2 Life in the high mountain.
1.1 The downward movement of nutrients
Mountains, despite their modest size on a planetary scale, are of great importance for many living organisms as elements in the control of the climate and the structure of the landscape. Many organisms are limited to a two-dimensional perception of the earth's surface, but mountains enrich their surroundings by adding a further vertical dimension; this vertical component is far smaller than the vast horizontal distance from the equator to the poles, but the variation shown over this short height is comparable to the entire variation due to latitude. The lower height limit of true mountain (or high mountain) areas has been located, in accordance with different criteria, at 1,640, 2,953, or 3,937 ft (500, 900, or 1,200 m), or at a (variable) altitude where certain changes in the vegetation and the landscape take place. From this point of view, a mountain is any elevation of the earth's surface characterized by the existence of species and communities adapted to the cold and other conditions prevailing at high altitudes. This would exclude, for example, the Arctic regions, which share only their coldness and some other climatic factors with the mountains, and would include raised plateaux within the concept of mountain.
The constant export of resources and organisms
Mountains, because they rise above the surrounding lowlands, are subject to constant loss of nutrients because of the downward movement of water and soil due to gravity. And not only nutrients are lost: cold air, water, fragments of rock and soil, seeds and organisms, populations and species are all transported to the neighboring lowlands. Even some peoples and cultures have moved from the mountains to the plains. This is because it is easier to move downwards than upwards.
Soil and sediment, the bulk of the inorganic materials exported from the mountains to the lowlands, are mainly transported by water, in both liquid and solid forms, although animals and wind are also important. From an ecological point of view, it should be noted that this transport impoverishes the mountain environment and enriches the lowlands. The mountain soils washed downhill before they attain any degree of maturity are always skeletal and impoverished when compared with the rich deep soils of the valleys. The low fertility of mountains is made even worse by the intense washing by rain, and their nutrients accumulate in the alluvial plains.
The export of organisms is less well known. In fact, from this point of view, the mountain's role is ambivalent. On the one hand, mountains are a cradle of species and populations and play a critical role stimulating the diversification of the organic world, but on the other hand they also provide shelter for species during their decline or house such relicts before they finally become extinct. Notable examples of this role are provided by the European bison (Bison bonasus) in the Caucasus and the mountain gorillas (Gorilla gorilla beringei) in refugia in the mountains of central-eastern Africa (see vol. 2, page 394).
The circulation of matter and energy
Biomass production in mountains is generally low, as it is restricted by the long periods of biological inactivity imposed by climatic conditions. In any case, mountains are very heterogeneous environments, and therefore it is difficult to generalize about their production capacity: a suitable gradient for example, may mean that in midwinter the sun shines on a slope at a high angle and provide as much energy per square meter as in the lowland during the summer, although of course for a smaller number of hours. This raises the case of examples that can be considered paradigms: in the most productive period of the year, an Alpine meadow or one in the Rocky Mountains may attain daily production of 132-220 lb (60-100 kg)/ha (1 hectare=2.47 acres), but the coldest and driest plateaux do not even produce 100 g/ha in a day. Yet most of the biomass produced in a mountain is underground, and the primary consumers only use a small part of the aerial production, and this is why it is considered that it can only support a biomass of 220-330 lb (100-150 kg)/ha (1 hectare=2.47 acres) of herbivores.
The values for biomass in the mountains show such great variation that it is impossible to try to define standard values, but it can be assumed that the quantity of accumulated biomass is greatly restricted by low productivity. In the Scandinavian mountains, for example, biomass values as low as 15 g/[m.sup.2] have been recorded around the glaciers, while in dense pastures, they reach 800 g/[m.sup.2]. Alpine meadows in Austria show much higher values that may reach 1,300 g/[m.sup.2]. The maximum aboveground plant biomass values, about 70 kg/[m.sup.2] have been measured in moist forests on tropical mountains with highly oxidized ferralitic soils. These forests are not representative of the typical montane environment but do exemplify the wide range of variation in biomass values shown by the high mountain plant cover.
Herbivores play a basic role in the circulation of matter and energy within the food web, for example, ungulates, rodents, birds, and arthropods. In absolute terms, the annual consumption of plant material by the total of animals in the most productive Alpine meadows may reach 2-3 t/ha. This value is low in comparison with consumption by the livestock in the valleys, but it is important as it is concentrated in the short summer. Because of the heterogeneity of the montane environment, in some areas the circulation of matter and energy is remarkable, while in others it is insignificant. Thus, most herbivores tend to maximize the consumption of highly localized resources when they are in an explosive phase of population growth, as happens with the irregular demographic fluctuations that are characteristic of relatively unstable environments.
1.2 The top layers and discontinuities
The lower level reached by the perpetual snows is very variable, as the temperature gradient depends not only on the altitude but on many other factors. In shady sites, the snow reaches lower levels than in sunny areas, because the snow melts less. In mountain ranges near the ocean, the level is lower on the windward side than on the leeward side. In the tropics the lower limit of the perpetual snow is at very high elevations that decrease with increasing latitude. Other factors, such as the wind regime, the luminosity, and the snowfall are also clearly affected by the altitudinal heat gradient, although this gradient is not linear, as it varies greatly with the influence of local topography.
The variety of montane environments
The zonation of the vegetation layers allows the presence of a great variety of environments on a single mountain, and a similar arrangement is shown in all mountains. Thus in the Alps, the basal zone, with crops, is replaced by first the montane zone, with mixed forests, then the Alpine zone, lacking trees, and then the snow zone, lacking vegetation. The Himalayas, because they are so high, have the greatest number of vegetation zones (as many as half a dozen) from tropical rainforest to perpetual snows. In the Andes, the nearby sea and the presence of plateaux complicate the situation with the succession of different zones on the western and the eastern slopes. Even in the mountains of Tierra del Fuego there are distinct zones of vegetation, although they are highly compressed, as a result of their high latitude, and the timber line is situated at an altitude of only 1,968 ft (600 m).
There is an area between every two layers, called an ecotone, with some characteristics and organisms from both. This area of transition is both similar to and different from the neighboring spaces, and the organisms living there must deal with difficult conditions, at the limit of their tolerances. Their high floristic and faunistic diversity mean that these border areas, or ecotones, are complex areas of considerable aesthetic value.
The limiting characteristics of the ecotones greatly condition plant life, especially at the timber line, where the cold is so intense that only scattered trees manage to survive. This strip has long been known by its name in German, Kampfzone or "zone of struggle," a term that illustrates the difficulty of establishment there. Trees grow only into shrubs or clumps, as cold and wind destroy the shoots that grow up too far. The tenacity and resistance of life in the high mountains is shown, for example, by the ancient specimens of Pinus longaeva, which have just a few live branches that resist the ice and snow in the Kampfzone of the White Mountains in California and which grow so slowly that they can only be compared with the dwarf willows of the tundra and the Antarctic lichens: with a trunk just about 8 in (20 cm) in diameter, one of these stunted and half dead pines may be more than 800 years old, young in comparison with its thicker neighbors, which may be 4,000 years old. The trunk of a deciduous oak in the forests of central Europe of about the same age may easily reach a circumference of about 26 ft (8 m).
The uppermost zones: the high mountain tundra
There are some similarities between the environmental conditions of the upper levels of the high mountain and the polar regions. Variations in water availability, precipitation, snowfall, and temperature are all smaller in the tundra than in the high mountain; sunshine, atmospheric pressure, and the inclination of the site permanently maintain different values in the two sites; yet the uniformity imposed by low temperatures is so great that most of the morphological and physiological adaptations shown by the organisms of the tundra and high mountain are adaptations to the cold. In the tundra, however, the intense winter cold can only be avoided by long migrations or by very marked morphological and physiological adaptations, while in the mountains the changes need not be so sharp, as there are periods when there is a positive heat balance. Thus, animals opt for ethological adaptations, such as migrations to lower layers in search of milder temperatures, a very common practice, as they obtain great benefits without having to move great distances. Furthermore, the absence of permafrost allows the existence of a habitable subnival space. The oscillations in air temperature reach no more than about 13 in (30 cm) into the snow layer, below which the snow cover acts as an insulating layer. At the same time, the soil heat melts the bottom snow and the resulting empty space remains at a temperature of about 32[degrees]F (0[degrees]C). This means that many high mountain animals opt to dig shelters by excavating down to this depth.
The similarity between the high mountain and the tundra cannot be reduced to adaptations to low temperatures by plants and animals. The Alpine layers of the mountains in the temperate zone share a number of species of plants and animals with the Arctic tundra. In fact, these species used to live within a climatic area that included both mountains and tundra, an area that later fragmented, thus separating populations of the same species. This type of discontinuity is also found in tropical mountains, although there the discontinuous areas correspond to the mountainous massifs and different valleys that are separated by barriers with different climates.
1.3 Refugia and centers of differentiation
Living organisms can colonize mountains by four different routes. The first route is direct emigration from similar environments in other mountain ranges. The second route is for the lowland species to adapt gradually to montane conditions without moving from their site of origin while it gradually rises over the millions of years that the folding of a mountain range takes. A third route is for lowland species to evolve slowly, adapting to increasingly montane conditions. The fourth route is for high mountain species to take advantage of a period of global cooling to spread from one mountain range to another through the valleys. Whatever route is followed, the result will be a reduction of the number of species living on the peaks, as environmental conditions are so different in the valleys and in the mountains that they prevent a species from having a continuous distribution range.
Mountains are areas with many endemic species, in the strict sense of the word, i.e. species with a limited distribution, restricted to just a few peaks or ranges. Often, the ranges are large enough to be considered as biogeographical units at the level of province. One widely known biogeographical division of the earth's land surface, adopted by UNESCO's MAB Program, recognises 22 mountain regions that deserve to be treated as biogeographical provinces, and probably a few more should be included. One of the reasons for this high number of endemic plant and animal species is the isolation of the organisms living there (more for climatic reasons than as the result of any real geographical barrier), which leads to a distribution comparable with that of an island environment.
Mountains contain many endemic plants and animals from a wide range of groups. The percentages of endemic species are relatively well known for the flora, but the situation is very different in the case of the fauna. As in most mountains, and especially tropical mountains, the entire fauna is not known, so it is impossible to calculate the number of endemic species present. Yet it can be accepted that the percentage of endemic animal species is proportional to that of endemic plant species, and that the number of endemic animals will be highest where there are most endemic plants.
The great biological diversity of the mountains is due to a series of factors acting together. In the first place, the high selection pressure exercised by the harsh environmental conditions favors the drastic elimination of badly adapted genotypes and the rapid fixation of adapted ones. In the second place, most of the organisms that colonize the high mountain follow opportunist strategies, with high rates of reproduction and of mortality, and their short generations follow each other in rapid succession, thus accelerating microevolutionary processes. In the third place, the number of individuals in mountain populations is usually low, or they show large oscillations that force them through bottlenecks that give rise to genetic drift with very marked effects and to the rapid differentiation of new forms. In the fourth place, the relative isolation of the populations of each species restricts genetic exchanges with other populations of the same species. Although mountains are not as strictly isolated as true islands, the other factors mentioned above act more intensely than they do in islands and lead to as much speciation as that occurring on islands. The fifth and last factor is not related to the evolution of species but to the role of the mountains as refugia, and also contributes to the high biological diversity of mountians: many animals that have gone extinct in the lowlands still have a few residual populations in the mountains, where the relative inaccessibility diminishes inter- and intraspecific competition.
