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2 Life in the tundra.

1. The ecological functioning of the tundra

1.1 The struggle for existence in adverse conditions

The tundra is covered in snow for most of the year, but in the summer, when the sun is above the horizon all day and all night, its heat is sufficient to melt the snow for at least two or three months, providing the conditions necessary for the life of plants and many other organisms. In addition to the populations of cyanobacteria, fungi, and lichens, the vegetation of the area consists mainly of mosses, rushes, and other plants with a similar growth form, and a few more flowering plants, including dwarf shrubs and even some normal-sized shrubs, but never trees. The northern limit of the taiga coincides with the southern limit of the tundra, although in some cases the two formations are separated by transition areas of tree tundra (that is to say, tundra with shrubs or small patches of trees).

The youth and immaturity of the tundra biome

The tundra is one of the planet's most recent biomes: its origins go back no further than the end of the last ice age, some 10,000 years ago, and in its present form it is only 5,000 years old. The elements of the tundra flora and fauna began to take form during the last glaciation in the regions that were then free of permanent ice, far to the south of the areas where they now live, and which were still covered in ice until the end of the last glaciation.

In functional terms, the biome is also immature, constantly being made and remade by the adverse soil and climatic conditions and showing low production and little accumulation of biomass. The processes of succession are very simple, with very few seral stages, and the close relations and the fluidity of the exchanges between the different ecosystems, including aquatic and terrestrial ones, clearly show the biome's inseparable unity. A good example of this is the transference of biomass from aquatic systems to aerial ones when the many insects whose larval forms live under water become adult and take to the air. Unlike the situation in more southern latitudes, where the contribution of biotic and abiotic factors to the functioning of the ecosystem is more balanced and regular in time, in the tundra these factors are interconnected in an unusual way: In the long term, it is climatic factors that determine production and development; but on certain occasions (for example, during the years lemming populations boom), biogeographical and biogenic mechanisms play a much more important role.

The gradient of biodiversity

The difference between the tundra and the understory of the lowland taiga is that the distribution of the plant and fungus cover is a discontinuous mosaic, and the moisture and the complex microrelief is irregularly distributed. In the understory of the boreal forest, where the moss layer in a pine-spruce forest, for example, may be so homogeneous that it is what characterizes the forest for large areas (for example, the forest with an underlayer of Polytrichum commune), but in tundra ecosystems it is possible to find dozens of moss species growing together in a single square meter.

Another important special feature of tundra communities is their great latitudinal differentiation, which is reflected in the impoverishment of the biodiversity of the flora and fauna at higher latitudes. In fact, several parameters, such as the number of species and their spatial distribution, or their population dynamics, vary from south to north. This phenomenon, similar to that occurring at the tops of mountains, is simply the local expression of a well-known general tendency--the decrease in species diversity with increasing distance from the equator. In the tundra, naturally, this tendency is more pronounced than in other areas and is intensified by penetration by boreal forest plants and animals into the southern tundra sectors.

The effects of human activity and global warming favor the entry into the south of the tundra of many species of plants, anthrophilous dipterans, and birds. The diversity is greatest in the tree tundra, where there are still patches of many taiga species together with the typical species of the southern shrub tundra. In this case, a mosaic of shrub communities, dense populations of bryophytes and patches of bare ground occupy the dividing lines between the drainage basins and the depressions, while the depressions are occupied mainly by peat bogs, by wetlands, and at the base of south-facing slopes where more snow accumulates owing to the action of the wind, by communities of herbaceous plants and small creeping mat-forming plants. The tree tundra and the shrub tundra have almost the same animal species.

Further north, woody plants start to become scarce, and this loss of biodiversity is not compensated for by the enormous variety of bryophytes or the rich groupings of invertebrates and protoctists that live in the mosses carpeting the soil. The typical tundra is dominated by bryophytes; the monotonous carpet is broken by only watercourses and their banks, lakes and boggy depressions. In some plateaux there are also spaces occupied by what is called "spotted tundra," where small patches of bare ground (generally 16-20 in [40-50 cm] in diameter) are surrounded by slightly raised separations covered by dense cushions of mosses, while other adjacent patches are separated by cracks or holes full of peat or by loose populations of mosses. There are few representatives of animals of southern origin.

Even further to the north, the Arctic tundra loses almost all traces of tree vegetation and the landscape is dominated by polygonal or spotted tundra, with an increasing dominance of bare ground over that covered with mosses. The fauna is now almost entirely restricted to species typical of the tundra. In the Arctic desert, the proportion of bare ground and moss cover is reversed, with only a few traces of vegetation in the middle of the immense frozen wasteland. The fauna is very poor, but slightly richer on the coastline, owing to the presence of marine mammals and seabirds.

Fluctuations in environmental conditions

Tundra landscapes are highly dynamic, if only because they undergo a complete change in their surface cover every six months. On one hand, there is an explosion of life following the spring thaw (and sometimes a little before); on other hand, the total inactivity under the snow cover during the winter. Variety is introduced into the apparently monotonous tundra landscape by variation in latitude, influenced by the nearby relatively warm or cold oceanic masses or currents by the influence of the microrelief, which is in turn highly influenced by the permafrost; and by the appearance of water wherever there is the slightest depression in the landscape. In the tundra, the amount of ice per unit soil volume is very high (in the Yamal Peninsula it may vary between 25 and 70%). Any small disturbance of the insulating cover formed by vegetation and soil may result in the formation of thermokarst (the melting of blocks of ice in the soil, with consequent subsidence, landslides, and surface collapse). Thus, over a short period, often no longer than a human lifetime, tundra landscape may change greatly in appearance and structure. The lack of a single component of the tundra community causes a series of knock-on effects and reorganization of the general structure, which may lead to change.

1.2 Sparse production and few consumers

It might be said that the tundra's defining characteristic is that everything is scarce. Limited sunlight, low soil nutrient reserves, short growing seasons, low temperatures, thin snow cover and low diversity are just some of the factors shaping the structural and functional features of the tundra. Yet these shortcomings are in sharp contrast with the overabundance of other factors that also impose severe limitations on most organisms: these include a long harsh winter with a short chilly summer, high humidity (despite the low precipitation) owing to the partial melting of the permafrost and its impermeability, broad open horizons with many rivers and lakes (in summer) or with a snow cover of variable thickness (in winter), the long daylength in summer, and the equally long periods of darkness in the winter. All these factors mean primary production is very low and biomass is also very low.

Modest biomass

This does not mean that there are no differences between the natural systems of the northern tundra and the southern tundra, not only in terms of their biodiversity but also in terms of their plant and animal biomass. The average plant biomass in the northern or Arctic tundra is about 11,013 lb/acre (5,000 kg/ha), and in the southern tundra it is 44,053 lb/acre (20,000-30,000 kg/ha). The annual production is around 2,203 lb (1,000 kg) in the Arctic tundra, and around 5,507-6.608 lb (2,500-3,000 kg/ha) in the southern tundra. The average animal biomass in a typical tundra is 154-198 lb (70-90 kg/ha), most of it corresponding to invertebrates, although the animal biomass may increase greatly when there is a population explosion of small rodents or if the proportion of vertebrates increases. The latitudinal differences in climatic conditions cause asynchronous cycles in the population levels of some animal species, to a certain extent unconnected between them, especially as a result of local conditions. The tundra also shows, from south to north, a simplification of vertical stratification and the reduction of diversity, especially among the specialized inhabitants of the herbaceous layer, such as for example larvae of springtails or simphytes (Tenthredo), which are very common in the southern tundra but much scarcer in the northern tundra.

Tundra organisms have to adapt to this short growing season. In these conditions, plants may not manage to produce seeds every year, which in the case of annuals may mean local extinction. This is why most tundra species are perennial (99% in the most northerly regions). Perennial plants, such as the cotton grasses (Eriophorum), sedges (Carex) and marsh cinquefoil (Potentilla [=Comarum] palustris) play a very important role in the functioning of the northernmost ecosystems. The short growing season also means that evergreen plants are abundant in tundra plant communities, for example marsh rosemary (Ledum palustre), cowberry (Vaccinium vitis-idaea and others), dryas (Dryas octopetala, D. punctata and D. integrifolia), and bog rosemary (Andromeda polifolia), which renew their activity as soon as the snow melts, without wasting time growing new leaves.

Production and recycling

The metabolic activity of the different producers of the tundra is highly variable, corresponding to their different nature and the variable density of their populations. Measurements to assess the carbon dioxide balance in different communities of the south of the Yamal Peninsula, in northwestern Siberia, have shown that the photosynthetic activity of sub-Arctic wetlands is no lower than that of comparable plant communities in more southerly latitudes. On the other hand, the shrubby tundra and peat bogs fix carbon dioxide more effectively in conditions of low light intensity than pool communities. From 75-100% of the carbon dioxide used in photosynthesis comes from the soil, and this is of great importance for the tundra's primary production, because the masses of polar air that reach the region contain comparatively low levels of carbon dioxide (0.016%, instead of the normal 0.03%). During the period of greatest photosynthetic activity, the different types of typical tundra produce 8,189 gal (31,000 l) of oxygen per hectare, per day (1 hectare=2.47 acres), shrub tundra produces 13,209 gal (50,000 l), while wetlands produce 36,192 gal (137,000 l). These data appear to confirm that tundra ecosystems also makes a significant contribution to the entire biosphere's carbon dioxide / oxygen equilibrium.

Very little is known about the biological activity of the tundra microorganisms, but it is very low, owing to the severe climatic conditions in the northernmost latitudes, though some forms of bacteria and fungi (yeasts) can grow at temperatures below 32[degrees]F (0[degrees]C). The low activity of the organisms is one of the reasons for the loose interrelationships between the organic and inorganic fractions of the tundra ecosystems. The organisms responsible for decomposing dead organic materials are larger organisms, such as wood fungi, soil mites (oribatids), and even thecamoebas and other soil protoctists.

2. The mycota, the flora and the plant cover *

2.1 The biological types

"The tundra is a forest without trees," in the words of a Russian saying. This saying illustrates the similarity in physiognomy (external aspect) between the tundra and the understory of the boreal forests. Dwarf shrubs (bilberries and raspberries) and a dense layer of mosses and lichens are major features they have in common.

