The characteristic communities of plants and animals that are found in different regions of the world are known as biomes. The differences between biomes are not necessarily related to the taxonomic classification of the organisms they contain, but rather to the life-form (the form, structure, habits, and type of life- history of the organism in response to its environment) of their plants and animals. This concept of the life-form was first put forward by the Danish botanist Christen Raunkaier in 1903. He observed that the most common or dominant types of plants in a climatic region had a form well suited to survive in prevailing conditions. Thus in Arctic conditions, the most common plants are dwarf shrubs and other low-growing plants; these have no extensive above-ground growth that would be broken by heavy winter snowfalls, and their buds are carried at or just below the surface of the soil where they obtain the maximum protection from cold and wind in the long winter. In warmer climates, the characteristic types of vegetation are trees or tall shrubs that carry their buds and reproductive structures well above the ground because they are rarely exposed to severe weather conditions. Deserts usually contain small plants, mostly quick- growing annuals, with little above-ground growth, and buds and survival structures below the soil surface, because of the risk of drought. Animals also show distinct life-forms adapted to different climates, with cold-resistant, seasonal, or hibernating forms in cold regions and forms with drought-resistant skins or cuticles in deserts. Nevertheless, animal life-forms are usually far less easy to recognize than are those of plants and, consequently, most biomes are distinguished by the plants they contain and are named after their dominant life-form.
Tundra is found around the Arctic Circle, north of the tree-line. Smaller areas occur in the Southern Hemisphere on sub- Antarctic islands. Alpine tundra occurs above the tree-line on high mountains, including those in the tropics. It is the most continuous of biomes and the easiest to define. Winter temperatures are ~57°c or lower: water melts at the soil surface in summer (air temperature is rarely over I5°c) but there is always a permanent layer of frozen soil underneath—the permafrost. There is a very short growing season, and only cold- tolerant plants can survive. Typical plants are mosses, lichens, sedges, and dwarf trees. Large herbivores include reindeer, caribou, and musk ox. Small herbivores include snow-shoe hares, lemmings, and voles. Many birds migrate there from the south in summer, feeding on the large insect populations in the tundra during that season. Carnivores are Arctic fox, wolves, hawks, falcons, and owls.

Northern Coniferous Forest (Taiga) forms an almost unbroken belt across the whole of northern North America and Eurasia—and is one of the most extensive biomes. Its northern margin with the tundra is sharp, being the Arctic tree-line, but its southern limit is less definite —taiga is also found on high mountains in lower latitudes, such as the southern Rockies. In the northern forests, the winters are long and cold, the summers short and often very warm. The soil in winter is mostly frozen to a depth of about 2 metres, but thick snow cover can keep soil temperatures as high as - 7°c. Trees are mostly evergreen conifers, able to photosynthesize all year and to resist drought (a result of strong winds and extreme cold) with their needle- shaped waxy leaves. They remain undamaged by snowfalls because of their overall shape. Taiga usually contains vast tracts of one or two tree species only, except along rivers. The soil is podsol and invariably contains a layer of ash- white sand, due to the leaching-out of bases and clays by humic acids (organic acids produced by the decay of plant material). Animals in this biome are limited by severe winters and the small number of different habitats. The most important large herbivores are deer—more species live here than in any other biome. Rodents are plentiful and can burrow under snow and survive harsh winters. Carnivores include wolves, lynxes, wolverines, weasels, mink, and sable; omnivorous bears are also found. Birds either are adapted to feeding in taiga, such as crossbill, or are summer migrants feeding on the vast seasonal swarms of insects.

Temperate forest Vast tracts of the taiga are still in the natural climax state, but little climax forest remains in the temperate forest biome.