The rate of speciation is higher in mountains than in lowlands, and so in order to maintain an equilibrium there must be an equally high rate of extinction in the mountains. Such a high rate of extinction has not been demonstrated, with a few exceptions, owing to the difficulties of conserving fossils in mountainous environments. The rates of immigration and emigration also intervene in this balance, but there are no data on them, either. Even so, it is accepted that there is a high turnover of species that is the result of separate rates of speciation, immigration, emigration, and extinction.
There are many endemic species in all mountains, but the latitude, the orientation of a range, the size of a range, and the climatic changes a range may have experienced all greatly affect the number of endemic species in a given area. The highest percentages of endemic species are found on highly isolated tropical mountains that rise in the middle of deserts and where a long time has elapsed since their colonization. To the contrary, in the northernmost mountain ranges there are virtually no climatic differences between the high mountain and the Arctic tundra; as a result, both areas have many species in common.
The orientation of the ranges also influences the number of endemic species: ranges with a north-south orientation, such as the Rocky Mountains or the Urals, have few endemic species, as their vegetation zones form corridors that connect with the northern lowland biome and make isolation impossible. The situation is very different, however, in the Caucasus Mountains, which run east-west and are separated from the Arctic tundra by wide steppes and deserts: more than 50% of species in the Caucasus are endemic. Even clearer examples are provided by the African tropical mountains, where the proportion of endemic plant species reaches 80%. Another factor influencing the level of endemism among the mountain species is the area available: larger areas have more species and thus more possible endemic species. Finally, the climatic changes that took place in the past, especially during the Pleistocene, when neighboring high mountain areas were alternately separated and then joined, favoring the processes of species differentiation and thus of a large number of endemic species. One type of mountain area that is especially rich in endemic species is the tepuy scattered among the forests of Venezuela, with groups as ancient as the amphibians and myriapods (millipedes) (see vol. 2, pages 32 and 359), or the upper layers of the central African mountains, with their distinctive populations of giant members of the Asteraceae (Senecio) and Campanulaceae (Lobelia), with leaves arranged in tuft-rosettes (similar to the frailejones [Espeletia spp.] of the Andes), and which have probably evolved from related species in the lower zones of neighboring forests.
The consequences of the glaciations
The situation in the mountains during the last glacial period varied greatly from range to range, and even from one face of a range to another. At high latitudes the mountains were completely covered by an ice cap, but at midlatitudes the highest mountains had smaller ice caps that only occasionally reached a large size. Thus in the Pyrenees, the residual glaciers now found at an altitude of about 9,842 ft (3,000 m) were then much larger, with tongues reaching elevations of 2,953 ft (900 m) on the southern side and of 656 ft (200 m) on the northern side. In the northern Andes, the perpetual snow line descended to between 3,937 and 4,921 ft (1,200 and 1,500 m), and other tropical mountains probably experienced comparable descents. The area of the periglacial environments was as large as that of ice: Tundra ringed the ice cap and also spread around the frozen peaks of mountain ranges. In Europe and North America, the unfrozen areas were occupied by periglacial environments similar to present-day tundra. In the Venezuelan Andes, it has been estimated that the area occupied by the periglacial areas was double the area they now occupy, a similar situation to that found in the African mountains.
The glaciations forced many species to retreat before the advancing ice and to seek shelter in warmer sites, or in other equally cold sites but without snow or ice. The expansion of thermophilic species was limited, and survival favored cold-adapted species that also withdrew before the advancing ice but remained in the proximities of the glaciers. They sheltered in periglacial areas without much snow. It is thought that the reason for the high species diversity of the Himalayan-Tibetan region and of the mountain ranges of central Asia is that the zone did not suffer extreme glaciations and was a refuge for a fauna that was preadapted to the cold, which later repopulated the modern tundra and high mountain biomes in the northern hemisphere.
The alternation of glacial and interglacial periods, with the consequent retreat of heat-loving and then cold-loving species to small refugia, was an additional factor accelerating the evolution of the populations affected. During the interglacial periods, the mountain species were confined to the high areas of the ranges in a situation similar to present day or perhaps even more restricted, with smaller, highly isolated populations. This situation was favorable for genetic drift and the differentiation of new taxa, but the number of species affected was not very large. To the contrary, there were many more warm refugia, and they were much more densely populated, and thus the speciation that took place in warm refugia during the glaciations was very intense.
The current distribution of different mountain species reveals details of their history and the effects of the glacial periods. The ptarmigan (Lagopus mutus), for example, has a distribution of the type called Arctic-Alpine, with a large range throughout the Arctic tundra as well as in some reduced, isolated areas in the northern hemisphere's mountain ranges. This type of distribution is the result of the fragmentation, due to global warming, of an area that was continuous before the last glaciation. Other less cold-resistant species lived through the glaciations in the southern tundra and the forests on its edge. Their areas of distribution also fragmented after the retreat of the ice, resulting in distributions known as boreal-Alpine that are later in origin than the Arctic-Alpine distributions. Montane distribution ranges were never connected with the Arctic tundra, and the only changes they underwent in the Pleistocene was the expansion and the contraction of their areas expanded then contracted. Some species with a montane distribution are of recent origin, and these species occupied the mountain during the Holocene and have differentiated very little from the other congeneric species in the lowlands. The species with a paleomontane distribution had differentiated before the beginning of the last glaciation and they remained in their mountains of origin or spread to other ranges. It is worth mentioning the paleo-xeromontane species, which differentiated in the most important center of speciation in the Paleo-Arctic region: the mountains of central Asia. They arose during the Tertiary and were able to survive in this region because there were many ice-free sites during the Pleistocene. In the warm intervals, these species spread to far-distant ranges.
2. The flora and plant life
2.1 Resisting the wind and under the snow
The first image suggested by the high mountain is rocky peaks, patches of scree, large herbaceous meadows, and dominating everything, incessant winds and shiny snow. The reality is much more complex, but wind and snow are major environmental factors on high peaks and two of the main factors conditioning plant life. More precisely, wind and snow are decisive both when there is too much and when there is too little, and act both directly and indirectly through other factors. The world's large mountains are diverse enough and the habitats in each massif are varied enough for these factors to express themselves in an almost infinite variety of ways.
The effects of wind and cold
Wind, often helped by mineral particles or ice crystals, acts directly as a mechanical factor that transports, damages, stunts, and in effect shapes individual plants and plant formations. The wind also has indirect effects, such as drying out the environment or preventing areas exposed to sunshine from warming up. Owing to the impossibility of avoiding the hostile conditions caused by the wind, plants possess clear morphological adaptations to them. Thus, the trees are reminiscent in form to a flag, with the branches blown in one direction by the dominant wind or bent by its force. Likewise, most of the herbaceous plants in alpine meadows hardly rise above the ground level, where the wind is milder, evaporation is less intense, and the plants can make use of the warmth of the soil. Many Alpine clump-forming plants, such as the Alpine azalea (Loiseleuria procumbens), spread their branches over the soil, forming dense carpets that bear the small flowering shoots and fruit. Some plants dare to rise a little more and then they grow forming pulvinules (rounded cushions); this aerodynamic form minimizes the impact of the wind, and the dense branches retain some heat and a moister environment. The dense-cushion growth form also retains the soil, adding mineral particles and dead twigs.
Cushion growth forms dominate the high mountains because of their adaptive possibilities. On Kilimanjaro, above an elevation of 13,123 ft (4,000 m), temperatures have been recorded within the cushions of 63-84[degrees]F (17-29[degrees]C) when the ambient temperature was only 45[degrees]F (7.2[degrees]C). In the Pamirs, Sibbaldia decumbens (Rosaceae) forms large cushions of up to 600 different individuals that, as an additional adaptation to an extremely dry climate, can store up to five or six times their own weight of water. Hummocks of moss campion (Silene acaulis) covered with pink flowers are as typical of the Alpine peaks as the hard cushions of the yareta (Azorella) are typical of the Andean puna. Several tropical mountains have examples of a very different type of adaptation: these large plants have a single columnar stem with a rosette of leaves at the end (a tuft-rosette), also known as the frailejone type, after Espeletia and other related genera of the Andean high paramo. Other rosette-trees include the giant lobelias (Lobelia and other related genera of the Campanulaceae) of the high mountains of central and eastern Africa.
The danger of desiccation
Dry climates may seem to be restricted to certain mountains, and some massifs are especially dry because rainfall is generally scarce or because water runs short during the summer when the plants need it most. Even mountain ranges with relatively rainy summers experience many periods, however short, when the water deficit exceeds the limits many plants can withstand. This occurs, for example, on summer days, when the sun is scorchingly hot and the air is transparent and dry, and when the soil is frozen and cannot supply the plant with the water needed for minimum transpiration. It is thus not surprising that the plants of the high mountain generally show characters or features to limit desiccation--such as succulent organs to accumulate water, leathery leaves, leaves with sticky surfaces and leaves covered in dead hairs that diminish transpiration and avoid overheating by the sun. This is shown by the dense hairs covering the edelweiss (Leontopodium) that are so common in the Himalayas.
The low temperatures, so typical of high altitudes, explain other characteristics, such as the great size of underground parts relative to meager aboveground parts. This is due partly to soil temperature conditions, because soil is much less affected than open air by low temperatures, but the effect of the light is also important, as it slows down the growth of aerial parts. It has been calculated that at an altitude of 9,186 ft (2,800 m) in Tibet, the underground plant mass represents 63% of the total plant mass, while at 14,763 ft (4,500 m) it accounts for 97%.
It is almost a cliche to say that despite their small stems and leaves, many high altitude plants have disproportionately large and brightly colored flowers (dominated by blues). This effect used to be explained as an influence of the climate on plant metabolism; but the relationship is not so direct, as shown by the fact in the mountain peaks in New Zealand, where there are no insects with long mouthparts, the plants have small flowers with flat corollas.
The protective effect of the snow
However scarce precipitation may be at the high peaks, the snow gradually accumulates because of the low temperatures. Outside the equatorial zones, high mountain plants in all other climates are under a thick cover of snow at some time of the year, directly affecting the plants through its weight (and its movement) and its low heat-conducting capacity. Low-growing herbaceous plants and prostrate or creeping clumps and shrubs are effectively adapted to help the plant to bear the weight of the snow without serious damage, thus avoiding the devastating effects of avalanches.
The snow is not always prejudicial as, for example, it protects against the harshest cold: under half a meter of snow, the temperature is around freezing point, even though the external temperature may be many degrees below freezing. Where abundant snow accumulates during the winter, the plants need not fear harsh frosts; but the situation is unfavorable for the plants that dare to live on ridges and slopes where snow slides downwards or is swept downwards by the wind. Apart from this, snowfall has such a large effect on plant life cycles and on the distribution of the plant communities that correct interpretation of a plant landscape is impossible without considering the snow's distribution on the ground and how long it lasts.
The annual cycle
Most climatic regimes show relatively clear seasonality. For the vegetation, this means there is an unfavorable period, generally very cold and often with snow, and a favorable period when the temperature is high enough for plants to grow (the growing season). Equatorial climates show little variation in temperature over the course of the year, and the existence of a dry period may indirectly favor the vegetation. Thus, in the paramos of the northern Andes, the low-growing plants flower and fruit during the driest season, despite the cold weather, as the clear sky allows bright sunshine during the day, with the consequent heating of the soil surface and of the plants, which may thus reach temperatures higher than that of the surrounding air (up to 104[degrees]F [40[degrees]C] in the soil, and from 50-60[degrees]F [10-20[degrees]C] or even 86[degrees]F [30[degrees]C] in the plants). Yet mountains normally alternate between a harsh winter and a mild summer.