Lichens

Lichens, together with bryophytes, are ubiquitous components of the tundra landscape. Tundra lichens show a surprising species diversity, especially considering the severe climatic conditions in which they live. In the tundra of the Taymyr Peninsula, 226 species of lichen have been identified. On Devon Island in the Canadian Arctic Archipelago, a further 260 species have been determined. The areas of tundra and tree tundra in eastern Europe alone contain about 500 species. This relatively high level of biodiversity is because lichens, like mosses, are undemanding in their environmental and climatic requirements. This is why they are distributed throughout the tundra area, from the northernmost to the southernmost regions, including the tree tundra at the edge of the taiga.

Lichens have traditionally been divided into three types on the basis of the morphology of their thallus. Crustose lichens form crusts that are very tightly fixed to the substrate they have colonized, and are inseparable from it. Foliose lichens form laminar leaflike structures joined to the substrate at just a few points or by some kind of small attachments. Fruticose lichens are fixed to the substrate at a single point, called the basal disc, and may be slightly or highly branched. All three types are present in the tundra. Crustose lichens colonize large stones and the surfaces of rocks. They vary greatly in appearance, forming bright patches of many different colors (orange, lemon yellow, olive green, apple green, etc.), or separated or grouped spots that are black, brown, or gray in color. Sometimes, they even give the impression of hieroglyphic script. Fruticose lichens are among the dominant plants of the tundra (together with some mosses and vascular plants), and they may form dense stands consisting entirely of lichens. Foliose lichens, however, do not play such an important role in the biocoenosis. Even so, their relatively large thalluses (up to 2 in [5 cm] or more in diameter), with their distinctive shades of gray and green, are very frequent in the moss and lichen tundra, in the shrubby tundra, and in tree-covered wetlands.

The growth of lichens, especially the crustose forms, is typically very slow but constant. If the size of the thallus is compared with its age, the crustose lichens of the tundra show a growth of 0.2-0.3 mm per year. There is even a species whose growth has been measured at only 0.004 mm per year, surely a record for slow growth, as this lichen would only grow 4 mm in a thousand years! Some lichens have reached sizes that suggest they have lived for many centuries. Radiometric dating confirms that, in contrast to long-established ideas, the most long-lived organisms are smaller ones (lichens) rather than the largest ones (large trees). As crustose lichens colonize stony substrates before any other plant, the size of the lichens that have colonized them can be used as a way of determining the age of many materials (or at least the time they have been exposed to colonisation by living organisms). This technique, "lichenometry", has revealed that the moraines in the southern Urals formed very recently, little more than 700 years ago, between 1230 and 1270 A.D.

The fruticose lichens of the genus Cladonia (known as reindeer moss as they are the reindeer's main food in winter) resume full activity as soon as the snow melts when spring arrives, when the temperature of the thallus is only 41[degrees]F (5[degrees]C). The reindeer mosses (Cladonia rangiferina, C. alpestris, C. sylvatica) never exceed 2 or 3 in (6 or 7 cm) in height even if they live for several decades, and their life cycle shows three growth phases. The rapid growth of the first stage may last for 5-25 years (on average about 10) and terminates when the thallus reaches its final length; growth starts at a rate of between 0.02 and 0.03 in (0.5 and 0.7 mm) per year, and at the end is up to ten times faster, 0.2-0.3 in (5-7 mm) per year. The second phase of stabilized growth, lasts for a few decades, sometimes up to a century, and although growth continues steadily, the lichen does not increase in height. As reindeer mosses need a lot of light to grow, the lower parts die back as a result of the shade cast by the upper branches. This phenomenon is especially clear when the substrate is wet and in years when summer rainfall is high. The third phase, the progressive inhibition of growth is characterized by the slowing down of growth, until it totally stops and the lichen gradually dies.

The growth of foliose lichens is comparable with that of fruticose lichens and furthermore, they both respond very clearly to changes in environmental humidity. Their thalli shrink and dry out in dry periods, but after heavy rains they become elastic, smooth and even give off a distinctive fresh smell. The branches of reindeer moss, for example, expand and the thalli of Peltigera aphthosa that are closest to the soil surface turn bright green. When water saturated, lichens lose their normal brittleness and are less prone to mechanical damage, from transport or blows, for example.

As they are slow but steady, the lichens are living, but silent, testimony to climatic change, environmental contamination and some sudden ecological catastrophes. There is an entire range of lichen species with different requirements and sensitivities, so that the damage caused to each one by a given pollutant is selective and distinct. This has allowed the development of techniques, based on variations in growth rates and knowledge of the different capacities for nonselective accumulation in different species of lichens, that give indications and measurements of the extent of environmental pollution and the pollutants responsible.

The typical mosses and the sphagnum mosses

Mosses are very widespread throughout the tundra, where moss species diversity is very high, sometimes with as many as several dozen species in a single square meter. The moss cover has a very important thermoregulatory function. In winter, together with the snow layer, the moss provides some protection from the cold for perennial creeping stems, rhizomes, roots and perennating buds (including dormant ones). In summer, intense evapotranspiration by mosses helps to create a more favorable microclimate. The wetter a habitat, the greater the nitrogen content and reserves in the tissues of the mosses. Thus, the habitats with a high availability of nitrogen favor the growth of both mosses and vascular plants. As a whole, mosses do not grow very fast either, and they account for 30-40% of the total annual production of the aerial plant biomass of the tundra communities. Even so, in highly favorable conditions of water availability, the Bryales (the typical mosses) may grow surprisingly fast: for example, on Devon Island in the Canadian Arctic archipelago, a specimen of Meesia triquetra living in a favorable site (near a watercourse) was observed to grow 1.2 in (3 cm), whereas the growth of the sedges in a nearby pool was only one tenth as great.

Bogs are widely distributed throughout the tundra and are dominated by sphagnum mosses (Sphagnum), towards the south to such a great extent that bogs of sphagnum mosses tend to replace the bogs of species of mosses belonging to the order Hypnobryales, which are the most common in the more northerly latitudes. Sphagnum, the only genus in its family, is a genus of highly specialised mosses with unusual features, including their rapid change in color in function of the seasonal changes in light intensity. The synthesis of red and dark pigments by sphagnum mosses was initially interpreted as an adaptation to protect themselves from the resonance of the light, but it is in fact more related to the daily cycle of environmental temperatures. It has been observed that reddening of the sphagnum is accompanied by the slowing of growth, whenever the nocturnal temperature falls suddenly from 36[degrees]F (2[degrees]C) to around 27[degrees]F (-3[degrees]C). When this temperature drop occurs, the sphagnum's chlorophyll content diminishes, and so its red color is a reflection of its physiological status. Repeated changes in color indicate an instability in the metabolic processes within the moss's tissue. At the end of the summer growth period, the sphagnum mosses in the peat bogs turn red (in fact, they turn a wide range of different shades between red and purple and are often remarkably attractive), a phenomenon comparable with the colors of autumn foliage of deciduous vascular plants before their leaves fall.

According to studies of the seasonal dynamics of sphagnum moss's photosynthetic activity carried out in Barrow, at the northern tip of Alaska, net photosynthesis increases between June and early August by a factor of 1.5-2 times per unit dry weight, and from 2.5-3 times per unit area. The reverse occurs in typical mosses: there is a smaller increase in net photosynthesis during the same period; in the moss Dicranum elongatum, for example, the difference is about 1.3 times per unit dry weight and 2.6 times per unit area. The conclusion that can be drawn is that the typical mosses and the sphagnum mosses respond differently to the changes in light intensity and the length of the photoperiod.

Sphagnum mosses always grow in acidic waters with a low mineral content, but they can also acidify the water they live in by ion exchange processes. This is due to the organic acids (mainly malic acid and citric acid) and polyuronic acids that make up 10-25% of the dry weight of the sphagnum.

Vascular plants

In addition to the peat bogs, with their typical layer of mosses, there are some hydrophilic communities where vascular plants play a role, for example, in wetlands with sedges, rushes, and other herbaceous plants, mainly the water meadows of the grass Arctophila fulva that are common on the banks of rivers and the shores of lakes. This grass is typical of the circumpolar region and is well known for its great polymorphism. It is a very important food plant for reindeer and waterbirds, and is well adapted to unstable habitats that are temporarily or permanently waterlogged. Its rhizomes have special cavities full of air so that the low aeration of the waterlogged site does not block the plant's respiration.

Shrub formations (normally willows, alders, or birches) in the tundra are usually in the floodable plains of rivers. The thermal effect means that these areas show a high rate of growth. The annual growth of the willow Salix dasyclados, typical of the southern tundra, may exceed 16 in (40 cm). The dwarf birch (Betula nana), known as yernik by the Russians, is a typical shrubby species of the tundra and also widely distributed throughout the tree tundra. Depending on environmental conditions, the yernik may adopt a semiprostrate growth form (up to 28 or 31 in [70 or 80 cm] in height), or a procumbent growth form (12-14 in [30-35 cm] tall) or a completely creeping growth form that roots in the moss layer in peat bogs (4-12 in [10-30 cm] above the mass of sphagnum). Yernik mainly grows vegetatively: At an age of 60 or 70 years the main stem and its root system dies. The remaining branches produce roots and small new shrubs form, and this usually happens more than once.

Graminoid herbs play an important role in the tundra. For example, the sedge Carex globularis is found almost everywhere. It has underground perennial rhizomes that emit epigeal vegetative shoots that last a single summer. The shoots of another sedge, Carex rotundata, on the contrary, may live three or four years, but only one or two of the upper leaves survive the winter; during its first years it grows an epigeal vegetative shoot that then flowers, fruits, and dies. This species belongs to the group of plants that remain green in winter and bear shoots of different ages. The water sedge (Carex aquatilis) and cotton grass (Eriophorum polystachyon) have the same sort of development of epigeal shoots as C. rotundata, but differ in that they may last one or two years longer.

2.2 Adaptive strategies

At latitudes where the harsh winter lasts three quarters of the year, organisms, especially plants, are obliged to adapt to the seasonal low temperatures and limited water availability. There is of course water present, but as far as the vegetation is concerned it is only available when it is liquid, and liquid water is scarce. Tundra plants, victims of functional drought although they are surrounded by solid water, have to spend most of their life in dormancy.