There are 4 basic types of temperate forest, (i) Mixed forest of conifers and broad-leaf deciduous trees. This was the original climax vegetation of much of north-central Europe, eastern Asia, and north-east North America—little remains today. (2) Mixed forests of conifers and broad- leaf evergreens. This once covered much of the Mediterranean lands but very little is left. It still occurs in the Southern Hemisphere, in Chile, New Zealand, Tasmania, and South Africa. (3) Broad-leaf forests almost entirely of deciduous trees. This formerly covered much of Europe, northern Asia, and eastern North America, and is found in the Southern Hemisphere only in Patagonia. (4) The rare broad-leaf forest consisting almost entirely of evergreens. This occurs throughout much of Florida, and also in north-east Mexico and in Japan. In the Southern Hemisphere it occurs on the southern tip of South Island, New Zealand. All these regions have very high rainfall, and the dripping forests have been termed "temperate rain-forests". In all temperate forests, there is frequently an understorey of saplings, shrubs, and tall herbs, which is particularly well- developed near the forest edge or where human interference has occurred. Temperate forests have warm summers but cold winters, except on western seaboards. Winter temperatures may fall below freezing-point. The deciduous trees escape these cold winters by losing their leaves; many plants have underground over- wintering organs. The fauna includes bears, wild boar, badgers, squirrels, woodchucks, many insectivores, and rodents. Predators include wolves and wild cats (on the decline), red foxes, and owls. Large herbivores are the deer. This biome is extremely rich in bird species, especially woodpeckers, titmice, thrushes, warblers, and finches.

Tropical rain-forestoccurs between the Tropics of Cancer and Capricorn in areas where temperatures and light intensity are always high and rainfall is greater than 200 cm a year (and is at least 12 cm in the driest month). Because of this, there is a great variety of trees: in some parts of the Brazilian rain-forests, there are as many as 300 species of trees in 2 sq. km. The popular image of the jungle—thick, steamy, and impenetrable—is borne out only in those areas that man has at some time cleared, especially along river margins; true climax tropical forest has very little undergrowth. The canopy is extremely dense; the light intensity below may be as low as i per cent of that above, and thus only a few extremely shade- tolerant plants can survive there. Life is concentrated in the canopy, where there is plenty of light. The crowns of the trees are covered with epiphytes—plants that use the trees only for support and are not parasites. Lianas—vines rooted in the ground but with leaves and flowers in the canopy—are also characteristic. Dead plants are rapidly decomposed, so there is little undecayed plant matter on the forest floor. The rate of turnover of nutrients is very high and the tropical forest has a higher productivity than that of any other terrestrial biome. The tropical rain forest biome contains the greatest variety of animal life of any biome, because of the richness of the food resources that it offers and the relative constancy of the conditions of the environment through the year. There is a great profusion of birds with many different diets—seeds, fruit, buds, nectar or insects. Many of the mammals are adapted to arboreal life (monkeys, sloths, ant-eaters, many small carnivores) but there are also many ground-living forms, including rodents, deer and peccaries. Amphibia, and reptiles, especially snakes, are important as predators of small vertebrates and invertebrates.

Temperate grassland occurs in regions where rainfall is intermediate between those of desert and of temperate forest, and where there is fairly long dry season. Temperate grassland has many local names—the prairies of North America, the steppes of Eurasia, the pampas of South America, and the veld of South Africa—but the dominant plants in all of them are the grasses, the most widespread and successful group of land plants. The soil always contains a thick layer of humus, unlike forest soils, but is more exposed than the latter, and therefore more likely to dry out. The dominant animals are large grazing mammals—on the North American prairies, vast herds of bison and prong-horn (which man had virtually wiped out by the close of the last century, but is now reintroducing); over the steppes of Eurasia, the saiga antelope, wild horse, and wild ass once roamed in herds; in the South American pampas, the natural grazer is the guanaco; and in Australia, the kangaroos fill this role. All these have been largely replaced by man with domestic grazing animals, often with disastrous results, as we shall see in Chapter 8, although grasses are adapted to withstand the effects of natural grazing.

Tropical grassland or savannah is a term applied to any tropical vegetation ranging from pure grassland to woodland with much grass. It covers a wide belt on either side of the Equator between the Tropics of Cancer and Capricorn. The climate is always very warm and there is a long dry season, and thus the plants often have drought-resisting features. The grass is much longer than that of temperate grassland, growing to 3 metres. There is often a great variety of trees, which also show drought-resisting features; a typical group is the acacias. The dominant animals are large grazing mammals, the African savannah having the greatest variety, and burrowing rodents are also found. Large carnivores, such as lions and hyenas, prey on the grazers.
Chaparral occurs where there are mild wet winters and pronounced summer droughts (known as Mediterranean climate), and in areas with less rain than grasslands. The vegetation is sclerophyllous (hard-leaf) scrub of low-growing woody plants, mainly evergreen, with hard, thick, waxy leaves—adaptations to drought. In the Northern Hemisphere it occurs mainly in countries fringing the Mediterranean basin, but also in north-west Mexico and California. Formerly this biome had a varied flora and fauna, with many herbivores such as ground squirrels, deer, and elk, and mountain lions and wolves as their predators, but this has been greatly reduced by man. In the Southern Hemisphere there are small areas of chaparral in southern Australia, southern Chile, and South Africa.