Mountain plants are adapted to low temperatures and freezing conditions, and they tolerate them, even if they only barely survive. The Alpine azalea (Loiseleuria procumbens) lives in cold windy areas in the Alpine layer by resisting temperatures as low as -33[degrees]F (-36[degrees]C), and the Swiss stone pine (Pinus cembra) of the Alps and Carpathians can withstand being completely buried in the snow as well as temperatures of -104[degrees]F (- 40[degrees]C). At these temperatures, plants are completely inactive. For example, the lowest temperature at which the alpine buttercup Ranunculus glacialis can photosynthesize is 19[degrees]F (-7[degrees]C). The winter is the season when plants rest and resist, while they wait for good weather to return so they renew their activity.
As there are very different habitats in the high mountain, the growing period and growing conditions vary greatly over very short distances. If there is abundant snowfall, as happens in Alpine mountains, the length of the growing period depends greatly on the snowfall. In sites where snow accumulates, active plant growth cannot start until the snow has completely melted, although air temperatures may be suitable long before then. Thus, the distribution and duration of the snow cover are decisive factors in the organization of the landscape in most mountain ranges. In areas where rainfall is low or where a serious summer drought occurs, plant life is endangered and then the melting of the snow layer is of great importance, as it supplies water and creates moist places. When summer water shortages do not pose a serious problem, the annual cycle of the vegetation is basically regulated by the temperature and the snow.
Some plants require relatively long growing periods, but others have adapted to very short growing periods; a good example is provided by the plants that grow where snowdrifts accumulate, where the growing period begins in the middle of the summer (when the snow has melted) and finishes with the first autumn snows. The shortness of the growing period in the mountain is a general problem, and this is why many plants develop leaf and flower buds in the autumn and renew their activity as soon as good weather returns. Most can make use of the little light and limited heat that passes through the snow layer, and so some species are active under about 5 ft (1.5 m) of snow and start to open their flowers before the snow has even completely melted. The most impatient species, such as the soldanellas (Soldanella), flower in waves that follow the retreating snow. There are a few species that flower later, like bulbs or rhizomatomous plants that can grow very rapidly because of their accumulated underground reserves.
In general, high mountain plants are prepared to renew active life as soon as the snow melts, which is why meadows become green so quickly and fill with flowers. Fruiting, however, takes longer. The fruit and the seeds develop while the leaves are yellowing. Some plants lengthen their fruit-bearing stalks, which thus rise above the snow layer and can make use of the heat the snow may reflect so they can finish maturing.
2.2 The altitudinal zones
The supraforestal zones include all the areas of montane vegetation sited above the upper limit of the forests. Logically, their elevation varies greatly from one mountain range to another, depending on the general climate of the area. In general, the highest altitudinal zones of vegetation are supraforestal, although this is not true in the drier ranges, such as the Chilean face of the Andes or the Hindu Kush, where there are no forests even at the middle latitudes. In any case, the timber line marks a very important biological frontier and for this reason has been the subject of many studies and much speculation seeking to relate its vegetation type and the greatest elevation at which scattered trees can grow. It should be born in mind that there are very arid massifs where there are no forests, and sometimes not even a single tree. There are mountains ranges and plateaux, such as the Pamirs and Tibet, where the rainfall is very low (4 in [100 mm] per year or less), and their vegetation is thus clearly desertic. Thus, the chenopod Krascheninnikovia ceratoides grows in the Pamirs and reappears in several arid areas in Europe, for example, in the Monegros region of the Ebro Basin, the westernmost point in its distribution.
Convergences and divergences
The high mountains differ greatly in their flora and vegetation and types of landscape, but some features are common to almost all the supraforestal areas in the world. The first feature is, as its name indicates, the absence of forests, although this does not mean there are no scattered trees or even small patches of forests in sites with favorable microclimates. At the zone's lower limit, there are often shrub formations, and in some places they are very common. Farther up, herbaceous or cushion communities dominate, and, depending on the massif's relief, they may occupy large areas or they may be almost completely restricted by rocky areas and screes. Towards the peaks, environmental conditions become more and more unfavorable, and vegetation becomes scarcer until there are only a few widely scattered plants in the shelter of the most protected sites. The plants that grow at the highest altitudes are the lichens, which reach great heights on the sunny and snow-free ridges. On Mont Blanc, there are two species of lichen growing as high as 15,420 ft (4,700 m), and on Mount Kenya at an elevation of 16,404 ft (5,000 m), there are several lichens and cushions of Grimmia, a highly resistant genus of mosses. Flowering plants do not reach such high elevations, although there are some exceptional cases, such as the presence of the alpine buttercup (Ranunculus glacialis) at the peak of the Finsteraarhorn (in the Bernese Alps) at an altitude of 14,026 ft (4,275 m), or Androsace microphylla (Primulaceae) in the Himalayas at an altitude of 20,833 ft (6,350 m).
The vegetation of the mountains does not correspond to any preset model, as each range has a set of individual features. One of the main causes of divergence is geographical location: within the mountains as a whole the first main distinction is between the Holarctic and the tropical ranges. Tropical mountains show great diversity in their floras and types of vegetation. In contrast, the Holarctic ranges located in the temperate or cold zones of the northern hemisphere show many characteristics in common. There are many montane genera, one of the most typical being Gentiana, that are present from the mountain ranges of North America to the Alps and the Himalayas. It may be noted too that there are many Arctic plants living in the Holarctic mountains, especially in the Eurasian ranges, where they account for 25-40% of the high altitude flora. Their landscapes also show considerable similarities.
The highest layers in the Holarctic mountains
Holarctic mountains with a moist climate, exemplified by the Alps, have supraforestal zones that consist of an Alpine layer, where the vegetation present may be dense; continuous meadows; and a subnival layer with a fragmented or dispersed plant cover. But many different environments occur in the high mountain, making the landscape a complex mosaic of communities. Thus, in the Alpine layer there are many different types of meadows, but there are also quaking bogs, and special communities of rocks and screes, sheltered slopes, snowdrift sites, and so on.
One special case is the large ranges of the Mediter-ranean Basin, such as the Atlas Mountains or the Sierra Nevada in Andalusia. The vegetation in these mountains is typically dry, spiny scrub formed by large thorn cushion plants such as the hedgehog broom (Erinacea anthyllis, Leguminosae) and the crucifer Vella spinosa, which in addition to protecting themselves from the wind and cold defend themselves from herbivores with thorns. The situation is different in the Himalayas, where the top layers of forest are replaced by a low forest of members of the Ericaceae, the celebrated giant rhododendrons, while the highest parts of the range have a herbaceous vegetation similar to that of the Alpine layer of the Holarctic mountains.
The highest levels in the intertropical mountains
Mountain ranges at midlatitudes show an alternation of seasons that always leaves one favorable period for plant growth, but in the equatorial high mountain the conditions of life are very unfavorable because there is no alternation of seasons and temperatures are low throughout the year (with only small daily variations). Thus, above the elevation where the average annual temperature is only 34[degrees]F (1[degrees]C) plant life almost completely disappears, and this is the beginning of a cold desert that occupies a strip of 328-1,312 ft (100-400 m) below the snow line. In this zone there are only lichens and some mosses (unlike in the Alps, where above the lower limit of the snow there are still 800 species of moss and more than 100 species of flowering plants).
It is difficult to make any further generalizations about the equatorial mountains because of the great diversity of their flora and vegetation. The zonation by altitude in the mountains of eastern Africa and of Madagascar for example, follows a very regular pattern, despite the considerable distance separating them: Above the dense forest there is usually a strip of giant bamboos with scattered trees; at a higher elevation, coinciding with a layer of permanent cloud, there is a tall scrub of heathers and other members of the Ericaceae, that is dense and hard to cross. This zone has many ferns, abundant lichens hanging from the branches, and ground covered by a dense layer of mosses. Above this level, the vegetation is increasingly low and scattered, and the only plants producing tall stems are the columnar giant senecios (Senecio) and the arborescent lobelias (Lobelia).
In the equatorial Americas, however, the Andes runs parallel to the meridians and has a great variety of climates and environments. In the rainier sectors, above the montane forests (Tierra fria, "the cold land") there is a cold wet paramo (Tierra helada, "the frozen land"), with clumps of frailejones (Espeletia), meadows and quaking bogs that are replaced at greater altitude by a very scattered vegetation. In the dry sectors, the paramo vegetation is replaced by the much drier puna. In the Peruvian-Chilean Andes, the puna consists of a lower layer of shrub vegetation, tolar, dominated by tola (Lepidophyllum quadrangulare) and several ericoid species of Parastrephia (Asteraceae), and an upper layer, the pajonal, occupied by open herbaceous communities with ichu (Stipa ichu, and other grasses, mainly of the genera Stipa and Calamagrostis) and the fescue paja brava (Festuca orthophylla) with patches of bofedales (see photo 146, page 241). Two of the most typical features of the Andean vegetation are on the one hand, the frailejones (Espeletia), that are scattered here and there in the desolate landscape of the dry paramos and puna, and on the other the thickets of the legume churqui (Prosopis ferox) and the rosaceous quenoa (Polylepis). Polylepis trees are found high above the normal timber line (up to 15,748 ft [4,800 m] in the case of P. tomentella), although they form thickets on the east- and west-facing slopes between large blocks, where warm air accumulates.
The forest layers
In the wet tropical and equatorial mountains, the forest vegetation terminates in a cloud forest (see volume 2, page 338). In temperate or cold climates, the higher levels are occupied by conifer forests, very cold-resistant trees that make the most of the short growing period in the high mountain. All the Holarctic mountains have roughly the same genera of needle-leaved conifers, in particular firs (Abies), pines (Pinus), and spruces (Picea), sometimes mixed with some broadleaf trees, such as birches (Betula). It is, however, very unusual for the upper limit of the forest to consist of broadleaf trees, as happens for example in New Zealand, in Scandinavia, and in the Range Mountains, California. In the southern Hemisphere at equivalent latitudes, the high mountain forests also mainly consist of gymnosperms, although they belong to different genera from the Holarctic conifers (such as Araucaria, Podocarpus, Agathis). The precise altitude of the timber line on a mountain depends on the geographical location and climate of each massif. The upper limit of the forest is generally higher in mountain ranges closer to the equator than in those in polar latitudes, in those with continental climates rather than maritime ones, and in large mountain masses rather than in isolated mountains. Thus, in the mountains in Alaska, the timber line does not go above 984 or 1,312 ft (300 or 400 m), in New Zealand it is at 3,281 ft (1,000 m), in the Appalachians, at 4,921 ft (1,500 m), in the Alps and Pyrenees, above 6,562 or 7,874 ft (2,000 or 2,400 m), and in the Himalayas, the timber line is at 11,483 ft (3,500 m). The forests and the highest trees reach their highest elevations between the 20th and the 30th parallels, precisely the latitude of the Himalayas. At lower latitudes, the timber line clearly descends due to the special conditions of the equatorial high mountains. Thus, on Mount Kilimanjaro, the timber line is at 9,842 ft (3,000 m), and in the Bolivian Andes at 10,499 ft (3,200 m).