Defenses against cold and the wind

Tundra plants protect most of their underground roots, rhizomes (in herbaceous plants), and buds and branches (in shrubs and bushes) by keeping them in the subsurface horizons of the soil, less than 8 in (20 cm) below ground level, where the thermal regime is less hostile. This is one way of ensuring survival to the next growing period, as these organs concentrate the food reserves produced during the previous summer. Even so, the underground rhizomes and stems with dormant perennating buds sometimes need additional protection from the cold. This is why the cotton grasses and sedges form many buds, many of which are not used, and the dead leaf bases remain on the stems.

Another type of adaptation to the cold is the production by shrubs of an excessive number of buds, both axillary and adventitious, that also produce a large number of leaf and flower buds. This form of "reinsurance" is clearly effective, especially in cases where cold, animal, or mechanical damage causes many branches or most of the plant to die. This form of protection, found in almost all the woody plants of the tundra, allows the plant cover to reestablish after any disturbance. Of course not all the leaf primordia will develop into leaves; a bud of the dwarf birch, or yernik, (Betula nana) contains about 21 or 22 leaf primordia, and may in fact produce 7 or 8 leaves; the marsh rosemary (Ledum palustre) only develops 12-14 of the 26-28 it could grow, and the other leaf primordia die.

Plants are also adapted anatomically and biochemically to low winter temperatures. The high concentrations of monosaccharides in the plant's perennating organs lead to a reduction in respiratory activity (very important to prevent the expenditure of the food reserves). Furthermore, monosaccharides prevent protein coagulation at temperatures below 32[degrees]F (0[degrees]C), by preventing the crystallization of water. These water-soluble sugars accumulate at temperatures below 32[degrees]F (0[degrees]C) as a result of the depolymerisation of the insoluble starch produced in photosynthesis. The change from cold to warm increases oxidation-reduction processes, leading to an increase in the plant's tissues of levels of hemicellulose, nitrogen of protein origin, vitamins, and pigments; this increase is accompanied by active production of enzymes (catalases, peroxidases, oxygenases, etc.). Plants lacking any biochemical resistance to the cold show a different sort of adaptation, the oxidation of all their food reserves.

The plants of the tundra need to protect themselves from the wind, and in this case, ecotopic selection acts in their favor. The small mats of plants with a cushion growth form, such as saxifrages (Saxifraga) and some minuartias (Minuartia arctica, M. macrocarpa, etc.) are especially widespread in stony habitats. They cling to the surface of the rock and scarcely rise above it, thus avoiding the full force of the wind.

The water economy

Tundra plants can grow as soon as the snow melts because their roots can function normally when the soil temperature is around 32[degrees]F (0[degrees]C) (in the case of the cloudberry [Rubus chamaemorus], for example, at temperatures above 28[degrees]F [-2[degrees]C]). The fact that below this temperature the frozen water is inaccessible to plants has given rise to the idea that "physiological drought" affects the plants of the tundra, and a different idea that postulates the existence of many plants with different xerophytic adaptations; but these ideas have now been abandoned because they could not be experimentally demonstrated. The true percentage of xeromorphic species in the tundra is surprisingly small; in the center of Taymyr, for example, it is only 9% of the species (15 species). However, it has been found that 90% of the species have a structure adapted to a moderate or an excessive humidity (mesomorphic, hydromesomorphic, or mesohydromorphic), which corresponds to the water regime of the basic kinds of ecotopes that are most widespread in the northern regions of America and Eurasia.

The differences in leaf anatomy and morphology, as well as their lifespan, determine the functional characteristics of the plants with different types of phenological rhythms. Thus, the average water content of the leaves of evergreen plants during the growing season is 51-66%, while that of deciduous species is 65-79%, with peak values during the period of most intense shoot growth. The ratios of transpiration of these two types of plant are 0.1-0.3 g and 0.4-0.8 g of water per gram fresh weight, respectively. The value for the intermediate group, semievergreen plants, is between 0.3 and 0.4 g of water per gram fresh weight, with a leaf water content of between 60 and 75% of fresh weight.

The ratio between the different forms of water in the leaves is also different in evergreen species and deciduous ones, although bound water dominates in both. When rapid growth halts, the ratio of free water to bound water is between 0.3 and 0.5 in evergreen plants and less than 0.2 in deciduous ones. When autumn arrives, these values increase to 0.6-0.9 in evergreen plants and to 0.3-0.4 in deciduous ones. The osmotic pressure of the cell sap is usually higher in semievergreen plants (29-40 atmospheres [atm]) than in deciduous ones (14-29 atm). Even so, during the period of intensive growth, comparable values (16-18 atm) are recorded in leaves of the same generation.

Life-cycle adaptations

The shortness of the growing season leads to the famous "explosion of life" that is so characteristic of the tundra spring. This burst of life is so brief that most flowering plants cannot flower and fruit in a single year, and have to delay fruit production until the year after the flowering. No strictly annual plant could reproduce or survive in the tundra. The fescues (Festuca brachyphylla, F. hyperborea and F. lenensis), sedges (Carex), bilberries (Vaccinium), members of the Polygonaceae, Ericaceae, and other flowering plants of the tundra are all perennials that live as long as possible so as to continue producing flowers one year and seeds the next.

The spring in the tundra is more a spring in terms of light than warmth. The plants of the warmer habitats flower first, as happens in the rocky surfaces that are free of snow one or two weeks before the adjacent areas. Within three or four days of the snow melting from the rocks on Devon Island at the beginning of spring, Saxifraga oppositifolia begins to flower. Seven to 20 days later, other species start to flower, such as dryas (Dryas integrifolia), Arctic heather (Cassiope tetragona), lousewort (Pedicularis lanata), and moss campion (Silene acaulis). The temperature conditions in rock cracks and stony places are very favorable to activate the phenology of the plants and their brief but abundant flowering. This diversity of conditions means that the percentage of individuals in flower varies greatly: on Devon Island, for example, in seven of the species studied only 30-50% of the specimens were in flower, whereas in five other species 70-100% of the specimens were in flower.

On sunny days without clouds, the plants may get hotter than the ambient temperature. A link has been found in several species between flower color and the fact that its temperature is higher than the ambient air. The smallest difference (about 34-34.7[degrees]F [1-1.5[degrees]C]) is found in plants with a white corolla, such as draba (Draba), Arctic heather (Cassiope tetragona); the largest difference (39.6[degrees]F [4.2[degrees]C]) is found in crazyweed (Oxytropis nigrescens, Leguminosae), whose flowers have an attractive violet-colored standard.

When it is cold, the flowering phase is longer. Thus, in crazyweed, a flower takes from three to five days to develop at an optimum temperature (more than 46[degrees]F [8[degrees]C]) and six days at lower temperatures (from 36- 37[degrees]F [2-3[degrees]C]). The purple saxifrage (Saxifraga oppositifolia) normally flowers for three or four days, but at low temperatures it only keeps in flower for two days. The respective values for other species are: one or two days of flowering in normal conditions and five days in cold weather, in Astragalus umbellatus; from two to three days, and seven days, for spotted dryas (Dryas punctata); and four or five days, and eight days, for some gentians (Gentiana algida). During the ripening of the seeds, there is a clear thermal effect caused by the hairiness of the lobes of the calyx.

Growth and reproduction

The rate of development of plants is determined by the climatic conditions, and also by when the sprouts appear and when completely developed. The fact that leaf and flower buds are completely formed by mid- or late July is an major adaptation to the tundra's harsh climatic conditions. The physiological maturity of the flower buds is clearly shown if a second flowering occurs, which can happen in unusual climatic conditions (such as an unusually hot autumn). The maturity of the leaf buds is shown by the rapid appearance of new buds after the plant is seriously damaged by grazing, mechanical damage, and so on.

Many perennial herbaceous plants come into growth again as soon as the sum of daily temperatures over 32[degrees]F (0[degrees]C) reaches 68-86[degrees]F (20-30[degrees]C). This is the case of the harebell (Campanula rotundifolia), globeflower (Trollius europaeus, T. apertus), willowherbs (Epilobium [=Chamaenerion] angustifolium, E. latifolium), and so on, all of them species with showy flowers. The buds grow on the branches (including the horizontal underground shoots) or on the rhizomes, and their growth is determined by their position and by the air temperature (in shrubs), by the temperature of the surface of the moss layer (in dwarf shrubs, especially prostrate ones), or by the temperature within the moss layer, or even that within the upper soil layer (in herbaceous perennials).

In a single region, the sum of temperatures above 32[degrees]F (0[degrees]C) needed may be distinct for different species of a single genus. This is true for bilberries (Vaccinium); the bog bilberry (V. uliginosum) requires 401-446[degrees]F (205230[degrees]C), but the bilberry (V. myrtillus) requires 446-500[degrees]F (230-260[degrees]C) and the cowberry (V. vitis-idaea) requires between 590 and 752[degrees]F (310[degrees]C and 400[degrees]C). The cowberry does not renew its leaves every year, but bears leaves of different ages on the same plant. Although the lifespan of a clump of this species is relatively limited (from 9-12 years), its vegetative system allows it to live for several decades and gradually spread to new areas. Rhizomes up to 3-5 ft (1-1.5 m) long have been measured. In the southern Urals, V. vitis-idaea prepares to flower for 9-14 days, and within 5-9 days of the first flowers appearing it has reached its flowering peak, which lasts for 14-20 days; the fruit ripen 50-55 days later. If there is a cold spell in the peak flowering period, it is very likely that all the flowers will die.

In the case of the bog bilberry (V. uliginosum) in the Yamal Peninsula, the annual growth of the shoots (0.19-2 in [0.3-3 cm]) takes 14-17 days, with each shoot producing 5-9 leaves, which are shed in the autumn. In wetter habitats, the leaves and fruits are larger, the shoots are longer and the growing period is longer than in the drier areas. The bilberry (V. myrtillus) is not as resistant to cold as the other two species mentioned and is unusual for its ability to restrict its annual sprouting period, both above the soil and below it. The time needed for the fruit to ripen depends on the climatic conditions in the year in question, and these conditions may vary greatly. In the northern Urals, this period requires a sum of positive temperatures of between 1,400 and 1,670[degrees]F (760 and 910[degrees]C).