Deserts are areas experiencing extreme drought. A good definition is those areas where rainfall is less than 25 cm per year, or—if higher—is mostly lost immediately by evaporation. Deserts can be divided into hot deserts (such as the Sahara) with very high daytime temperatures, often over 5O°c, and low night-time temperatures below 2O°c with relatively mild winters, and cold deserts (such as the Gobi Desert in Mongolia) with very severe winters and long periods of extreme cold. Typical desert has large areas of barren rock or sand and very sparse vegetation. Desert plants are adapted to drought in various ways: some have drought-resistant seeds; others have small thick leaves that are shed in dry periods; yet others, such as the New World cacti, are succulents, storing water in their stems. Desert animals are mostly small enough to hide under stones or in burrows during the intense daytime heat in hot deserts. Certain rodents are well adapted to desert life—they live in cool burrows, are largely nocturnal, and waste very little water in their urine. Insects and reptiles lose little water, having waterproof skins and excreting almost dry, crystalline urine. Deserts spread when wind carries the top sand away, or when man encourages his domestic animals to overgraze their edges.

Freshwater biomes are far less self-contained than those of the surrounding land or the open sea. They receive a continual supply of nutrients from the land, but much of this is washed downstream in the rivers and there is an overall loss of organic material. Thus they are generally far less rich in nutrients than oceans, and usually less productive than either sea or land environments. They are more changeable than ocean or land biomes; rivers gradually wear away the land through which they pass and thus the river biome itself gradually changes, and many small ponds are seasonal, drying up in summer. There is a wide range of freshwater environments ranging from small ponds and streams to vast lakes and wide rivers. At the lower end of the scale, they are often better considered merely as a wet extension of the surrounding terrestrial biome.

The dominant plants of larger lakes and slow rivers are phytoplankton, but larger floating and rooted plants cover considerable areas. Many of the animals are restricted to the freshwater habitat; amphibians, though living on land, need fresh water in which to breed; land animals use fresh water for drinking and bathing; and many birds are adapted to the freshwater habitats. Animal communities in large lakes correspond to planktonic, nektonic, and benthic communities of the oceanic biome (see below). Some large lakes have well-defined shores, constituting sub-biomes: examples are the Great Lakes with their dune systems, or Lake Victoria with its muddy shores. Marshes (salt marshes and freshwater marshes) are best considered as intermediate between marine or freshwater biomes and the surrounding terrestrial biome, and estuaries are transitional both between freshwater and marine biomes and also between the water and the land. They have a very complex structure and are highly productive, with a great variety of plant and animal life. Freshwater habitats have suffered greatly from pollution by man— toxic industrial wastes, detergents, and vast quantities of sewage are dumped into rivers and lakes, and cause the extinction of all but a few resistant forms of life.

Marine biomes Land covers only 29 per cent of the earth's surface, whereas the oceans take up 71 per cent, with an average depth of 3900 metres. It is impossible to distinguish regional biomes in the seas, because of the uniformity of the marine environment and of the ease of distribution of its inhabitants. On the land, animals and plants of different latitudes have different life-forms; in the sea, animals do have distinctive forms, but these vary according to the depth at which they live, rather than according to latitude—for example, deep-sea animals are of a life- form especially adapted to cope with high pressures and total darkness. Water has a higher specific heat than soil or rock, and even the warmest oceans never reach the high temperatures of tropical forests or hot deserts. Similarly, the coldest seas are never as cold as the tundra or northern forests. The surface temperature is never greater than 3o°c and rarely falls below o°c. Marine organisms obviously have no problems in obtaining sufficient water, but light is a limiting factor. The tiny photosynthetic plants (phytoplanktori) are restricted to the upper photic zone (the uppermost 200 metres); virtually no light penetrates below 500 metres. Atmospheric oxygen and carbon dioxide are plentiful at the surface and these gases are also dissolved in the water. Pressure is an important factor limiting the downward movement of shallow-water species. Sea-water is much richer in nutrients than fresh water, and these are recycled to the photic zone by upwellings of deep currents. In some other areas, surface waters converge and descend. Where these are already exhausted of nutrients, the area of descent forms a "desert", such as the Sargasso Sea in the southern North Atlantic. Such areas are the only virtually unproductive parts of the surface waters.