Often, at the timber line, the forest is stunted and more open and then gradually dies out. The timber line, on the other hand, is not regular but curves up or down depending on the climate and the topography of each site. The timber line usually rises on sunny sides and falls on shady ones, avoiding inhospitable peaks and rocky areas. In general, the forests in the Holarctic mountains grow on the hilly areas that are free of snow earlier and not in the depressions, where the longer snow cover shortens the growing period. To the contrary, in the mountains in the southern hemisphere, where snowfall is less, the forests grow in the valleys and in the depressions, which are protected from the cold and have deeper soils. Many different suggestions have been made to explain why the timber line occurs at a particular elevation but there are not enough data, nor is any one cause applicable in all cases. Thus, the cold, the snow and the short growing period often occur together and occur almost everywhere. Wind, persistent mist, soil migration, and even some biotic factors (such as the presence of wild herds or natural pests) may sometimes be of great importance, but they are only local factors and cannot be applied generally to all mountains. The current timber line is often more due to human influence than to anything else, and is now lower than it used to be.
3. The fauna and animal life
3.1 Life in the mountains
Like the character in the Hemingway novel who found the dry frozen corpse of a leopard in the snow near the peak of Kilimanjaro and wondered what had made the animal go so far up the mountain, it is not always easy to explain what the animals that live on the mountain are looking for (or have found). For most animals mountains are barriers to their movements, cold spaces with very diverse environments where food resources are generally scarce, although they may occasionally be abundant. Mountains limit the expansion of some animal species and populations while favoring the expansion of others, yet due to their relative inaccessibility they have acted as refugia where some taxa have survived that are now extinct in the lowlands.
Adaptations to the cold, and temperature oscillations
The animals that live in the mountains find safety and food, but they pay a price, as they are subject to harsher environmental conditions, especially the cold. Mention has been made of the similarities (and differences) between mountain and tundra environments, especially in their temperature regimes. In the mountains, unlike the tundra, the daily temperature oscillation is large and permits cycles of melting and freezing. This means that the animals can still be active during periods when the average daily temperature is below 32[degrees]F (0[degrees]C), as long as the sunshine ensures higher temperatures for some hours during daytime. Thanks to the proximity of lowland areas with higher average temperatures, the high mountain may contain a much more varied and complex fauna than that of the tundra.
Mountain animals show adaptations to the cold that are not restricted to acquiring the ability to withstand the low winter temperatures. In the high mountain, even in summer, the nights (and even some days) are cold, so that other less pronounced adaptations are necessary, generally based on thermoregulatory habits to deal with untimely cold spells.
An initial adaptation to the cold is basking in the sunshine to raise body temperature. The need to make the best use of this radiation during the day is the explanation for the differences in pigmentation between homoeothermic (warm-blooded) animals and poikilothermic (cold-blooded) ones in the mountains. Homoeothermic animals acquire winter coats and plumage, generally white, that they change with the seasons; poikilothermic animals spend the winter in diapause and opt for dark pigments so they can absorb the sunshine better. Melanism is one of the most frequent adaptations in this environment, as dark pigments not only help the animal absorb heat-bearing infrared radiation better but also act as a barrier to harmful ultraviolet radiation. Thus, mountain butterflies have to be very active during the brief reproductive season, and therefore, would be at a great disadvantage if they could not reach high body temperatures, and so their coloration is darker at higher elevations. The black salamander (Salamandra atra) of the Alps is another notable example of mountain melanism, although it has other adaptations to shorten its active period.
Shortening the active period is one of the most widespread adaptive strategies among mountain animals, as remaining active during the cold season would require an excessive expenditure of energy. Making the best use of the short hot season may be achieved not only by concentrating all the phases of the life cycle in a short period but also by dividing the life cycle into different phases over several years. This adaptation is common in both amphibians and invertebrates. Thus, for example, many mountain amphibians grow slowly and do not complete metamorphosis in their first year of life; they remain in a larval stage for at least two years and reach a greater size than lowland larvae of the same species. In other cases, fragmentation of the life cycle affects the adults, that only reproduce in alternate years. Among the insects, many beetles spend several years as larvae, slowing approaching the moment for metamorphosis. The opposite strategy, shortening the life cycle, occurs in many dipterans whose lifespan is so short that several generations may live and die in the short mountain summer.
Another adaptation frequent among mountain species is the presence of vivipary in groups that are usually oviparous. A high percentage of amphibians and reptile that live in mountains become viviparous to shorten their annual cycle and adapt it to the shortness of the growing season. In some species, this adaptation is gradual and depends on the harshness of the environmental conditions. Thus, the common, or viviparous, lizard (Lacerta vivipara)--a stenothermic species found in northern Eurasia--is oviparous at the southern edge of its distribution in the Pyrenees and other mountains of the Iberian Peninsula, where the hot season is longer. In some cases, viviparous animals may also suppress part of the life cycle, as does the black salamander (Salamandra atra) in the Alps. Not only do the females of this species bear live young, thus suppressing the egg stage, but their young have already undergone metamorphosis, and are born as small adults. Modifications of reproductive behavior are also common and may even go as far as parthenogenesis (reproduction without the need for fertilization), as happens in some reptiles in the Caucasus, including Lacerta saxicola, and others in the Andes. The reproductive process is thus shorter and the production of offspring are guaranteed, but genetic flexibility is diminished and the species may even become extinct if there are major environmental changes.
Some species modify and adapt their range of preferred temperatures in function of the range of environmental temperatures. Many high mountain insects can withstand very low temperatures (some can even resist temperatures close to absolute zero) and may even need cold temperatures to survive. Some butterflies that live in quaking bogs cannot survive if the average temperature of the month of January is above freezing point. For other insects, the limiting factor is the average temperature of the warmest month, which must not exceed 52 or 54[degrees]F (11 or 12[degrees]C). In the Alpine zone of the European mountains there are grasshoppers that cannot live where average temperatures exceed a threshold of 46[degrees]F (8[degrees]C) during the summer; the notopteran insects of the Rocky Mountains in Canada live on the surface of the snow and in captivity must be stored in a refrigerator. The small lizards found in mountains are much more resistant to low temperatures than are other species of the same genus in the lowlands and lose the ability to move only at temperatures below freezing point.
Adaptive changes as a response to the cold are sometimes more subtle, including simple, but not very obvious, physiological or ethological modifications. Controlling the position of hairs or feathers to increase the layer of air insulating the skin, regulating heat loss by evaporation, improving the insulation of the limbs, and modifying basal metabolism are typical examples of this type of physiological adaptation but are much less conspicuous than melanism or modifications of the life cycle. Ethological changes, which are always more immediate and simple in any process of adaptation, include the search for and construction of underground shelters, migration to lower altitudes, or the use during the winter of food reserves accumulated within the body as fat or hidden as food stores.
Some adaptations are not directly related to the cold but to some of its consequences, such as snowfall. To cope with the snow, several species of mountain animals have a protective covering of hair or feathers around the legs that are absent in lowland species of the same genus. Similar adaptations can also be seen in the winter coats and plumage of some animals such as the stoat (Mustela erminea) and the ptarmigan (Lagopus mutus). Their winter coverings are thick and provide excellent insulation, as well as being white as snow, making them less easily seen by their prey or predators. The snow may also give rise to adaptations in an animal's conduct that help the animal to defend themselves against the cold; the Tibetan gazelle or goa (Procapra picticaudata), for example, protects itself from blizzards by digging a cavity in the snow and sheltering behind the pile of snow that forms.
Many mountain animals also show characteristics that appear to be adaptations to conditions with large daily temperature oscillations. In some animals, the extreme fluctuations in temperatures force them to have a fixed behavior, starting activity before the environment becomes too hot and retiring to their shelter at the end of the afternoon when the temperature goes down. On sunny days, many animals attain high internal temperatures by basking on stones, as the stones get hot quickly in the sunshine. In tropical mountains temperature differences of 185-194[degrees]F (85-90[degrees]C) between midday and midnight have been measured on the surfaces of dark stones. Other species, however, remain in environments that vary little in temperature, for example under buried stones in the middle of herbaceous clumps, or near permanent snowdrifts, where the snow keeps the surrounding air cool. In fact, all high mountains have a characteristic nival microfauna that is not affected by wide variations in air temperature.
Adaptations to wind
Adaptations to wind are less well known but, generally for topographical reasons, winds blow regularly and intensely in the mountains. Animals shelter from the winds during the winter by hiding behind rocks or rock walls, which act as screens, while in the summer they use them to cool down and to avoid being attacked by troublesome insects. The capacity of mountain animals to resist wind without moving is much greater than that of lowland animals. This resistance is due to the covering of hair or feathers that, because of the position chosen by the animal with respect to the wind, covers the parts most exposed to the blizzard. While most animals advance headfirst into the wind, others such as the yak (Bos grunniens) turn their backs to it to protect their hairless muzzles with their thick, woolly rumps. The chamois (Rupicapra rupicapra) can remain motionless for hours in a blizzard at a temperature below -14[degrees]F (-10[degrees]C).
Yet the clearest examples of adaptations to the wind are those that affect flying animals, especially insects. It is hard to fly in the middle of a strong wind, and thus many montane insects have lost their wings. In the Himalayas, half the insects that live above the timber line have lost the ability to fly, and among those that live near the snow line, the proportion of nonflying species is 60%. In many genera of grasshoppers with species occurring at different altitudes, it has been shown that species living at higher elevations have smaller wings than lowland species and may even have lost them. Even flying montane insects usually make only short flights low over the soil to avoid being blown away by the wind. In sharp contrast to insects, large birds of prey make use of the wind in their movements, gliding effortlessly and even riding the constant currents to perform long flights.
Adaptations to scarcity of oxygen
Probably the best known physiological adaptations to the high mountain are those related to the low atmospheric pressure and the consequent low partial pressure of oxygen. In animals, the modifications that resolve the problems related to hypoxia are linked to the genes and thus are true adaptations.
Mountain mammals show both physiological and anatomical adaptations: They have larger lungs and hearts and a greater quantity of blood; their red blood cells are smaller and more numerous; hemoglobin availability is much greater and its affinity for oxygen is higher than it is in lowland animals. These adaptations are clearly shown where several species of a single genus live at different altitudes. For example, in the Caucasus, there are five different species of vole (Microtus) with different preferences for the altitudinal zone they live in; the size of the erythrocytes varies inversely with the altitude of the zone that they tend to inhabit.
Mobility on ledges and screes
Cliffs, gravels, and screes are possibly the least welcoming areas in the mountains. Only extraordinarily agile species or those able to fly can move over them with ease, and this gives them great advantages. The cornices and ledges of cliffs may have more nutritious grass, and because access is also difficult for predators, they are ideal places for many herbivores to feed. Even very small surfaces may serve as a base for large birds of prey to build their nest. It is not surprising that the golden eagle (Aquila chrysaetos), the griffon vulture (Gyps fulvus) and the lammergeier (Gypaetus barbatus) normally nest on small ledges protruding from rocky walls.
Gravels and screes are very suitable shelters for some groups of animals, as the gaps between the stones form a protective network of tunnels. Many of them have lost the tunneling ability of other lowland species of the same genera. For example, the snow vole (Microtus nivalis) digs tunnels when living in areas with well-developed soils but ceases to do so when living in screes. The Alpine marmot (Marmota marmota) excavates warrens in the Alps, but in the Pyrenees (where it was reintroduced recently), it usually lives in areas where there are accumulations of rocks. Pikas (Ochotona) are also typical animals of screes and nest between the stones of the rock debris in the mountains of Asia and North America.