Other small-leaved prostrate clumps, such as crowberries (Empetrum hermaphroditum, E. nigrum) and European cranberry (Vaccinium oxycoccos), have a similar morphology, but the environments where Vaccinium oxycoccos can grow are considerably more restricted than those of crowberries; it lives in the peat bogs and flooded open spruce forests. Both Empetrum nigrum and the swamp blackberry overwinter with live green leaves. When spring arrives, the "old" leaves start photosynthesizing as soon as the snow melts, and then the buds sprout and produce the new leaves. In the swamp blackberry, the axillary buds may remain vegetative for up to three years. If they have still not developed by the end of the second year, the plant produces apical flower buds that give rise to flowering shoots the following year. Thus, the main stem has short stems between one and three years old, and when one dies, a new one grows. The European cranberry flowers relatively late, in the middle of July, and this normally does not allow the fruits to completely ripen until the following spring. This is why the European cranberry can only be eaten in the tundra in the spring.

3. Fauna and animal life

3.1 Faunistic variety

In the harsh conditions of the tundra, the processes that mold faunistic groups and the dynamics of animal populations depend as much on the combination of environmental factors present as on the nature of interspecific relationships. Migratory waterbirds are the principal faunal group present in the tundra during the summer months. They capitalize on the explosion of life in the rich tundra marshes during the Arctic summer but spend the winter to the south in more hospitable climes. Some mammals, however, can survive the long tundra winter. Cold-blooded reptiles and amphibians, on the other hand, are scarce and are only active in summer. All tundra fauna has to endure the rigors of the climate, yet at the same time the fauna has to be in a position to take advantage of the relative advantages it offers at certain times.

Birds

The great variety of bird species and their high population densities is a reflection of the ability of birds, more than other vertebrate groups, to thrive in the tundra summer.

Almost 285 species of bird breed in the tundra and along the coasts of the Arctic Ocean, although they are neither distributed equally throughout the biome nor contribute evenly to the biome's trophic chains. For example, of the 45 marine bird species that live along the Arctic Ocean, 29 species are not directly linked with any of the tundra ecosystems. The group of birds native to the sub-Arctic regions (those that live exclusively or preferentially in the tundra or tree tundra) consists of 57 species, nearly a quarter of all the breeding birds of the area, if marine birds are excluded. As far as habitat is concerned, the majority of tundra birds are confined to marshy or fluvial environments and only a few nest in rocky or mountainous habitats. Very few species withstand the harsh tundra winter.

Endemic birds

Many of the birds are endemic to the tundra; that is, they live and breed in no other biome. For example, the snow goose (Anser [=Chen] caerulescens), Brent goose (Branta bernicla), Canada goose (B. canadensis), red-breasted goose (B. [=Rufibrenta] ruficollis), long-tailed duck (Clangula hyemalis) and snowy owl (Nyctea scandiaca) do not breed outside the tundra. Other common breeders such as the ptarmigan (Lagopus mutus) and dotterel (Eudromias [=Charadrius] morinellus), however, have much broader distributions and are not found exclusively in the tundra. Both of these species prefer mountainous and rocky areas.

The willow grouse (Lagopus lagopus) and ptarmigan (L. mutus) are the only two species of galliforms (pheasants, partridges, grouse, etc.) to inhabit the tundra, and a number of different subspecies are recognized in both species. The willow grouse can be separated, for example, into the red grouse (L. lagopus scoticus) found in Britain and Ireland, L. lagopus koreni in Siberia and L. lagopus alascensis in northern Alaska. Likewise, a number of different subspecies of ptarmigan have been identified, including the nominate L. mutus mutus from Scandinavia and the Kola Peninsula and L. mutus kelloggae from the Ural Mountains and northern Siberia.

Widely distributed birds

The next most important group consists of 18 species with much wider geographical distributions, that are found from the northernmost limits of the tundra through, to, and beyond the limits of the taiga. Typical birds of this group include the black-throated diver (Gavia arctica), pintail (Anas acuta), and ruff (Philomachus pugnax) as well as even more cosmopolitan species such as the peregrine falcon (Falco peregrinus), short-eared owl (Asio flammeus) and raven (Corvus corax).

Another numerous group consists of 38 species that reach only the southernmost limits of the tundra. These include the wigeon (Anas penelope), velvet scoter (Melanitta fusca), Baikal teal (Anas formosa) and wood sandpiper (Tringa glareola). Owing to their abundance, many of these species play a very important role in the ecosystems of the southern limits of the tundra as well as in the more southerly areas that represent their true habitat.

One final group (almost 100 species) is composed of species which enter the tundra along ecological corridors, principally the wooded river courses which penetrate the southern part of the tundra. Their role in the tundra ecosystem is secondary.

Bird populations

Tundra ecosystems are not characterized by a great diversity of species although certain species may be present in huge numbers. In sub-arctic regions, animal populations maintain optimum levels by intense breeding activity or by increasing resistance to unfavorable conditions. Populations of predatory birds, for example, fluctuate periodically in response to the abundance of lemmings, their main prey. These fluctuations are most pronounced in the snowy owl (Nyctea scandiaca) and the rough-legged buzzard (Buteo lagopus). Clutch size increases in years with abundant food supply, although in a family such as the skuas (Stercorarius) which only lays one egg, demographic fluctuations depend purely on mortality levels.

Amongst the most numerous birds in the tundra are ducks and geese, waders, passerines, willow grouse and ptarmigan. The breeding density of willow grouse (Lagopus lagopus) in the Eurasian tundra may reach 30 pairs or more per [km.sup.2]. The long-tailed duck (Clangula hyemalis) is found breeding in densities varying between two and seven pairs per [km.sup.2] (1 square kilometer=approx. 247 acres), whereas the pintail (Anas acuta) reaches densities of between one and three pairs per square kilometer.

Mammals

Far fewer mammals (around 60 species) than birds live in the tundra, although many mammals are much better adapted to the rigors of the polar winter. Mammals, in general, remain in the tundra all year around and because of their relatively limited capacity to undertake long distance migrations, migrate only short distances. The surface of the tundra during winter appears lifeless, yet many mammals lead a very active life under the snow layer.

Endemic mammals

Only around a dozen of the 61 mammals that live in the tundra can be considered to be truly endemic. The Arctic fox (Alopex lagopus), reindeer (Rangifer tarandus), musk-ox (Ovibos moschatus), lemmings (Lemmus, Dicrostonyx), and Middendorff's vole (Microtus middendorffi) are some of the commonest and most typical of all circumpolar mammal species.

The Arctic fox (Alopex lagopus), a circumpolar canid, is endemic to the tundra. The Eurasian subspecies (A. lagopus lagopus) and the American subspecies (A. lagopus unnuites) have a head and body measuring between 18 and 29 in (46 and 73 cm), a long bushy tail of up to 20 in (50 cm), relatively short legs and short rounded ears. In summer their coats are grayish-brown, but in winter most individuals turn white. Some larger animals with blue-tinged coats are considered by some authors to belong to a third subspecies (A. lagopus beringensis); other authors believe that individuals with bluish fur are merely a color phase that is present in varying percentages in certain populations. A fourth subspecies has been described with especially thick and fluffy fur from Novaya Zemlya, Spitsbergen, Franz Joseph Land, and Greenland.

The reindeer, or caribou (Rangifer tarandus), is a deer with a circumpolar distribution that lives in the tundra and taiga of the continental mainlands and islands of the Arctic, North Atlantic, and Pacific Oceans. The most important subspecies in the tundra are R. tarandus tarandus and the Greenland reindeer (R. tarandus groenlandicus). Both males and females of the tundra reindeer have a relatively small body with broad hooves, short legs, and wide, spreading antlers. These characteristics are especially marked in the Spitsbergen reindeer, a massive-hoofed race weighing only around 101 lb (50 kg) which some zoologists consider to be a separate species (R. platyrhynchus). The coats of most nordic subspecies are pale (almost white in the subspecies R. tarandus peary from the north of Greenland, Ellesmere Island, and some neighboring islands), whereas the coats of more southern races are more tawny-brown. In winter reindeer feed by scraping food from under the snow with their hooves, a habit that has earned them the name caribou in North America: The word khalibou in the Micmac language (a North American Indian language belonging to the Algonquin group) means stirrer of snow.

Of all the endemic tundra animals, the musk-ox (Ovibos moschatus), found throughout areas with a dry polar climate, is the most characteristic. In summer it can be found in areas of relatively abundant vegetation; in winter it frequents areas where the snow layer is not too thick. Its circumpolar distribution and perfect adaption to life at high altitudes have encouraged the belief that this species originally evolved in Arctic regions. However, this assertion is contested and many authors believe that although the musk-ox may have originated in the area of the Bering Straights during the first Pleistocene glaciations, its ancestors were originally, in fact, from the mountainous regions of Central Asia.

The common lemmings (mainly Lemmus sibiricus and L. lemmus) and collared lemmings (principally Dicrostonyx torquatus and D. groenlandicus) are small rodents (4-6 in [10-15 cm] excluding tail) with short tails and limbs and ears that barely extrude from the animal's dense coat. The soles of the feet are also covered by dense hairs. The upper body hair is brown-gray, the underparts are somewhat paler, and there is a dark stripe, extending from the back of the head halfway along the back, which in the collared lemmings also spreads down the sides of the face. In winter, the third and fourth claws grow noticeably to permit the excavation of runs under the snow. Both the common lemmings and the collared lemmings are spread throughout the tundra proper and the wooded tundra of Eurasia and North America and fulfil a vital role in these ecosystems. Lemmings feed principally on sedges, cotton grasses, and various species of moss.

Widely distributed mammals

Another very important group of tundra animals includes a number of species found in biomes as varied as the tundra and the deserts of North America and Eurasia. Members of this group include the red fox (Vulpes vulpes), wolf (Canis lupus), stoat (Mustela erminea), weasel (M. nivalis), and musk-rat (Ondatra zibethicus).

Mammals from the taiga can penetrate into the tundra only along ecological corridors. The most important of these to do so are the brown bear (Ursus arctos), elk (Alces alces) and northern redback vole (Clethrionomys rutilus). Only two species of hare--the mountain hare (Lepus timidus) and the snowshoe hare (L. americanus)--penetrate into the tundra and actually play a significant role in its ecosystems.

Mammal populations

The most notable characteristic of the dynamics of sub-Arctic mammal populations is the ability of certain species to increase their numbers dramatically over a period of only one or two breeding seasons. Subsequently, populations return to their original levels as rapidly as they grew in the first place. This pattern is most notable in lemmings: Every three or four years lemming numbers grow abruptly until there are thousands of lemmings per hectare (1 hectare=2.47 acres). In these "lemming years," the Arctic fox (Alopex lagopus) has litters of 1015 cubs, a good example of how the population cycle of a predator (the Arctic fox) coincides with that of its prey (the lemmings).

