it lives in many other desert regions of the United States (Nevada, Utah, California) and Mexico (Baja California, Sonora, Sinaloa).
[Photo: John Cancalosi / Auscape International]
78 The sidewinder (Cro-talus cerastes) from the Sonoran Desert is the Amer-ican snake best adapted to desert conditions. One subspecies (C. cerastes lateriorepens) lives in the most arid part of the desert-in northern Baja California and near the Gulf of California. Because of the horn-like scales above its eyes, it is sometimes called the horned rattlesnake, though its distinctive lateral locomotion makes it a sidewinder. It is similar to the species of northern Africa and the Near East, but unlike these Old World snakes, the sidewinder is only moderately poisonous and does not represent a serious risk to human beings.
[Photo: Jim Merli / Natural Science Photos]
79 The roadrunner (Geo-coccyx californianus) is the best-known bird in the deserts of the southwestern United States and northern Mexico. It does not fly well, as its wings are very short, but it runs very well and fast, reaching almost 25 mph (40 km/h). Its long legs, which are more than half a meter long and are folded when it is resting, have four toes on each that help it to run; its tail, which is as long as its legs, acts as a rudder and a brake. When it runs, the roadrunner lowers its head and holds its tail vertically; steering is achieved by turning the tail slightly to the left or right; to stop, it raises its tail into a fan, thus braking itself. This unusual bird belongs the same family as the cuckoos (Cuculidae, the cuckoos, such as Cuculus and Clamator), but unlike its nonrunning relatives it is not a brood parasite.
[Photo: Michael & Patricia Fogden]
80 The turkey vulture or turkey buzzard (Cathartes aura) is not restricted to deserts, but it is widespread in the desert biome. In the Saguaro National Monument (Arizona, United States), it can often be seen silhouetted against the sky, perching on the top of a cactus, as in the photo on top of a saguaro (Carnegiea gigantea). In fact, this New World vulture can easily be seen in a variety of habitats, from the driest deserts through savannahs to dense tropical forests, and it also occurs in many temperate forests in North America. Groups of turkey vultures are sometimes seen searching for food among garbage, and the species seems to be more abundant in ecosystems disturbed by humans than in relatively intact ones.
[Photo: Jim Brandenburg / Minden Pictures]
81 The Australian thorny devil (Moloch horridus) is an agamid ant-eating lizard that occurs in central and western Australia. Despite its aggressive appearance and intimidating spines, it is a timid solitary organism that needs nothing more than a clump of spinifex for shelter and a few ant nests for its food. This lizard is so well adapted to desert conditions that it does not need to drink-and does not even know how to drink. An occasional stroll through the morning dew and it is sufficiently hydrated. The secret of this excellent adaptation lies in the microstructure of the spines on its feet, full of tiny channels that collect the water of the dew by capillary action and rapidly transport it throughout the entire body.
[Photo: Otto Ruge / ANT / NHPA]
82 The splendid fairy wren (Malurus splendens) lives mainly in southern Australia, though it also frequents the thickets of riverbanks in the continent's interior. Its distribution coincides with that of the white-winged fairy wren (M. leucopterus). In addition to these fairy wrens, there are other Australian wrens (family Maluridae), including the emu wrens (Stipiturus) and the grass wrens (Amytornis), which recall the common wren (family Troglodytidae) because they are small, with a long erect tail. They differ, though, in that the males are brightly colored, as shown in these double naturalized specimens that exhibit the plumage of the male (right) and the female (left) of the splendid fairy wren. (This photo is an artistic montage.)
[Photo: Rod Williams / Bruce Coleman Limited]
83 The bilby or rabbit-bandicoot (Macrotis lagotis) is a long-eared bandicoot. An unusual marsupial of the arid regions of NW Australia, it used to be widespread but is now in danger of extinction. This strange animal differs from other bandicoots in its long ears, snout, and limbs, its long hairy tail, and its digging habits. There are no known close relatives of the rabbit-eared bandicoot, apart from another species of the same genus (M. leucurus) that lives in the center of Australia. The relationship between these species has become obscured over time.