In the interstitial spaces of the gravels, in addition to the animals that take shelter there, there are communities of skiophilous animals, which need relatively constant conditions of temperature and humidity and would not withstand high external levels of light or cold. These communities consist mainly of arachnids and wingless insects, ecologically comparable with the members of the fauna of caves. The stones create an environment where conditions vary little, essentially by buffering temperature and humidity. At a given time, under the stones the daytime temperature does not exceed 57[degrees]F (14[degrees]C), while the external air temperature is around 50[degrees]F (10[degrees]C) and the outer surface of the rock may be as hot as 89[degrees]F (32[degrees]C). Similarly, the nighttime temperature under the rocks may be 39[degrees]F (4[degrees]C), while the rock surface and external air may be below 32[degrees]F (0[degrees]C). The relative humidity under the stones remains constant and high, between 65 and 90%, although outside it may range from 20-80%.
3.2 Animal population dynamics and interactions
The rhythm of animal life in the mountain is marked by different "clocks," but because temperature is the dominant environmental factor, biological rhythms are synchronized to heat rhythms, so that animal communities adjust their changes to day-night or annual cycles. Superimposed on these rhythms are those derived from reproductive activity, when the generations are too short to correspond to the annual cycle or to others with a longer period, such as those associated with succession and with changes in animal communities or with the distribution of the zones of vegetation.
Daily cycles are the most obvious, and in tropical mountains are almost the only noticeable cycles. Many mountain species of insects and invertebrates adjust their timetable of activity to make use of the time when temperatures are best, hiding under stones or in cracks when the environment is too cold.
In the mountain, underground micromammals and reptiles leaves their shelters at dawn, when the sun starts to warm things up and they can recover from the low night temperatures. They have attained an adequate temperature within a few hours, when the predators awake, and they have returned to the safety of their burrows. At midday, when temperatures are highest, they restrict their activities to the minimum in order to avoid unnecessary loss of body water. Later, when the heat has passed, they may show a secondary period of activity. Their shelters may be cracks in the solid rock, sites under stones or between blocks on screes and protected areas of vegetation. The Mount Kenya swamp rat (Otomys orestes), for example, shelters at night under the dry leaves of the tree Senecio and sometimes in cavities they dig in the plant's stem.
Many caulirosula, or rosette trees, (such as the South American frailejones and the African tree species of Senecio and Lobelia, with a tuft-rosette of leaves at the end of a tree-like stem), show a daily rhythm, opening their green leaves during the day and closing them at night. This has led many species of insect to use this nighttime shelter by adapting their activity to the plant's, on which, in a notable example of coevolution, they also feed. Of course, the animals that depend most on external temperatures, such as reptiles and insects, show the clearest rhythms in their activity. Reptiles, owing to their ability to generate large amounts of heat, often complemented by dark pigmentation, maintain body temperatures higher than that of the air temperature, but that vary in parallel with the air temperature. Thus, changes in internal temperature in the lizards of the genus Liolaemus in the Andes follow a daily rhythm with a peak and a lowpoint at the same time as those of the ambient temperature but always 46-68[degrees]F (8-20[degrees]C) warmer.
It is not surprising that animals should make use of the indirect effects of the circadian cycle, such as changes in temperatures and light, morning dew, and evening storms on summer days, in order to evade excessive heat or cold, troublesome insects, and enemies and even to obtain food. The circadian changes in the mountain's wind regime are also used to organize daily activity, especially longer movements. At midday in summer, the mountain ungulates avoid the excessive heat by moving to shady places, where they can be seen resting at the same time every day. Sometimes they expose themselves to the wind on the peaks for the same purpose or to free themselves of insects. The large birds of prey use the daily rising thermals to reach great heights.
The annual cycles
Cycles with a period greater than 24 hours but less than one year are the consequence of oscillating demography. They occur, for example, among mountain rodents, although in the montane environments they are less common than in the lowlands, because the extreme environmental conditions impose moderate rates of reproduction and growth and also because the intensity of the summer-winter variation forces them to synchronize their reproductive cycle to the annual cycle. Thus, after the daily cycle, the annual cycle is the main cycle regulating animal life in the mountain.
Despite its great importance, cold is not the greatest limitation on animals in the mountain: Food shortage (especially in winter) is caused by the dying down of vegetation. Thus, for example, the period when the young are born can be made to take place at the same time as the climatic succession, since it has to coincide with the time of the year that is most favorable in terms of heat regime. The short duration of this favorable period means that many species start the breeding season very early, when the soil is still covered with snow and the air temperature is below that which, in autumn, would induce them to retire to their winter shelter.
For many animals the easiest way to survive the harsh winter mountain conditions is to avoid it. Migrations to milder areas are easier in the mountains, as there is no need to travel great distances. Descending to lower elevations is possible for birds and also for the large mammals and many flying insects. The winter movements start with the arrival of the first snows--chamois (Rupicapra), mouflon, urial (Ovis vignei) and argali (Ovis), deer (Hippocamelus, Moschus, Pudu), and wild boar (Sus scrofa)--all move down to the milder lower forests, although they sometimes move only to the sunny slopes, as environmental conditions are very similar in both sites.
At the beginning of summer, many animals invade the mountain to take advantage of the hot season. Migrations within the mountains are gradual; the animals do not move directly to the summit but follow the retreating snow, moving from one vegetation zone to the next when temperatures permit and retreating if conditions deteriorate. Little by little, the animals reach the meadows above the forests. The first to arrive are the birds, although they may have to shelter within the forest at night. Later, well into the summer, insects arrive and must shelter under the stones because of the low nighttime temperatures. The large mammals also climb mountains in search of food; the ungulates follow the retreating snows, grazing on the tender shoots growing where the snow has melted, while the carnivores hunt the mice and marmots that wake up when the snow melts. Migratory birds following mountain routes cross the mountain passes when the peaks have lost their snow cover; they normally migrate later than birds that fly over the sea, because they have to wait until the climate lets them cross the highest mountain passes.
Other animals, such as amphibians and small mammals, also perform seasonal movements related mainly to reproduction but also to food shortages. In many amphibians, only one sex disperses, or if both sexes disperse, each one does so in a different way: before reproduction, males and females gather in the often transitory pools formed by melting snow. After spawning, the males (and the females too in some species) abandon the site where the eggs are laid and disperse, thus avoiding competition for food with their numerous offspring. This also happens in small mammals, for whom postreproductive dispersal is one method of ensuring the resources of a given site are not exhausted. These migrations occur in species with high rates of reproduction that would otherwise show excessive population growth in just a few sites.
The option of hibernation
Animals may also hibernate to deal with the winter cold, and depending on the harshness of the winter, hibernation will be more or less intense. Thus, in areas with mild winters, many animals undergo a period of drowsiness that is easily reversed if temperatures increase. This is not true hibernation, which is a complex and lasting physiological phenomenon that cannot be interrupted by occasional increases in the temperature. In addition to bats, hibernation occurs in several dozen mammals from the temperate latitudes, the marmot being the best known.
The factor that triggers hibernation in the marmot (Marmota marmota) is the shortening of the photoperiod in the autumn which causes a complex hormonal response that leads to reduction of the general metabolic activity of the organism. The animal retires to its warren, previously lined with grass and deep enough to ensure a constant temperature above the freezing point. While hibernating, the body temperature falls by about 86[degrees]F (30[degrees]C), the heartbeat slows down to about a fifth, and respiration decreases by at least a half. Other physiological changes occur, for example, in the composition of the blood and in the consumption of the fat reserves accumulated during the summer. Sleep is not continuous, as it is broken from time to time in order for the animal to eliminate bodily wastes. In spring, the marmot's endogenous clock brings into operation the machinery needed for return to normal activity. When the animal wakes up and goes outside, it has lost more than a quarter of its weight.
Poikilothermic (cold-blooded) animals are clearly dependent on the environmental temperature, as their metabolic processes are directly influenced by the external temperature: If it is cold, these processes are slower and the successive phases of the annual cycle are retarded; when it is hot, they speed up. This direct dependence on the temperature means it is possible to consider cold-blooded animals as heat accumulators that can perform some activities when they have absorbed the requisite number of calories. Because the important thing is the total number of calories to be absorbed, it does not matter to them if they are obtained on a few hot days or on many cool days. Greatly simplifying, it could be said that a species that completes its growth in four months at temperatures of 45[degrees]F (7[degrees]C), would only take two months at temperatures of 57[degrees]F (14[degrees]C), and just one month at 82[degrees]F (28[degrees]C).
Cold-blooded animals need to accumulate a given number of calories before they can start activity explains the great difference between the number of cold-blooded animals living in mountains and the number living in regions with cold climates with little variation. In Tierra del Fuego, for example, where the average annual temperature is about 45[degrees]F (7[degrees]C), with little variation between winter and summer, only a few animals manage to accumulate enough calories to complete their annual cycle, and this is why there are few cold-blooded animals. Yet in many mountainous areas, the average annual temperature is very low, below freezing, but temperatures vary greatly with the seasons, and summer days may be hot, with average temperatures above 54-57[degrees]F (12-14[degrees]C), that allows them to accumulate enough calories in the short summer season to complete their annual cycle. This is why insects invade these areas in summer, although they have to spend the winter in diapause.
The variety of ecological niches
Plant production in the high mountain contrasts with that in the tundra. During the Arctic summer the plants can make use of the almost permanent light and not so cold temperatures to produce organic matter continuously, while in the mountain the lack of light at night is compensated for by the higher daytime temperature. The result is that both the biomass and plant production in the Alpine high mountain and in the Arctic are similar (from 500-1,000 g/[m.sup.2] as an annual average, in both the Arctic tundra and the Alpine layers). But mountain herbivores seek quality rather than quantity and prefer the tender spring shoots to the hard dry stems of late summer. Thus, both permanent and migratory residents follow the retreating snow in order to eat these fresh shoots, and reach the highest levels, just below the snow level, in high summer.
In the mountain, the carnivores are specialized in feeding on the few types of prey available: ungulates, reptiles, rodents and insects. The ungulates are large prey but hard to catch (except for the young, or as carrion after death). Despite the difficulty of catching them, they attract the large carnivores from the lowland areas. The puma (Felis concolor), the leopard (Panthera pardus), and the specialized snow leopard (P. uncia) can even catch prey in the perpetual snow layer. Small carnivores, such as many small and medium birds of prey and some reptiles, mainly eat rodents, whose abundance compensates for their small size. The group of small carnivores includes specialized hunters, such as the stoat (Mustela ermina), whose thin body is flexible enough for it to enter underground rodent galleries. Insectivores do not usually live permanently in the mountains, but are summer visitors that take advantage of the brief abundance of insects; the only insectivores living permanently in the mountain (carabidas, spiders and other small invertebrates) are so small that they can find enough prey outside the periods when insects are abundant.
Except for carrion-eating birds, decomposer organisms are less abundant in the mountain than in biomes where the production of organic matter is much greater. The cold limits the production and also slows down the rate of decomposition, especially in the poor, acidic, and stony soil types that are typically found in mountains. This slower rate of decomposition implies the formation of peat bogs and waterlogged areas where organic remains accumulate without decomposing. The soil fauna is, however, abundant and varied. A single litre of Alpine soil contains between 500 and 2,500 soil insects, from 500-3,000 acarid mites, up to 1,800 rotifers, 3,000 tardigrades, 18,000 nematodes and between 500,000 and 1,200,000 heterotrophic protoctists.