Amphibians and reptiles

In the tundra there is less diversity among amphibians and reptiles than among mammals, birds, and fish. This is to be expected given the consistently low temperatures and the brevity of the summer, often too short for amphibian larvae to complete their metamorphosis into adults. Information on amphibians and reptiles and their distribution in the tundra is incomplete and very contradictory. Five species of amphibian are thought to penetrate into the North American tundra and four into the Eurasian tundra. Nevertheless, in the whole circumpolar region only four species actually manage to form stable populations: three species of frog--Rana arvalis, R. temporaria (the common frog), R. sylvatica--and the Asian salamander Salamandrella [=Hynobius] keyserlingi. R. arvalis is normally found in small pools up to 19.7 in (half a meter) deep in the scrubby or wooded tundra, whereas R. temporaria inhabits only the tundra proper. The Asian salamander (S. keyserlingi) is an uncommon species that lives on the banks of rivers and lakes. The occasional viviparous lizard (Lacerta vivipara) can sometimes be found in pools and other similar habitats.

Invertebrates and rhizopods

In terms of both the number of species and total biomass, invertebrates are the most abundant group of organisms in the tundra and in total there are thousands more invertebrate than vertebrate species. To be able to survive in such a hostile environment invertebrates have to hibernate in a state of anabiosis and are able to inhibit all vital bodily functions to such an extent that all signs of life disappear. As in any other biome, the populations of invertebrates in the tundra have special taxonomic characteristics that include many varied types of creature (worms, molluscs, arthropods, and so on). The greatest diversity occurs in arthropods, largely found living in mosses or in the thin, warmer surface soil layer. Some of the most significant species include the springtails (Collembola), barely 0.02 in (3 mm) long, and the even smaller soil mites (Oribatidae).

There is also notable diversity in insect and arachnid populations. The commonest beetles are terrestrial predators (Carabidae, ground beetles and Staphylinidae, rove beetles); butterflies and moths (Lepidoptera) are also well-represented. Typical species include the nocturnal microlepidopteran moths of the family Tortricidae and both nocturnal macrolepidopteran geometer moths (Geomatridae) and diurnal species, such as many butterflies, especially the Arctic ringlet (Erebia disa) and the cranberry fritillary (Boloria aquilonaris).

Blood-sucking (hematophagous) insects represent one of the most typical animal groups in the tundra. They belong to two main dipteran groups: the black-flies (Simuliidae) and mosquitoes and gnats (Culicidae) and both can seriously affect human activity in the summer. Black-flies are restricted to the wooded tundra, whereas mosquitoes penetrate further north, although by 72[degrees]N they disappear almost completely.

Thecamoebas are now considered to be protoctists and not animals, although traditionally included in the invertebrates as protozoans. They play a very important role in the formation of hydromorphic soils and are avid predators of soil bacteria. They belong to the Rhizopoda and are very abundant in the bogs and the pools of the tundra.

3.2 Adaptive strategies

Animals have adapted to the extreme conditions of the tundra in a number of ways. The most widespread species have opted for ecological adaptations (behavioral changes, migrations to more favorable environments, increase or decrease in activity and refuge in safer areas), whereas species with a greater degree of endemism rely more on morphophysiological adaptations. It is worth remarking that in the harsh conditions of Arctic regions similar morphophysiological and ecoethological adaptations are repeated not only in particular taxonomic groups but also among members of different animal classes.

Morphophysiological adaptations

The climatic conditions and the ecological characteristics of the tundra could lead to the apparently logical conclusion that the principal physiological trait that all tundra animals share is a high metabolism. Nevertheless, an examination of morphophysiological indicators shows that this is not so. The species that are best adapted to life in the tundra can function at low metabolic levels, even when conditions demand high mobility.

Thermal insulation

The use of an insulating layer of fat to protect the body from the cold is not an exclusive privilege of tundra animals: northern boreal populations of widespread species also possess similar fat layers. Tundra forms of a species are not infrequently smaller bodied than their boreal counterparts and have less-developed hairy coats.

With the exception of the Arctic fox (Alopex lagopus) and a few other species, the isolating properties of the hairy coats of tundra animals are not superior to those of boreal forest animals. A reindeer's coat (Rangifer tarandus), for example, gives no more protection than does a sheep's. This does not imply, however, that a reindeer's thick winter coat with its long, air-trapping (and therefore delicate) hairs on the back and smoother, more curly hairs on the belly, is not good protection in winter. The musk-ox's coat is also very dense and additionally has very thick underfur. In winter it is almost black and in summer dark brown.

One characteristic of tundra animals is the concentration during the short summer of processes such as reproduction and molting of fur or feathers which are costly in terms of energy. In more southerly latitudes, birds and mammals molt and breed in different periods of the year.

Growth and sexual maturity

Tundra animals have developed the ability to grow quickly when they receive 24 hours of sunlight a day. The growth rate of young reindeer during the continuous daylight of midsummer is much greater than in other ungulates, and commercial breeding of lemmings has shown that this ability is hereditary. Reindeer do not reach sexual maturity until they are two or three years old, and females generally give birth to only one calf, very occasionally to two. In the northernmost limit of their distribution, calves are born from the end of April until the beginning of May.

Increased fertility and sexual precocity are also characteristic of many tundra animals, although the northernmost populations of widely distributed species tend to be more fertile than the typical endemic tundra species. The majority of female stoats (Mustela erminea) begin breeding the year they are born, although at lower latitudes breeding does not take place until the second year. This precocity also occurs in female shrews (Sorex). Shrews do not normally reach sexual maturity until their second year of life, yet in the tundra almost 30% of female shrews are ready to breed in the year they are born.

Seasonality in diet

Another important biological characteristic of tundra animals is their diet. Alimentary sources vary from one season to another, although winter is the season that demands greatest specialization. The wolf (Canis lupus), for example, has a very varied diet in summer, yet in winter depends almost entirely on reindeer meat. On the other hand, musk-ox (Ovibos moschatus) feed throughout the year on the leaves and buds of willows, sedges, grasses, rushes and other Arctic plants, and occasionally supplement their diet in winter with mosses and lichens. In winter the diets of willow grouse (Lagopus lagopus) and ptarmigan (L. mutus) consist largely of the leaves and buds of willows and dwarf birches. However, in summer, when there is no snow cover, their diet is more varied, and they feed on the leaves, buds, and fruit of a variety of shrubs such as bilberry (Vaccinium myrtillus), bog bilberry (V. uliginosum), cowberry (V. vitis-idaea), cloudberry (Rubus chamaemorus), and crowberry (Empetrum nigrum). They are ground-feeders and only occasionally feed in trees.

In other cases specialization is an all-year feature of an animal's life. The health of Arctic fox (Alopex lagopus) and snowy owl (Nyctea scandiaca) populations depends on the abundance of lemmings. Branta geese base their diets on cotton-grass (Eriophorum) and various species of meadow grasses (Poa). The gyrfalcon (Falco rusticolus) survives the winter on a diet of ptarmigan and willow grouse. A consequence of such extreme dependence is that the abundance of one species is essentially the product of the abundance of another, independently of whether the former species is the principal prey of the latter or the principal predator. This relationship also determines the distribution of animals throughout the biotopes that favor each species. It should be noted that in most cases the alimentary specialization of tundra animals has no negative effects on other aspects of their behavior.

Nutritional specificity leads to certain gastrointestinal morphological peculiarities in some tundra animals. Particularly affected are the length of the intestine and cecum in herbivores. The length of the intestine of a lemming from the Obi region, for example, is twice as long as its body. This extra length permits the animal to assimilate food more efficiently, a very important capacity in the tundra where the most abundant plants are fairly indigestible (sedges, mosses, etc.). In contrast to alimentary specialization, many mammal species in the tundra have evolved the capability to considerably broaden their diets in extreme situations. For example, reindeer (Rangifer tarandus) normally feed almost exclusively on lichens (Cladonia, Cetraria, etc.) in winter. However, to obtain more phosphorus and sodium they will sometimes turn to toadstools, eggs, and even small vertebrates (lemmings and birds). Some northern species such as the Arctic fox (Alopex lagopus), snowy owl (Nyctea scandiaca), and polar bear (Ursus [=Thalarctos] maritimus) are capable of accumulating reserves of energy for the winter in the form of unsaturated fatty acids which are convertible into assimilable fats. In the subarctic tundra it is essential to have stored fat reserves that can be rapidly assimilated if the unstable summer climate provokes a food shortage. These reserves consist basically of liver glycogen and explain the abnormally large livers in the northernmost populations of many species of mammals, birds, reptiles, and fish.

Ethological adaptations

The most important adaptations found in the fauna of the tundra correspond to behavioral traits that enable animals to confront successfully the harshest aspects of life in the Arctic whilst benefiting also from all the advantages the biome has to offer. Essentially, this amounts to an acceptance of the fact that it is easier to avoid the winter cold than to try to resist it.

Migrations

One of the most important ethological adaptations found in tundra fauna is the ability to migrate large distances in search of food. Data shows that migrations are the most rational and complete method of exploiting the nutritional resources of the tundra.

Large seasonal migrations of reindeer (Rangifer tarandus) are a good example. They are forced to live in two different biotopes, the tundra proper in summer and autumn and the wooded tundra and taiga in winter, because if they stayed in one or other of these biotopes permanently they would inflict considerable damage on the pastures. In the summer reindeer live in the northern areas of the tundra, feeding principally on wild herbs as well as on the leaves and buds of Arctic willows (Salix arctica) and dwarf birches (Betula nana). In this part of the tundra they find sufficient food and suffer less from the onslaught of blood-sucking insects. In autumn when the food sources have been exhausted in the most northern areas of the tundra, reindeer migrate south to the wooded tundra and to the northernmost areas of the taiga where they feed on lichens. They are capable of migrating up to 932 mi (1,500 km).

The migrations of the Arctic fox (Alopex lagopus) are also well documented. The length and direction of its migrations are determined by the availability of its principal prey, the lemming. In years when lemmings are extremely scarce, Arctic foxes may migrate thousands of kilometers along coasts and rivers or may even take up residence in the northern parts of the taiga. Lemming and fox population explosions occur on average once every three to four years, although as few as two years or as many as four may separate one eruption year from the next.