[Photo: Kathie Atkinson / Auscape International]
84 Gueltas are pools of water from the water table that form in the middle of the desert and do not evaporate because they are protected by rocks or other features of the relief. Gueltas allow the growth of a varied vegetation and are a gathering point for many desert animals, who could not survive without these water reserves. Their surface area, depth, morphology, salinity, and other characteristics vary greatly with the relief, the chemical composition of the subsoil water, the rate of evaporation, etc. These factors determine the biota of each guelta, which normally include regionally endemic forms. The photo shows the Kata Tjuta gueltas in Mount Olga, in the Uluru-Kata Tjuta National Park (Australia).
[Photo: J.M. La Roque / Auscape International]
85 The immense salt de-posits of Lake Eyre (Aus-tralia) and many other lakes in the deserts of central Australia form when the water input from rivers ceases and the calcium carbonate precipitates, forming a hard crust on the soil and covering the scarce plants. For most of the year, episodic saline lakes are dry, but when they fill, the water that lasts for a few weeks may cover an area of hundreds of square kilometers and reach a depth of 20 ft (6 m).
[Photo: Reg Morrison / Aus-cape International]
86 The crustacean Triops granarius is remarkably well adapted to the temporary nature of pools in deserts. The photo shows a specimen in the ephemeral flood pools of the deserts of Saudi Arabia. When the water evaporates, the animal dies, but before the pools dry out completely, it lays its eggs-the resistance forms that survive the long, hot dry period. The eggs of T. granarius can resist temperatures of 194[degrees]F (90[degrees]C) for several hours and hatch after burial in mud for 15 years. This crustacean also shows ethological adaptations to the difficult conditions in the temporary pools of water in deserts, one of which is to swim against the current to avoid being stranded.
[Photo: Hans Christian Heap / Planet Earth Pictures]
87 When the pool it lives in dries out, the Imirhou catfish (Clarias gariepinus), which lives in the Imirhou wadi in the Sahara (at the base of the Tassili Massif), can crawl overland to a nearby pool. This large carnivorous catfish has adapted to the temporary nature of bodies of water in the desert and the frequent isolation that living in episodic waters sometimes implies. Other Saharan fish of the same family are also able to crawl from one pool to another, showing that in the past the Sahara Desert was wetter than it is now.
[Photo: Nick Greaves / Planet Earth Pictures]
88 Oases occur at the few points in the deserts where there is permanent water. Many oases, especially in Saudi Arabia and northern Africa, are of great importance as stopping and resting points on communication and trade routes. The availability of water has allowed permanent human settlement in these points, obviously depending on factors such as the availability of fertile soil, communications, and safety. In fact, many oases are the only permanently inhabited sites in the desert, and they have been so greatly changed by human beings that it is difficult to know what they were like before. The original flora probably consisted of tamarisks (Tamarix aphylla, T. gallica), oleanders (Nerium oleander), and other shrubs, but they were replaced long ago by date palms (Phoenix dactylifiera), fruit trees, and vegetable plots (see figure 118). Most agricultural production in desert regions comes from oases. But they are not only important for their agriculture; oases are to some extent a reversal of the general tendency of deserts to increase.
[Photo: Hans Christian Heap / Planet Earth Pictures]
89 The waterwheel is a traditional manual system used to extract water for irrigation or for drinking. It is still used frequently in arid regions throughout the world, where the presence of sufficient underground water at a reasonable depth makes it possible for sedentary populations devoted to irrigation farming to settle. The waterwheel consists of a horizontal wheel geared to a vertical wheel, which, in turn, holds a chain of pitchers long enough to reach the water table; the pitchers are fastened in such a way that they can bring up the water and pour it into a tank or channel. An animal moving in a circle around the axis of the wheel transmits the movement by means of a spar to which the animal is attached. The gearing of the two wheels transforms the horizontal rotation into vertical rotation and allows the water to be brought up by means of the pitchers attached to the continuous chain. Water-wheels have been decisive in making use of the underground water in oases, which is generally far more abundant than the surface water, turning these oases into veritable gardens in the midst of apparently more adverse environmental conditions.
[Photo: Bernard Regent / The Hutchinson Library]