The trophic webs
In the high mountain, species diversity is very low and food webs contain few components. One of the most representative examples is provided by a study of the Alpine zone of the Rocky Mountains (at an elevation of between 10,827 and 12,467 ft [3,300 and 3,800 m]). The animals identified were 13 species of herbivorous mammals, three shrews, six species of herbivorous and insectivorous birds that normally nest in the area but emigrate in winter, eight species of carnivorous mammals, and another eight species of birds of prey: altogether a total of about 40 species of terrestrial vertebrates with more than 700 possible interactions. The study showed that the most euryphagous species (those that utilize or tolerate a wide range of food) were the birds of prey, as they hunt any animal of the right size; also the wolverine (Gulo gulo) and fox (Vulpes vulpes), species that mainly live in forests but also hunt in the Alpine meadows and are very unselective. The small predators cannot capture medium-sized prey, such as the yellow-bellied marmot (Marmota flaviventris), while the large prey, such as the Mountain goat (Oreamnos americanus) or the red deer (Cervus elaphus) can be caught only by the wolverine and the brown bear (Ursus arctos). The herbivores feed very selectively: most underground rodents eat leaves and storage organs, such as roots and bulbs. Cottontail rabbits (Sylvilagus) and ground squirrels (Spermophilus) are folivores (leaf-eaters), and the diet of the pikas (Ochotona) consists mainly of fruit, and seeds.
In the Alps and the Pyrenees, the food webs are relatively similar. Voles (Microtus) occupy the niche of the underground rodents in the Rocky Mountains, Alpine marmots (Marmota marmota) occupy that of the American species of the same genus, while ground squirrels (Spermophilus) are absent. There are few predators, mainly the fox (Vulpes vulpes) and the small carnivores. The bear is virtually extinct and the only large omnivore is the wild boar (Sus scrofa). A superpredator like the wolverine is lacking.
In South America, toco-tucos (Ctenomys) are the underground rodents, chinchillas (Chinchilla laniger) are the equivalent of the pikas (Ochotona), and the main ungulate is the guanaco (Lama guanicoe). The only large predator is the puma (Felis concolor).
In Africa, the animals occupying the niche of the pikas (ochotonids) are the hyracoids, such as rock hyrax (Procavia), and the most important predator is the leopard (Panthera pardus). In the Himalayas there are many species of underground rodent, some of intermediate size, such as the marmot (Marmota caudata), and there are also several ungulates and two large predators, the Asiatic black bear (Ursus [=Selenarctos] thibetanus) and the snow leopard (Panthera uncia).
4. Life in lakes and streams
4.1 Cold waters with low mineral levels
Outside the areas of perpetual snows, the high mountain landscape often contains many very unusual aquatic environments. In addition to high precipitation as snow or rain due to the effect of the relief on the circulation of air masses, the most frequent types of rocks (generally igneous, metamorphic or magmatic) are relatively impermeable and resistant to erosion by water. Thus as water flows over the surface streams and torrents accumulate in basins and valleys and form pools.
The origin and formation of lakes and pools
High mountain lakes are often the result of volcanic activity or the effects of the Pleistocene glaciations, which were responsible for most of the world's present-day lakes, in some cases remodeling basins that already existed. Some lakes are still periglacial and receive the direct influence of the cold and normally turbid glacial waters. There are mountain lakes in North America (Rocky Mountains, Sierra Nevada, Appalachians), South America (Andes), Eurasia (Scandinavian mountains, Pyrenees, Alps, Tatras, Caucasus, Pamirs, Hindu Kush, Karakoram, Himalayas), Africa (Kenya, Ruwenzori, Kilimanjaro), and Oceania (Southern Alps in New Zealand, the Central Range of New Guinea). The importance of the lakes in the landscape the and life of the mountain has given rise to local names, different from the normal words for lake.
The altitude at which mountain lakes occur is highly variable and depends on the latitude and how continental the climate is. It ranges from an altitude of a few hundred meters in the highest latitudes, to over 9,842 ft (3,000 m) in the tropics. The world's highest lakes are some in central Asia at an altitude of about 17,716 ft (5,400 m). They are normally located above the timber line and are often in sites with little vegetation, sometimes almost deserts, and so the water is relatively poor in humic materials, distinguishing them from the lowland lakes of northern Europe and Canada, with which they otherwise have much in common. Despite the low plant biomass and production in the basins, decomposition is very slow, so that it is not uncommon to find, especially in lakes at lower elevations of the ranges, quaking bogs and even sphagnum bogs. Mountain lakes generally cover small areas, although there are some notable exceptions, such as Lake Tahoe in the Sierra Nevada in California, some lakes in the Tibetan Himalayas (Tso Morari, Pangong Tso) that cover several hundred square kilometers, or the remarkable Lake Titicaca in the Andes, with an area of more than 3,089 mi2 (8,000 [km.sup.2]). Mountain lakes are, however, usually deep in relation to their area. The ratio of the area of the drainage basin and the volume of the water mass determines some characteristics of the lakes. The larger the lake, the higher the levels of salts in its water and the higher the inputs of nutrients and of organic and mineral particles from the basin, and these aspects determine the quantity of organisms it contains. The lake's morphology, especially the ratio between its volume and the surface area of sediment in contact with the water, and the periodicity with which its water is mixed--all condition the differences between the biota of lakes and their dynamics.
Lakes and pools
The variety of characteristics of the lakes in mountain ranges is largely related to the altitudinal gradient. Lakes located in glacial cirques, craters, or near ridges are likely to be more rounded in shape, to be deeper in relation to their area, to be less productive, and to show low inputs of materials originating from outside and low biological diversity. Lakes sited in valleys at lower altitudes will probably receive more inputs of all types and these will enrich the possibilities of life.
In any case, compared with lowland lakes, mountains lakes always show low production, with cold, clear, and soft water. Owing to the low biological activity, oxygen levels are not far from saturation, although in lakes at very high altitudes (more than 13,123 ft [4,000 m]) it is possible to find oxygen profiles similar to those of very productive lowland lakes, with a sharp decline in oxygen levels in the deep layers during the summer. This is because atmospheric pressure is much lower than in lowland areas (in some of the highest lakes, maybe only half the atmospheric pressure at sea level), so that less oxygen dissolves, and although there is less activity in the sediment, the bottom layers may show oxygen deficiency similar to that occurring in productive lowland lakes.
The level of phosphorus, the main element limiting plant production, is as low in mountain pools as it is in the central areas of the oceans (see volume 1, page 154). In arid mountain ranges, such as some on the periphery of the Himalayas, it seems that the washing of glacial sediments and moraines provides a little more phosphorus, and as a result the lakes are slightly more productive. Nitrogen concentrations vary greatly with geography. In European mountains, atmospheric pollution means levels of nitrogenous compounds are high and continuously increasing, whereas in the Andes, they are almost undetectably low, and so nitrogen-fixing cyanobacteria are very important, and their nitrogen fixation has been thoroughly studied in Lake Titicaca.
Sometimes mountain lakes are in inflowing (endorheic) basins, so there is no way the water can drain down to the lowlands. This, especially in relatively arid mountains, gives rise to brackish waters with very unusual ionic ratios, generally with more potassium ions and sulfates than chlorides. There are notable examples in the peripheral ranges of the Himalayas, where there are many lakes in which salts precipitate on the banks. Many of these pools have undergone oscillations from the original level, normally, but not always, reductions. The saltiest ones contain species typical of lowland lakes in arid countries, such as the brine shrimp (Artemia salina) or the rotifer Brachionus plicatilis.
Outside the tropical regions, it is very common for high mountain lakes to freeze for part of the year, and this greatly affects animal life, especially where a thick layer of snow falls on top of thick ice, as the snow absorbs and reflects the light, leaving the lake in the dark. This layer of snow and ice creates an unusual environment for microorganisms, especially at the end of the cold period, when light and external temperature increase. There is no well-documented proof of the existence of permanently frozen lakes, but it appears to be true of Lake Ororotse-Tso in Tibet, the bottom of which is covered by a carpet of filamentous chlorophytes. In the case of mountain lakes of volcanic origin, the composition of the water is very different from the majority of those of glacial origin. Far from having special organisms, these waters generally have sparse populations of algae and a low diversity of consumers.
Torrents and streams
In mountains, streams are generally abundant, with low volumes of flow, and are highly uniform. Typically they show low temperatures (their waters rarely exceed 50[degrees]F [10[degrees]C] at any latitude); steep slopes (often more than 15%) that in some cases lead to spectacular waterfalls, sudden changes in volume of flow according to the seasonality of the precipitations (with extremely sharp contrasts in monsoon areas) or the timing of snow melts; low levels of mineral salts (due to the low chemical weathering of rocks); and high oxygen levels, the result of low temperatures and low biological activity.
The streams connect the other aquatic environments that occur in the mountain, pools and quaking bogs and have a large surface in contact with the terrestrial ecosystems as a whole. The characteristics of the organisms and of the materials born in the waters are good indicators of the natural systems in the basins they drain.
4.2 The flora and fauna
In general, it can be said that the world's mountain aquatic systems are both physically and biologically very uniform. They contain a larger number of cosmopolitan species and show fewer biogeographical differences than do lowland aquatic systems.
Life in river environments
The communities of streams, such as those of almost all the high mountain aquatic environments, contain a high number of cosmopolotin species; ones that are widely distributed throughout areas with similar ecological conditions. The characteristics of the environment make a high rate of turnover and a high dispersal ability necessary. The macroinvertebrate fauna of the Pyrenees, for example, closely resembles that of the Alps, and both resemble lower areas far to the north. There are few studies of Asia's mountain ranges and these studies contain little taxonomic detail, but it appears that the relative abundance of families is also the same from one site to another. Thus, in the rivers in the Himalayas, mayfly larvae, especially those of the baetid family, account for 60% of the species. They are followed in abundance by caddisfly larvae, which dominate the highest streams, mainly species of the limnephilid family.
In seasonal streams in the headwaters, located above the tree line, conditions make it difficult for organisms to grow. Because of the steep slope, the water flows with great force and sweeps everything away, preventing the diversification of microenvironments and thus of communities. The risk of being swept downstream means that the species, regardless of the taxonomic group to which they belong, show similar ethological and morphological adaptations. The fluctuations in volume of flow and in temperature are also very marked, and it is not uncommon for there to be dry periods that lead to sudden situations of danger. In waterfalls, the force of the current means that the fauna, mainly oligochaetes and gastropods, is closely linked with the bryophytes. On stream banks, the diversity of the flora and fauna increases on the moist surfaces of stones.
The autochthonous production of these streams is based on the mosses that grow above the water level and on the algae adhering to the stones, which grow on the few nutrients contained in the waters. The fauna basically consists of omnivores, generally small rapidly growing animals with short life cycles that usually undergo population explosions when melting occurs and are a source of food for predators, mainly plecopterans and planarians. Normally in these streams there are no fish.