Large-scale migration is not confined to large mammals; lemmings and willow grouse also undertake considerable migrations. Lemmings (Lemmus and Dicrostonyx) can cross large rivers and are capable of traveling 9 mi (15 km) a day. Willow grouse (Lagopus lagopus) migrate southwards in large flocks once the breeding season is over.

Breeding behavior

The harsh conditions in the tundra explain the high mortality rates found in many tundra species. However, many species possess compensatory mechanisms that increase breeding success. Some of these mechanisms have physiological origins and have already been mentioned (increased fertility, sexual precocity, etc.), whereas others are ethological adaptations. Good examples include starting to mate very early in the year (lemmings begin to pair in February) or exploiting all the possible ways of successfully raising a clutch, as is done by most birds.

Although some Arctic foxes (Alopex lagopus) begin to mate in March and April, most individuals do not actually mate until May or June. Each litter usually consists of 7-10 cubs although in lemming years between 20-25 cubs may be born. The earths used in the breeding season are highly complex: Each has various entrances and may be used for a number of years.

In the winter the musk-ox (Ovibos moschatus) lives in relatively sedentary herds of a hundred or more animals that undertake short-scale migrations. However, during the breeding season (August and September) breeding males separate from the herd and gather together harems of a dozen or more females. Once the breeding season is over, all sexually mature animals return to the herds for the winter. Fertility is low and females give birth to only one calf every two years. Reindeers (Rangifer tarandus) are also gregarious animals, although females and their young graze separately from the males in summer. In autumn all sexually mature individuals join to form a single herd.

The first ptarmigan (L. mutus) and willow grouse (Lagopus lagopus) chicks are born in June, although the peak arrives in July. Clutch size generally varies between five and seven eggs and exceptionally up to 20 eggs. After breeding, willow grouse flock and migrate south to the wooded tundra or even to the taiga. Ptarmigans on the other hand, prefer to breed in more mountainous areas and do not undertake large-scale migrations.

4. Life in rivers and lakes

4.1 The abundance and diversity of bodies of water

The tundra's water regime is governed by its relatively low rainfall, no more than 12-16 in (300-400 mm) per year. Yet this is enough to create an excess of moisture, as the summer is so short and there is almost no evaporation occurs. In addition, the presence of permafrost isolates the beds of the lakes and rivers in their lower stretches, preventing subterranean water flows into rivers and lakes and impeding flow from rivers and lakes to the water table.

Continental lakes and lagoons

The insufficient evaporation and the lack of filtration of water to the water table lead to the formation of a huge number of small lakes and bodies of water: lakes, pools and ponds, many of them the result of the formation of thermokarst. For example, in the Bolshezemelskaya tundra (a moraine plain to the north of the Arctic Circle, between the valley of the river Pechora and the northern Urals) there are more than 4,000 lakes per 386 [mi.sup.2] (1,000 [km.sup.2]), which in 95% of the cases cover less than 0.2 [mi.sup.2] (0.5 k[m.sup.2]) and are less than 7 ft (2 m) deep.

The great majority of shallower lakes and pools are frozen solid almost the entire year. Life is only possible for three or four months a year and only for those organisms that can survive in a latent state; there are usually no vascular plants or fishes. There are many planktonic organisms that can survive the long winter in an inactive state. The deepest lakes are of glacial or tectonic origin (tectonic lakes are located mainly in mountainous areas) and are covered for eight or nine months of the year with a thick layer of ice (which can be 7 ft (2 m) or more thick), under which many organisms can live without problems, including fish, in a normal state of inactivity.

Rivers and streams

The tundra is criss-crossed with small streams supplied mainly by the water of the melting snow. The spring melt is followed by major surges and a major increase in the water level not only in the streams but also in the lakes. Yet at the end of summer, the flow of water diminishes, and the shallow streams and small natural channels between the lakes may dry out completely. In autumn, the rainy season, the water level rises again. This surge of water is not so large but is very important for the aquatic organisms, because it supplies the nutrients that the summer rains had washed from the surface of the thawed soil. The spring surge, in comparison, usually contains fewer nutrients. In general, the water of the tundra's rivers and lakes shows low mineral levels and its salt content does not usually exceed 15-30 mg/l. In boggy regions, the water is usually a dirty yellow color owing to the humic compounds it contains.

The tundra is also traversed by the lower stretches of some large rivers that cross the tundra running north towards the Arctic Ocean. The largest ones in Eurasia are the Pechora, Ob, Yenisey, and Kolyma, while in North America, the largest is the River Mackenzie. These water masses act as huge heat buffers, and for this reason they take longer to freeze than shallow streams do. The spring melt begins in the upper stretches, furthest to the south, and gradually works its way down to the mouth. Water level increases substantially at the time of the melt, but the most important surge does not occur until the snow melts in the boreal forests and the tundra. When the water level subsides, it leaves many shallow pools behind. After a while, some dry out and become meadows, while the others are left as small permanent bodies of water connected to the river only when water levels are high. These waters contain abundant phytoplankton and some fish, such as the whitefish (Coregonus peled), enter these bodies of water to feed. As these rivers empty such a huge quantity of fresh water into the sea, the large Arctic rivers greatly lower the salinity of the coastal waters of the Arctic Ocean, which are a major breeding center for many species of fish of great commercial importance.

Coastal lagoons

The lagoons and pools scattered throughout the low-lying coastal plains are a special case. The strong winds that blow from the sea bear a lot of salt. Near the coast there are also deep lakes that were formerly connected to the sea but were later separated by an arm of land.

One of these lakes is Lake Moguilnoie on Kil'din Island (69[degrees]30'N, 34[degrees]E). It is separated from the sea by a narrow natural dyke through which seawater reaches the lake at high tide and leaves at low tide. The deepest part of the lake is full of seawater, which is denser, while the surface layers are only slightly saline and are sometimes consist entirely of freshwater. The lake contains typical freshwater species, such as the rotifer Keratella quadrata, along with typically marine ones, such as crustaceans, for example Centropages hamatus, and even fish, such as cod (Gadus morhua), which is represented by a special race (G. morhua kildinensis) that is restricted to the waters of Kildin Island.

4.2 The aquatic organisms

The total freezing of the tundra's shallow rivers and the shores of lakes does not allow the growth of macrophytes. The only ones present are a few specimens of the water sedge (Carex aquatilis) on the banks or, in not very deep water (6-8 in [15 to 20 cm]), the grass Arctophila fulva. In fact, these plants are not strictly aquatic but amphibious, and they are only important as primary producers in shallow pools and ponds.

Primary producers

In the tundra lakes, most primary production is by the phytoplankton ** which is highly diverse and often abundant, despite the shortness of the growing season. For example, the species list in some shallow lakes on the coast of Barrow Point (71[degrees]15'N, 156[degrees]40'W), at the northern tip of Alaska, include about 120 photosynthetic algae and bacteria, many of them common in temperate waters. The phytoplankton biomass varies between 0.025 and 0.400 mg/l (only in very rare cases does it reach 1 mg/l), values that are normal in waters at high latitudes, but considerably lower than those found in temperate waters. Most phytoplankton organisms found at this site are very small forms, such as the Cryptophyte alga Rhodomonas minuta, whose abundance in summer varies between one million and six million cells per liter. In the Arctic area, as happens in temperate regions, the composition of the phytoplankton in neighboring lakes may be dominated by different species. Thus, in the same area of Alaska as the previous example, in one of two neighboring lakes the phytoplankton is dominated by a diatom of the genus Synedra, while in the other, the dominant forms are Chlamydomonas and Pyramidomonas.

It is worth pointing out that these two lakes both show a peak abundance of phytoplankton in the spring, immediately beneath the ice layer. This abundance does not occur everywhere or in every year. When the ice is very thick and covered in snow, photosynthetic activity reaches its peak when the ice is absent. Lake Taymyr is a large lake in the Taymyr peninsula, the most northerly area of Eurasia, with an ice-free period of about three months. The phytoplankton found in Lake Taymyr consists mainly of diatoms of the genus Melosira, and it only grows for two months a year, reaching its maximum abundance in September, when temperatures are already beginning to fall but also when additional nutrients enter the lake with the autumn surge.

The quantity of organic material produced each year by the phytoplankton depends not only on the nutrient input but also on the depth of the water mass. In shallow pools that freeze solid in winter, active photosynthesis is only possible for two to three-and-a-half months in summer. In deeper lakes, however, photosynthetic activity may take place under the ice, and the growing season may last for eight or nine months. Thus, the annual production of the phytoplankton in shallow lakes is only 0.3-0.8 g C/[m.sup.2], while in lakes it is 3.6-8.5 g C/[m.sup.2]. Where there is an additional nutrient input (from natural sources, such as the excrement of bird colonies or because of human activity), production may increase substantially, reaching tens of g C/[m.sup.2].

In shallow waters, in addition to the phytoplankton, there are sometimes also microscopic algae attached to the bottom and forming a continuous layer that contributes significantly to the total primary production. In rapids, these algae that fix themselves to the stones of the bottom (epilithic algae) are the only primary producers and their production is limited by the low concentration of nutrients, mainly phosphorus. The regular supply of phosphorus-rich fertilizers creates up to a tenfold increase in production.

The zooplankton

The zooplankton of the tundra waters (here taken to include all the heterotrophic organisms that live in lakes and pools in the tundra, whether in suspension or fixed to the substrate) is highly varied and includes representatives of the same taxonomic groups as the waters of temperate areas; protoctists, rotifers, and some groups of crustaceans, such as filter-feeding branchiopods (fairy shrimps and water fleas), peracarids, and copepods.

The number of zooplankton species varies from one lake to another, basically as a function of climatic conditions. Although in some lakes in the Bolshezemelskaya tundra there are 12 species of rotifers, 22 species of cladocerans (water fleas) and 12 species of copepods, in the lakes of northern Alaska there is only a single species of water flea (D. middendorffiana), six species of copepod and a few species of rotifer, all of them scarce. The plankton of the island in Lake Taymyr is dominated by rotifers (such as Kellicottia longispina or Notholca striata) and copepods (Eurytemora lacustris, Eudiaptomus gracilis, Acanthocyclops vernalis, and others), but only two species of water flea are present, and they are not very abundant.

In shallow coastal lagoons that freeze in winter and fill with salt water in summer, some species of fairy shrimp (Anostraca) are common, such as Branchinecta paludosa and Polyartemiella hazeni. Some of the large tundra lakes, including Lake Taymyr, contain some species that are relicts from the glacial periods, such as the copepod Limnocalanus macrurus, the mysid Mysis relicta, and the amphipod Pontoporeia affinis. According to certain zoogeographical hypotheses, these forms now living in freshwater are of marine origin. When the glaciers retreated, the rising of the freed landmasses and the consequent retreat of the sea meant the dispersed populations of these species adapted to life in freshwater, and some have even been able to penetrate up the headwaters of the River Volga to the Caspian Sea.

The dominance of copepods in large northern lakes is easily explained, as unlike water fleas (cladocerans) they can feed successfully even in very dilute concentrations of planktonic algae. Copepods are also able to accumulate body reserves of energy-rich compounds that allow them to survive long periods without food. The cladocerans (Daphnia, for example) require more concentrated food, and without it they have trouble surviving. This is why the daphnias in the tundra tend to colonize small lakes or pools of both fresh and salt water. To survive the winter the females produce eggs with ephippia, a modification of the cuticle detached from the carapace that protects the eggs and whose special structure favors dispersal. This type of egg is only produced after fertilization of the females by the males. In temperate latitudes and during the reproductive period, Daphnia reproduces parthenogenetically, that is, without fertilization by a male gamete, several times. The males only appear at the beginning of the cold season, in the autumn. In Arctic latitudes, however, the females of some species of Daphnia (especially D. middendorffiana) start to lay resistant eggs (with ephippia) in the first generation and can do so without males. Sometimes, part of the population continues reproducing parthenogenetically, while others produce eggs with ephippia; this strategy ensures the survival of the species in case of a sudden cold spell or the death of all the adults. The daphnias that live in less severe conditions in the Bolshezemelskaya tundra produce three or four parthenogenetic generations in summer, while the males and the eggs with ephippia, are not produced until later.

The benthic fauna

Compared with temperate areas, the fauna of the bottom of tundra lakes is poor, both in terms of species diversity and abundance. For example, on the shores of Lake Taymyr, as the water level falls sharply in the autumn, and the coastal area freezes solid throughout the winter, there are almost no vascular plants and thus all the kinds of fauna that depend on them are also absent. Furthermore, the animals that live in the sediments of the coastal area are not very numerous either, and only increase in abundance at greater depths (20-26 ft [6-8 m]), where the water never freezes solid. The dominant benthic forms in this lake, as in many other tundra lakes, are chironomid larvae (e.g. orthocladinid midges), although there are also oligochaetes, nematodes and molluscs. The average benthic biomass in Lake Taymyr is about 1 g/[m.sup.2], while in lakes in the south of the Bolshezemelskaya tundra it exceeds 6 g/[m.sup.2].

Many benthic animals that live in the tundra waters, including chironomid larvae, can survive being frozen solid for long periods and then thawing. The growth of Chironomus larvae in the shallow lakes of northern Alaska takes seven years. These larvae are thus frozen solid for more than half the year about seven times in their lifetime. Each time, they thaw out successfully and continue growing during the short Arctic summer.

Many invertebrates live in the rivers of the tundra. In addition to the chironomid and caddis-fly larvae that are usually found in rivers with strong currents, other organisms are also very typical of these sites, such as blackfly larvae (simuliids), a major prey for invertebrate predators and for fish entering the river.

The fish fauna

Most bodies of water in the tundra have no fish, as they are so shallow that they freeze solid in winter. Yet in large pools, lakes and rivers, fish may be abundant, especially in more southerly regions. The number of fish species diminishes from north to south. In the rivers and lakes of the north of the Yamal Peninsula, for example, there are only seven species of fish, whereas the more southerly regions have twice as many species.

The composition of the fish fauna of the waters of the tundra is very unusual in comparison with that of other areas. There are, for example, no members of the carp family (cyprinids) so common in temperate latitudes, although several species of the salmon family (salmonids) are abundant.

In the southern area of the tundra, the fish species found include pike (Esox lucius), perch (Perca fluviatilis) and arctic grayling (Thymallus arcticus). Burbot (Lota lota) is found everywhere. Arctic chars (Salvelinus) have a circumpolar distribution and are found in the basins of all the rivers that flow into the Arctic Ocean. The anadromous forms are usually considered to be the original form from which the semianadromous and purely lacustrine forms are derived from.

The chars are essentially predators, whereas whitefish (Coregonus muksun, C. nasus, C. lavaretus, C. peled, and C. sardinella) mainly feed on benthic and planktonic invertebrates. Some whitefish, especially C. lavaretus, have a circumpolar distribution, with many subspecies (33 in Eurasia alone); other species, such as C. peled, are found only in central Eurasia, from the River Mezen in the north of European Russia, to the River Kolyma in northeast Yakutiya. C. peled is typically a predator of plankton and highly typical of tundra lakes, even small ones. C. peled spawns in late autumn, when the water temperature is little more than 32[degrees]F (0[degrees]C), and the eggs lie on the stones at the bottom. In the spring, they start growing and reach the rivers in the melt period, and in the case of lakes about one month before they melt.

The ecological plasticity of the ichtyofauna

In the waters of the tundra, in addition to the truly freshwater species, there are anadromous and semianadromous species. Anadromous species spend most of their life in the sea, where they grow and feed until they reach maturity, migrate to the headwaters of the rivers and their tributaries to spawn. These species include the Atlantic salmon (Salmo salar) and some populations of Arctic char (Salvelinus alpinus). The semianadromous species are euryhaline species that feed in the lower stretches of the rivers and in the brackish waters of the estuaries but return to the headwaters to spawn. These include the Arctic whitefish (Coregonus autumnalis) and some forms of the Siberian sturgeon (Acipenser baeri). In some cases, some species may have anadromous (or semi-anadromous) forms and others that live permanently in rivers and lakes. The exclusively freshwater species include the pike (Esox lucius), Gymnocephalus [=Acerina] cernua, Hucho taimen and the arctic grayling (Thymallus arcticus).

In any case, many fish of the tundra typically show great ecological plasticity. Thus, members of a single species may feed on plankton in some waters, exploit the benthos in others and be predators of other fish in other sites. It is difficult to imagine a similar situation in the tropics, where each of these ecological niches would be occupied by different species and, more often than not, by species of different genera. This ecological variability is related to their morphological characteristics, and this is the reason for the taxonomic debates over the classification of some groups of fish typical of the high latitudes. For example, some specialists distinguish more than 40 species of char (Salvelinus), whereas others consider there are only 3-5 true species and the rest are local intermediate forms.

18 Low clumps of bilberries (Vaccinium) and dwarf willows (Salix), here photographed in Denali National Park, Alaska and showing their

typical reddish autumn coloration, are characteristic of the tundra's woody vegetation and are among the tallest flowering plants found in the biome. Wind-blown ice limits the growth of plants that dare to rise above the carpet of mosses and other plants with similar growth forms (see figure 25). During the short summer, when the snow layer covering the vegetation melts, its flowers add a touch of color to the tundra landscape.

[Photo: Antoni Agelet]

19 Distribution of the number of plant species in the tundra, according to stem type, together with the percentage of the local flora they represent. For climatic reasons, the presence of procumbent plants in the tundra is greater than in other biomes.

[Source: data provided by the authors]

20 The thick snow cover is often a good refuge for the small, warm-blooded animals that live in the tundra. Ptarmigans (Lagopus) dig holes in the snow where they rest until they feel threatened; then they fly out rapidly through the exit opposite to the one they entered. Lemmings and other small mammals spend most of the winter in the burrows they dig in the snow, which they line with leaves and grasses. They also dig galleries to feed, as the habitats under the snow are the only place where they can find some plants to eat in the winter.

[Drawing: Jordi Corbera, from several sources]

21 Cotton grasses (Erio-phorum) form meadows in wet sites, as shown by this photo taken in the Kent Peninsula, Canada. In these wetland meadows, the silky fluff of the seeds carpets the whole area in white after the winter snow has melted. Cotton grasses produce perfectly viable flowers, fruits, and seeds but put more trust in their rhizomes to reproduce and disperse, like many other perennial tundra plants. As they are very common plants, they play a very important role in ecosystem functioning; their fleshy roots are one of the main foods of lemmings and other animals.

[Photo: John Eastcott & Yva Momatiuk / Planet Earth Pictures]

22 Fruticose, foliose, and crustose lichens in the Arctic tundra, from eastern Russia, Norway, and Northwest Territories (Canada). Lichens, as they are now known, are lichenized fungi, that is to say, fungi associated with photosynthetic bacteria or protoctists to form a compound organism with all the features of a fungus but also able to obtain energy from sunlight. In the tundra there is a wide diversity of species of lichens, because the plants that can adapt to this extremely harsh environment can spread and diversify greatly. The reindeer moss (Cladonia rangiferina, upper photo), which appears in the form of yellow patches, maintain fresh, juicy structures throughout the winter that can easily be bitten, chewed, and ruminated by reindeer (Rangifer tarandus) and musk-ox (Ovibos moschatus). Cetraria nivalis (center photo), shown under a thin layer of ice is able to reproduce vegetatively by fragmentation of its branches and these broken parts can take "root" by means of rhizoids or other fixing organs. Lichens grow very slowly, but crustose lichens (lower photo) eventually cover rocks and can be used to date them.

[Photos: Yuri Shibnev / Planet Earth Pictures; J.L. Klein & M.L. Hubert / Bios / Still Pictures, and John Eastcott & Yva Momatiuk / Planet Earth Pictures]

23 Peat-bog vegetation consists basically of sphagnum mosses. Sphagnum mosses acidify their environment, as they contain high levels of organic acids, and plants that grow in peat bogs have to adapt to this acid environment. When mosses (such as Dicranum elongatum, Sphagnum squarrosum, S. palustre, and Tomenthypnum nitens) have formed a more or less stable soil, some flowering plants can root in the peat bog. These include sundew Drosera rotundifolia, cross-leaved heath (Erica tetralix), and cotton grass (Eriophorum angustifolium). The lichens that are so abundant in the tundra are not present in peat bogs, because there is too much water for them (see figure 12).