Downstream the immediate effects of meteorological phenomena decrease and the distribution of water flow is more regular over the course of the year. The environment becomes more heterogeneous and the communities form a complex mosaic. The population of herbivorous browsers increases, mainly mayfly larvae and chironomid larvae, which feed by scraping off the algae that grow on the stones. In areas with less steep slopes, there are macrophytes, and the afore-mentioned organisms are joined by gastropods and caddisfly larvae. The diversity of the predators increases in parallel with that of the herbivores; they are mainly plecopterans and planarians, as in the seasonal streams at the high altitudes, but here they are represented by different species with longer life cycles. One special environment is the outflows from the lakes, which are dominated by filter feeders, black-fly and chironomid larvae that eat the plankton and detritus born from the lake. When the streams enter an area with tree cover, the increase in the input of plant remains means that grinding organisms, such as some caddisfly larvae, play an important role. The fall of trunks, branches, and leaves increases the heterogeneity, creating natural barriers where fine particles of organic matter accumulate and are used by foraging organisms, such as some families of plecopterans (especially nemourids) or ostracods. Filter feeders (blackflies and caddisflies, such as Hydropsyche) are found where the current is strongest. The spring melt causes a washing of materials and organisms. When surges occur, many larvae survive on the banks and in the interstitial spaces of the riverbed. The return to photosynthetic activity by the algae that adhere to the stones causes an increase in browsers. Many omnivorous species turn to a more carnivorous diet and genuine predators increase; there is even some degree of cannibalism due to the increase in the number of individuals damaged during the winter or swept away by the current.
Many streams occur in suspended lateral valleys that fish cannot reach naturally. In many countries, however, fish have been introduced into these environments, either local species, or more frequently fish from other areas or even from other continents. Often, introduced species do not reproduce. Normally, the fish that live in mountain streams are salmonids, mainly trout and char (Salmo, Salvelinus) and eat invertebrates. There are, however, herbivorous fish in the mountain rivers in Nepal, such as the cyprinid Garra, which eat sessile algae.
Life in lake environments
There are fewer biogeographical differences between the flora and fauna in mountain lakes in different areas of the world than between that of lowland lakes. Their recent origin and extreme conditions have not allowed much diversification. Even the tropical ranges, such as some parts of the Andes, contain many cosmopolitan taxa. Nonetheless, there are some endemic forms, such as some species of the crustacean Boeckella in the lakes of the Andes, or the differentiation of melaniid molluscs and some fish in Lake Titicaca. It is even possible to identify some disjunctions, such as the distribution of the different boreoalpine components in the different mountain ranges in Europe, and to identify separate colonization routes for different groups, as in the case of crustaceans in the Pyrenees, in which the diaptomids are more closely related to the species in the Atlas than to central European species, while the opposite is true in the cyclopinids.
Many high mountain lakes clearly lack macroscopic organisms as a result of the extreme physical and chemical conditions. Even so, they contain a great deal of microbial life, although this was considered irrelevant until the introduction of more thorough study techniques in the mid-20th century. Small and flagellate forms dominate the plankton: 80% of the microscopic algae measure less than (20 [micro]m).
The annual periodicity of the phytoplankton population does not follow a general pattern in all lakes. In temperate latitudes, chrysophytes usually grow in spring, after the ice melt. The summer population is more varied and cryptophytes, chlorophytes, and dinoflagellates appear in the water column, separated in space. Peak production is generally in the autumn, and the role of diatoms is much more variable than it is in lowland lakes. Under the ice, where there is little light and temperatures are low, the algae persist, and some species grow actively in very dim light. If the snow is so thick that the mass of water is in the dark, the biomass of algae diminishes slowly but progressively, and the colorless flagellates that feed on bacteria and dissolved substances become more important. In tropical areas, the seasonality is less sharp, but there are normally profound changes in the communities during the rainy period. Very often, especially in pools with well-developed vegetation, many of the organisms found in the open waters are organisms of the shoreline that are there more or less by chance. This is true of many desmids and diatoms found in the Andean lakes in Colombia.
The production of the microscopic algae is the basis for a web of consumers and decomposers in which small organisms such as bacteria, ciliates, and rotifers play an important role. The population of planktonic crustaceans is, however, poor. The sort of composition normally found is a single cladoceran filter feeder and one or two cyclopinids, together with some diaptomids in more favorable periods.
The benthos and the shoreline vegetation
The shores of many high mountain lakes are little more than piles of rubble. The vegetation consists of mosses, often species typical of streams, that also colonize the lakes by changing their morphology: they lengthen their stems, the leaf arrangement is looser, and the rhizoids are thinner and weaker. There are species that reach considerable depths, 98 ft (30 m) or more, as deep as there is sufficient light to meet their respiratory requirements. The softer waters contain sphagnum mosses, in some cases forming completely submerged populations. The other benthic are microscopic algae, diatoms and many cyanobacteria that replace one another as the depth increases. In the first few meters, the ultraviolet light, which reaches levels as high as on high mountains, makes it difficult for the rocks to be colonized by algae. It is quite common to observe that on the lower surface of stones, where the light reaching the stones is more diffuse, the lower surfaces are richer in algae than are parts that are exposed to direct sunlight. The desmids, for example, are abundant in lakes that are in contact with peat bogs or with a shoreline rich in mosses, but there appear to be regional differences that have not been completely explained. For example, the population of desmids is richer in the Colombian Andean lakes than in the lakes of the Alps and is much richer than the population of the lakes in the Pyrenees.
The lakes with more developed banks may have submerged aquatic macrophytes. In some ranges, such as the Alps, it is difficult to see why they are scarce or absent. In the Pyrenees there has been relatively detailed research into the causes determining the distribution of the different species, and it has been observed that the differences in populations are mainly due to the water's mineral content. Although this mineral content is always very low, the differences are enough to cause changes in the species composition and in the trophic stage, and thus, for example, lakes with very soft waters have communities dominated by Isoetes. In slightly harder water, this community is replaced by a "meadow" of Potamogeton, Ceratophyllum, Myriophyllum, and water crowfoots (Ranunculus). In very productive pools, generally those frequently visited by domesticated livestock, there are populations of macrophytes typical of nutrient-rich waters, such as water starworts (Callitriche, Callitrichaceae). There are similar communities that have similar characteristics in the lakes in the Andes and the Asian mountain ranges.
Flowering plants rarely live at depths of more than 16 ft (5 m). In deeper waters, the only macrophytes present are a few charophytes, macroscopic green algae, mainly of the genus Nitella, or the mosses mentioned before. The growth of flowering plants is not limited by the lack of light, as the plants cease to grow at depths where radiation intensity is above the compensation point (where respiration and photosynthesis proceed at the same rate and there is no net gain or loss of carbohydrate). There may be several reasons, for example changes in the distribution of the sediment due to the slope. The ultimate cause, however, when all the other factors are favorable is that the concentration of oxygen in the air spaces within the plant increases at greater depths. The greater water hydrostatic pressure allows a greater partial pressure of gases (including oxygen) inside the air spaces, thus favoring an increase in the levels of oxidizing free radicals that attack the plant's cell membranes. Plants without air spaces, such as mosses and some marine flowering plants, can reach greater depths.
Lakes with more developed vegetation usually show a clear distribution in concentric belts from the surrounding meadow vegetation to the floating water plants. In the Pyrenees, for example, the transition between the Alpine meadow and the submerged communities consists of a community of sedges (Carex, Eriophorum) and rushes (Juncus), together with other characteristic plants, such as butterwort (Pinguicula) or grass of Parnassus (Parnassia palustris). This community is only temporarily flooded, but water is always available thanks to the substrate of sphagnum and mosses it grows on. Within the lake, and thus permanently flooded, there may be another belt of virtually a single species of sedge (Carex rostrata). This typically aquatic community reaches a depth of a few meters, with quillwort (Isoetes lacustris), awlwort (Subularia aquatica), water crowfoots (Ranunculus aquatilis, R. pseudofluitans) and Potamogeton. Bur reed (Sparganium angustifolium) with its long strap like leaves grows above these small plants.
In small shallow pools with a lot of aquatic vegetation, high daytime photosynthesis may cause the water's pH to rise to surprisingly high values, which makes carbon dioxide even scarcer, and the problem is worse if the water becomes warm and if it is at a high altitude. This is the reason underlying the strategy of, for example, the quillworts (Isoetes), which fix carbon dioxide at night in the form of organic acids for use in photosynthesis during the day. This photosynthetic metabolism diminishes the risk of excessive water, as the stomata open at night, and was formerly thought to be characteristic of land plants adapted to arid environments (such as the members of the family Crassulaceae, in which it was first discovered).
Half of the invertebrate fauna living in the shore zone are insects that do not perform their complete life cycle within the lake. Many insect larvae are the same species as those present in streams and springs. The invertebrates that do complete their entire life cycle within the water include many beetles, molluscs (often with very thin shells because of the water's low calcium content), sponges, bryozoans, coelenterates, planarians, and crustaceans. There is normally a greater diversity of benthic crustaceans than planktonic crustaceans, especially the cladocerans of the Chidoridae group. The benthic fauna of the deepest parts lacking vegetation is less dispersed than that of the coastline and is limited to chironomids, midges, oligochaetes, ostracods, and occasionally bivalve molluscs.
Fish and amphibians
Mountain lakes show very little diversity of fish. Generally, there is only one species of trout (of the genera Salmo, Oncorhynchus, or Salvelinus, or in the southernmost areas of the southern hemisphere, Galaxias). Because angling is one of the few ways these lakes are used, the introduction of species is common and trout are often introduced into lakes where they were not previously present. Insect larvae or adults trapped on the surface film of water are the trout's main food, but in periods of scarcity they may eat almost anything, even the eggs or smaller individuals of the same species. Adult amphibians are not common in the lakes, although their larval stages, which last for 20 days, sometimes fill the banks, or more frequently, the small pools on the banks, where they shelter from fish predators.
130 The mountain goat (Oreamnos americanus) is one of the large mammals that is perfectly adapted to the high mountain environment. It can often be seen on steep scarps and cliffs in areas with low temperatures and abundant snow. Its thick, smooth and silky coat effectively protects it from the cold, and its strong, sharp hooves allow it to walk on rocks and ice. This animal, closely related to the true goats is famous for its agility when climbing: it can climb up 1,640 ft (500 m) in only 20 or 25 minutes.
[Photo: Eric Dragesco / Jacana]
131 The rhododendrons (Rhododendron) of the Himalayas, with their attractive and conspicuous flowers, are usually tall, robust species that form small thickets. In the mountains of Europe and Asia, rhododendrons, together with other shrubby plants, such as junipers (Juniperus), form a layer of low shrubs that occurs at an elevation higher than the coniferous forests and lower than the meadows.
[Photo: Jaume Altadill]
132 Pinus longaeva [=P. aristata] is a pine of the North American high mountain that grows very slowly and never gets very thick but is extraordinarily long-lived (the reason for its name). Some individuals are 5,000 years old, making them the oldest living organisms on earth. The slow growth of this and other high mountain plants is mainly due to the low annual average temperature, as this shortens the growing season. But pines (Pinus) are able to grow so well in the high mountain and the boreal forests--both biomes with short growing seasons--because they are evergreens, and can begin photosynthesis as soon as spring arrives.
[Photo: Jeff Foott / Auscape International]
133 The 300 or so species of the genus Saussurea (Asteraceae) grow mainly in the mountains of temperate Asia, although some species also occur in Europe. Some species are cultivated as ornamental, others have medicinal properties and others are used in perfumery for their strong and lasting smell. The photo shows a specimen on Annapurna in the Nepalese Himalayas at an altitude of 16,076 ft (4,900 m).
[Photo: Jaume Altadill]
134 Mountains are home to some endemic bird species, such as Charmosyna papou, a parrot endemic to the mountains of New Guinea (Indonesia) that lives in the forests at altitudes between 4,757 and 9,842 ft (1,450 and 3,000 m). This specimen was photographed in the North Arfak mountains in Irian Jaya, at an altitude of 6,562 ft (2,000 m). Birds are the largest animal group in New Guinea, and the parrot family (psittacids, the lories, macaws, parrots, cockatoos and other similar birds) is very well represented. The bird fauna has diversified in the mountains, and the different species have adapted to different altitudes to avoid direct competition.