[Drawing: Jordi Corbera, from several sources]

24 The plants of the tundra start flowering surprisingly early, as soon as the snow starts to melt, such as willow (Salix herbacea, lower photo), although some flower while there is still snow on the ground, such as the Arctic willow (S. arctica, upper photo). The upper branches may start flowering while the base is still covered with snow. The overwintering plants already bear fully formed flower buds, and as soon as the temperature approaches 32oF (0oC), the reserves are rapidly mobilized from the underground storage organs, allowing early flowering. This period of transition, which begins when the first snows bury a plant with flower buds and ends with the new abundant flower production. This had never been observed by humans until recently, as it is very difficult to carry out year-round observations owing to the harsh climatic conditions. The photos show specimens from Iceland (below) and Canada (above).

[Photos: B. & C. Alexander / NHPA and Eckhart Pott / Bruce Coleman Limited]

25 One very common plant community in Finland's tundra is formed by dwarf birch (Betula nana), identified by its reddish leaves in the photo; cowberry (Vaccinium vitis-idaea), shown with its red berries; and reindeer moss (Cladonia), seen in this photo taken in autumn. There are many different species of Vaccinium distributed throughout the colder countries of the northern hemisphere and far into the Arctic. Some are cultivated in large fields in northern Europe, Canada and America, but the fruit of other species is generally poisonous.

[Photo: Eero Murtomaki / NHPA]

26 As soon as spring arrives, the tundra is filled with color because the short growing season means all the plants flower at the same time. The large-flowered wintergreen (Pyrola grandiflora) bear spikes of pink flowers next to dryas (Dryas integrifolia), in Hudson Bay, Canada. The Latin name Pyrola is the diminutive of Pyrus, meaning a pear tree, and refers to the plants' pear-shaped leaves. The large-flowered wintergreen is the only species of the genus that is found in the tundra, where it is very widespread.

[Photo: Jeff Foott Productions / Bruce Coleman Limited]

27 The snowy owl (Nyctea scandiaca) is one of the few birds that lives permanently, both summer and winter, in the tundra. Nevertheless, when conditions are most adverse in northern regions, birds often wander southwards in search better conditions. Thus the snowy owl's distribution can change from one year to another and individuals have often been observed in Scotland and, in exceptionally bad years, as far south as France, Yugoslavia, Austria, and the northern United States.

[Photo: Francesc Muntada]

28 Most of the bird species endemic to the tundra are wildfowl (Anatidae). A perfect example is the long-tailed duck (Clangula hyemalis), a species that migrates far south in winter, shown in the photograph hidden in the vegetation on Ellesmere Island in the Canadian Arctic. Curiously, Arctic birds tend to be smaller than their southerly relatives, exactly the opposite of the situation in mammals.

[Photo: Stephen Krasemann / NHPA]

29 The ptarmigan (Lagopus mutus, above) in winter plumage, and the willow grouse (L. lagopus, below) in summer plumage, are very similar species. In the male, the black lores of the ptarmigan represent the only morphological difference in winter. In both species the overall plumage of the males is white in winter and patchy brown or gray in summer, although both the white wings and the black outer tail feathers, only visible in flight, are retained throughout the year. Both species have black bills. The main differences between the two species are in their habitats and behavior. The ptarmigan prefers montane tundra and is essentially a sedentary species. The willow grouse, on the other hand, spends the summer in the shrub tundra and in the tundra proper. It only sporadically reaches the northern Arctic tundra but never actually breeds there. At the end of the breeding season it forms large flocks to migrate southwards but almost never crosses the border between the tundra and the taiga.

[Photos: Hellio & Van Ingen / NHPA and Erwin & Peggy Bauer/ Auscape International]

30 The hooves of the ungulates that live in the tundra are perfectly adapted to soft surfaces. The musk-ox (Ovibos moschatus) and the reindeer (Rangifer tarandus) have broad, flat hooves with separate digits that allow them to move on snow in winter and over soft waterlogged soils during the spring thaw. When reindeer or musk-ox walk on a slippery surface or a soft patch of snow, they separate their digits to provide a broader base. The patches of stiff hairs clustered between the digits also help to maintain balance. In contrast, the hooves of ungulates that live on rocky mountain slopes, such as the Spanish ibex (Capra pyrenaica) or guanaco (Lama guanicoe), are deeper and narrower. The soles of their feet are softer and more elastic and adhere better to rocky surfaces. Furthermore, they have harder and more pointed hooves to brace themselves when walking and climbing in rocky areas (see also fig. 145).

[Drawing: Jordi Corbera from various sources]

31 The musk-ox (Ovibos moschatus) became extinct in Eurasia in the 18th century and in Alaska in the 19th century. By the beginning of the 20th century only very small populations remained in northeast Canada and Greenland. Nevertheless, from the mid-20th century onwards its range has been expanding rapidly owing to reintroduction programs carried out in Spitsbergen (1929), Alaska (1930), and the Taymyr Peninsula (1974). Today, the musk-ox also lives in the northernmost regions of mainland Canada, the islands of the Canadian Arctic, and northern Greenland. Herds of as many as a hundred animals form primarily in the summer (the photo shows a herd on Nunivak Island, Alaska), although most herds are normally much smaller and consist only of one or two dozen animals.

[Photo: Fred Bruemmer / Bruce Coleman Limited]

32 Dipteran flies are abundant in the tundra and may even make life intolerable: some, such as the bloodsucking forms, because of their painful bites and the diseases that they transmit; other dipterans, such as the chironomids shown in the photo, because of the dense swarms they form.

[Photo: Janos Jurka / Natur-fotograferna]

33 A completely white winter coat is a characteristic shared by almost all the mammals that live permanently in the tundra. While some species maintain their white winter coats in summer, most species molt into a darker coat at the beginning of spring. The Arctic fox (Alopex lagopus), for example, molts into a white coat by October which it maintains until the following April when it grows its dark summer coat. Individuals in more southern regions where the climate is not so harsh keep their dark coats throughout the winter (see also figure 55).

[Photo: Erwin & Peggy Bauer / Auscape International]

34 Thermal gradient in the coat of an Arctic fox (Alopex lagopus) and reindeer (Rangifer tarandus). Many large terrestrial Arctic mammals have thick hairy coats (between 1 and 3 in [30 and 70 mm]). They consist of fine but dense hairs that isolate the animal very effectively from the cold by preventing the turnover of the layer of warm air in contact with the body. The thermal gradient that is established between the body temperature (at the surface of the skin) and the ambient temperature (the air) reduces heat loss from the body to the exterior.

[Diagram: Jordi Corbera, based on Davenport, 1992]

35 In spring, reindeer or caribou (Rangifer tarandus) migrate in huge herds from the forests where they winter to the pastures of the tundra. Southern populations migrate only a few hundred kilometers, but the most northern populations often travel over 621 mi (1,000 km), at an average daily speed of between 12 and 34 mi (19 and 55 km) per day. They are the longest recorded migrations by any terrestrial mammal. The calves are born on the journey north, and within a few hours they can run well enough to keep up with the rest of the herd. The enormous energy expenditure of this migration is compensated for by the high-quality spring pastures in the tundra.

[Photo: Kennan Ward / Natural Science Photos]

36 The periodic mass migrations of lemmings (Lemmus, Dicrostonyx, etc.) are legendary, although stories of migrations ending in mass suicide are generally incorrect. Nevertheless, it is true that roughly every three or four years lemming populations boom. As the tundra does not offer a wide range of foodstuffs, any increase or decrease in the food supply has direct repercussions on lemmings and all other species in the biome. Lemmings, however, show much more extreme population variations than other species because they can reproduce very quickly. Lack of food, climatic factors and, to a lesser extent, predators, normally keep lemming populations in check. However, a particularly good year in the tundra can lead to a rapid increase in lemming numbers and, when certain critical population levels are reached, large numbers of lemmings disperse. Most individuals die on these movements, and populations return to their normal levels. The years in which these migrations occur are known as lemming years: the photograph was taken in October of the lemming year of 1963.

[Photo: Goran Hansson / Naturfotograferna]

37 In the southeast of the Taymyr Peninsula, in northern Siberia, temperatures almost never rise above 39.241oF (4-5[degrees]C), and in summer it is not uncommon for temperatures to fall below 32[degrees]F (0[degrees]C) and for snow to fall. In these conditions, the ice in the region's many lakes and pools never melts completely, and intense cryogenic action gives rise to polygonal soils, like those that can be seen on the center-right of the photo. These severe conditions, however, have not eliminated plant and animal life. The planktonic organisms that pass the winter in a latent state populate the waters of the lakes, and there are even fish that live under the permanent ice layer in the deeper lakes. The surface layer is inhabited by unicellular algae, worms, and insect larvae, while lichens, mushrooms, mosses, and herbaceous plants grow on its surface. Some of them form small clumps or cushions that hug the soil (see also figures 7 and 118).

[Photo: Peter Prokosch / WWF / Still Pictures]

38 The peracarid Mysis relicta is present in some large lakes in the tundra, and is the only freshwater member of the order Mysidacea. It can be recognised by its typical humpback appearance. The illustration shows a male with the typical highly developed abdominal appendages (pleo-pods).

[Drawing: Jordi Corbera]

39 The Arctic char (Salvelinus alpinus) is a typical species of cold waters that can live in lakes, such as these specimens in Lake Uniakovik in northern Labrador, and in rivers. The river specimens are migratory and spend the summer in the sea; at the end of autumn or the beginning of winter they return upstream to spawn. The sedentary populations in lakes spawn in both spring and autumn.

[Photo: G. van Ryckevorsel / Planet Earth Pictures]

* In accordance with the taxonomic system adopted in this work (see volume 1, page 175), fungi and lichens, organisms common throughout the tundra, are not plants but belong to a different kingdom, the fungi. Bearing in mind their importance in the tundra landscape (especially lichenized fungi, or lichens), and unlike other volumes where they are not discussed or only briefly mentioned in the section on the flora and plant cover, the title and content of this chapter refer to the mycota, that is, the fungal population, represented basically in this case by lichens.

** The taxonomic system adopted in this book is based on the division of living beings into five kingdoms (see volume 1, page 75) proposed by Robert H. Whittaker in 1959, in accordance with which most phytoplankton organisms are not plants (as the name suggests) but protoctists. But the traditional term phytoplankton is still valid to designate the planktonic primary producers, despite the taxonomic inexactitude of its etymological meaning of "plant plankton." The same is true of the term zooplankton, which also includes many protoctists, although in this case the proportion of true animals is high.
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