[Photo: Ian Craven / WWF / Still Pictures])
135 The clumps of yareta (Azorella yareta, Umbellife-rae), a hard-cushion species that forms masses that hug the soil, are very typical in the mountains of Lauca in the Chilean Andes. This plant is used as fuel because it produces very little smoke and because there is nothing else to burn. Note the intense effect, at an altitude of 13,123 ft (4,000 m), of the ice, which has fragmented the rocks, a phenomenon that is very typical of high mountain.
[Photo: Estudi Ramon Folch]
136 The remarkable tuft-rosette of leaves of the frailejon Espeletia pycnophylla is a typical feature of the plant landscape of the higher levels of the wet Andes, where these plants cover immense areas above the timber line. The tuft-rosette of hairy leaves is born at the end of a thick stem, and the different species (in this case Espeletia pycnophylla) form a very unusual plant landscape. It is comparable with the wet formations typical of the very high areas of tropical eastern Africa, which also consist of columnar plants with a terminal tuft-rosette of large leaves, such as some species of Senecio and Lobelia (see photos 182 and 183).
[Photo: Tony Morrison / South American Photos]
137 For this screw pine (Richea pandanifolia, Pan-danaceae) from the mountains of Australia and Tasmania, this snow represents a return to well-known conditions. Although this family is usually associated with tropical coastal areas, the species of this genus evolved in a cold climate when the Australian continent, together with Antarctica, was near the south pole.
[Photo: Reg Morrison / Auscape International]
138 The timber line and the upper limit of vascular plants is at a greater elevation in tropical regions and descends with increasing latitudes. Thus in the polar regions, the forests grow only at sea level, whereas in dry subtropical zones, where they reach the highest elevations, they reach 14,764 ft (4,500 m). The factors determining the level of the timber line on a mountain are not the same in every region. Heavy snow, strong winds, poor or excessive drainage, undeveloped soils, diseases and volcanic eruptions, for example, may prevent tree growth. Using the upper limit of the forest and the lower limit of the periglacial processes (basically solifluction) as the criteria to separate the high mountain from the midmountain, this threshold occurs in Scandinavia at only a few hundred meters elevation (and in southern Iceland is almost at sea level), between 5,249 and 5,577 ft (1,600 m and 1,700 m) in central Europe, at about 7,218 ft (2.,200 m) on the southern slopes of the Pyrenees, at about 10,827 ft (3,300 m) in the Rocky Mountains (at the 40th parallel), at 14,764 ft (4,500 m) in the Equatorial Andes, and at about 16,404 ft (5,000 m) in northern Chile and western Bolivia.
[Drawing: Jordi Corbera, from Price, 1981]
139 The blue poppies of the Himalayas are surely the best known flowers from this massive range because of their large and colorful flowers. They belong to the genus Meconopsis and are related to the poppies (Papaver) that are so common in cultivated fields in the northern hemisphere. This specimen, possibly M. aculeata, was photographed at more than 13,123 ft (4,000 m) in Garhwal Himla in Uttar Pradesh (India).
[Photo: Gerald Cubitt / Bruce Coleman Limited]
140 Thorn-cushion plants are very typical of the Mediterranean high mountain. The ones in the photo, taken near Beraton at the base of Moncayo (Iberian Peninsula), are Erinacea anthyllis, sometimes known as the hedgehog broom in English. They are members of the Leguminosae and grow--always on calcareous sites--from the mountains of North Africa to the dry peaks of the Pyrenees. Between April and May, these plants color the landscape with their delicate blue flowers.
[Photo: Ernest Costa]
141 The insular distribution of the puna and the scattered distribution of the patches of paramo in the Andes are clearly shown in the map. They are the result of the confinement of these formations to the highest elevations in the Andes, between 9,842-10,499 ft ([3,000-3,200 m]-8,202 ft [2,500 m] in the paramos in Venezuela) and 14,760-15,748 ft (4,500-4,800 m). The typical paramo, known as "jalea" in Quechua, with distinctive species such as the "frailejones" (Espeletia), has a cloud form and a moist form in Colombia and Ecuador (up to 79 in [2,000 mm] annual average rainfall) and a more xeric (requiring only a small amount of moisture) form in Peru and Venezuela (from 26-71 in [650-1,800 mm]). The typical paramo is a clearly equatorial formation, adapted to a photoperiod that is stable over the course of the year and average temperatures that are usually above 500F (10[degrees]C), despite the great altitude. To the contrary, the southern or temperate paramo, at a latitude of about 40-50[degrees]S, experiences great temperature and photoperiod variations over the course of the year, but converges with the typical paramo because of the abundant rainfall caused by the oceanic influence. The puna is a tropical mountain formation and is colder and drier than the paramo. On the Amazon (or Atlantic) face of the Andes, there is a relatively wet puna (from 18-24 in [450-600 mm] annual precipitation), while the puna on the Pacific face is usually drier, or even arid (from 2 in [60 mm] to 14-18 in [350-450 mm]).
[Drawing: Editronica, based on Monasterio]
142 The distinctive landscape of the typical Andean paramo is clearly shown in this photo (taken in May 1932 on the slopes of the Colombian volcano Tolima, 12,467 ft [3,800 m]). It is one of a set of photographic plates taken by Josep Cuatrecasas during his now classic research in the Andes into equatorial mountain flora. The photo shows many columnar specimens of Espeletia hartwegianum and the meadows of Calamagrostis rectosum around the expedition's guide and pack animals. Cuatrecasas distinguished between the typical paramo, with large fasciculate grasses (Calamagrostis, Festuca) and columnar frailejon candle-trees (dozens of different species of Espeletia) from the subparamo of heaths and other more or less ericoid shrubs (Guatteria, Befaria, Vaccinium, Pernettya, Bac-charis, Senecio, Hypericum, etc.) and from the high supraparamo immediately below the perpetual snows, with many endemic species covered in woolly hairs (Senecio canescens, S. santanderensis, S. cocuyanus, etc.).
[Photo: Josep Cuatrecasas (1903-1996) / Jardi Botanic de Barcelona]
143 When the cold season arrives, the markhor (Capra falconeri) grows a long, soft winter coat like the one on this adult male specimen from the mountains of Afghanistan. The color of the hair also varies with the seasons.
In the summer it is reddish, while in the winter it turns a more grayish color. The markhor is a mountain animal that lives from where the forest finishes up to great heights, and may easily be seen in gorges with scarps and in rocky areas. To survive in the mountainous regions that form its habitat, this goat has to perform seasonal migrations over heights of several hundred meters. In spring and summer, it feeds on the fresh herbaceous plants of the high pastures; in winter it is forced to browse the leaves of trees and shrubs at lower levels.
[Photo: Roland Seitre / Bios / Still Pictures]
144 The European ibex (Capra ibex), such as these specimens taken by surprise in the Valais (Switzerland), dominate the great heights near the glaciers, the most important geological agents responsible for mountain landforms. The European ibex generally lives at elevations greater than 5,577 ft (1,700 m), and it is one of the few species of large mammal that can live all year round in the harsh prevailing conditions above the timber line, where there are few shelters and little food.
[Photo: Oriol Alemany]
145 The Canadian bighorn (Ovis canadensis) is one of the few species able to move with agility and speed over screes and rocky walls thanks to the sharpened bases of its hooves and adherent pads (see figure 30). This means it can graze on areas inaccessible to other animals and escape from predators unable to climb up the rocks. The only disadvantage for these and other ungulates that also live among the ridges is that they are an easy target for hunters, as they live in high sites without vegetation.
[Photo: Leonard Lee Rue / Bruce Coleman Limited]
146 The contrast between the wet "bofedales" and the dry rocky areas of the puna is clearly visible in this photo, which also shows four chinchillon or vizcacha serrana (Lagidium vizcacha). It was taken at Lauca in the Chilean Andes at an altitude of 12,795 ft (3,900 m). The "bofedales" are patches of waterlogged land at the edge of which there is a cover of plants with a cushion growth form, which protects them from the harsh weather. The chinchillon often goes to this area to feed as the vegetation is relatively abundant.
[Photo: Estudi Ramon Folch]
147 For animals that live in the mountains at midlatitudes the winters are especially severe, since the amount of food available is considerably reduced and the climatic conditions are very rigorous. Even so, some organisms are active in the winter season. Others cannot resist the cold and must either migrate to areas with more favorable conditions, or reduce the physiological requirement for food by hibernating. One of the animals that hibernates is the Alpine marmot (Marmota marmota), such as these specimens in the Alps, but squirrels and some small mammals also hibernate.
[Photo: Hans Reinhard / Bruce Coleman Limited]
148 The trophic pyramid in the Andean paramo exemplifies the simplicity of the food webs in mountainous regions, with only a relatively low number of species; three necrophagous birds, only one large natural predator, the puma (Felis concolor)--dogs were introduced by human beings--and a larger number of small predators and plant eaters.
[Drawing: Jordi Corbera, based on Del Llano, 1990]
149 The puma (Felis concolor) is a large cat that lives in the Americas from southern Yukon and Nova Scotia to Chile and Patagonia. It can be found in very diverse habitats (tropical forests, coniferous forests, wetlands, dry scrub, etc.) and from sea level to elevations of more than 13,123 ft (4,000 m). It goes furthest up in the summer, following the seasonally migrating ungulates that are its main prey.
[Photo: Tom Walker / Jacana]
150 Many lakes in the mountains are of volcanic origin, such as this one in Kamchatka, Russia, which formed when the crater filled with water. This is the simplest type of lake formation due to volcanic action, but this type of lake may also form when the ash and lava of a volcanic eruption block the drainage of a valley and the waters are trapped.
[Photo: Jaume Altadill]
151 The frogs of the genus Telmatobius, which live in the lakes and streams of the high Andes are well adapted to the thin air of the great heights. They are completely aquatic species that have greatly developed cutaneous (skin) respiration. Their skins have many small folds and wrinkles that increase its surface area, and thus the area over which oxygen can diffuse, allowing them to spend their entire life underwater. This development of the skin is a clear adaptation to altitude, as shown by the fact that of the 29 species of the genus Telmatobius, only those living in the highest areas have folded skin, while lowland species have smooth skins.
[Photo: Tony Morrison / South American Photos]
152 The fish of the genus Orestias, of the family Cyprinodontidae, occur in the lakes in the high mountain of South America. In Lake Titicaca on the Andean altiplano, this genus has undergone an extraordinary adaptive radiation, giving rise to a score of species with different characteristics and adapted to different life styles. These specimens, photographed in Chile, are members of the species Orestias parinacotensis.
[Photo: Xavier Ferrer & Adolf de Sostoa]
153 The characteristics of the phytoplankton in the lakes in the Andes in Argentina, at temperatures of 42-44oF (5.5-6.5[degrees]C) (data from 1969). The samples from lake Mascardi were taken at two very close points (both in the research station). In general terms, the phytoplankton of these three lakes shows many similarities with those of high altitude lakes and polar regions, both in their species composition and their relative concentration and productivity. The phytoplankton species of high mountain lakes are very small forms (not exceeding 20 [micro]m), and both their biomass and activity are very low owing to the scarcity of nutrients and the special climatic conditions. Productivity (expressed in carbon fixation per unit volume and time) is thus usually very low.
[Source: Margalef, 1983]