There are three principal marine biomes. (i) The oceanic biome of open water, away from the immediate influence of the shore. This is further divided into the planktonic sub-biome containing free-floating plankton (mostly microscopic organisms with buoyancy mechanisms); the nektonic sub-biome of active swimmers, including fish, squids, turtles, and marine mammals; and the benthic sub-biome, whose fauna is especially adapted for life on the sea floor. (2) The rocky shore biome, dominated by large algae that show zonation up the shore (see Fig. 21). Life here is in constant danger of desiccation when uncovered by the water. (3) The muddy or sandy shore biome where mud and sand is constantly being washed ashore by the sea, and the animals are often in danger of being buried. The main plants are thin green algae growing in flat sheets on the shore, such as the sea-lettuce (Ulva). Animals include burrowing worms, and also wading birds, which are important predators of the invertebrate fauna.

Soil The distribution of soils is another important aspect of bio- geography. Terrestrial plants are mostly rooted in the soil and obtain from it water and their nutrients, such as nitrates and phosphates. Because all animals are ultimately dependent on plants for food, they are also in turn dependent on the soil. Many factors influence the ability of plants to root and take up nutrients from the soil. Most important are the structure of the soil and its texture (the size of the particles of which it is composed), the amount of nutrients actually present, and the quantity of water with gases dissolved in it, and air spaces, that it contains. These are different in different types of soil. It is not surprising, therefore that soil has a strong influence on the distribution of living things. However, the relationship between organisms and soils is complex because the soil is largely produced by interaction between the organisms that live in it and the local climate.

Habitats and microhabitats

In the British Isles, the badger is most common in southern and western England and rarest in East Anglia and parts of Ireland and Scotland. Where they occur, badgers most frequently make their burrows orsetts in woods and copses, especially where these give easy access to the pastureland where badgers often feed, and where the soil is well-drained and suitable for digging. The biologist calls these places where the badger lives its habitat. Nearly all animal and plant species seem to have such recognizable habitats where they are found more often than in other places.

For many organisms, especially larger ones, distribution can be conveniently considered in terms of such units of habitat as "woodland," "grassland," or "seashore." But most species have specific distributions even within such units of the environment as the habitat. The woodland habitat, for instance, consists of a host of smaller microhabitats—the humus and leaf-litter layer of the soil, rotting logs, the ground flora zone, the various levels of the tree canopy, tree trunks, and beneath the bark of living trees. Certain characteristic species of animals and plants are found in each of these microhabitats, and so the distribution of these species in woodland coincides more or less closely with that of their microhabitats. Some species are found in more than one of these areas, but generally each species has a particular microhabitat that may be termed its "headquarters," in which it occurs most frequently and in the highest numbers. Even within habitats that are simpler in structure than woodland—such as grassland—many different microhabitats occur and, as will be explained later, the number of microhabitats present is an important factor determining the number of species that may live in a habitat.

Many quite large and active animals show a tendency to confine themselves to certain parts of a large habitat. The spider monkeys (Ateles') of the lowland forests of central and northern South America are active climbers, able to jump long distances, and family groups move about the forest a great deal. If the monkeys are observed over a period of time, however, they can be seen to spend most of their time in the lower parts of the high canopy of the forest, and especially on the smaller peripheral branches of the trees. The reason for this is clear; over 90 per cent of the diet of spider monkeys consists of fruit and nuts, and these food resources will naturally be found most abundantly on the smaller, fast-growing parts of the trees. It is probably best to call areas like these, preferred by a largish, active animal, minor habitats, rather than microhabitats, and to keep the latter term for subdivisions with more clearly defined boundaries.

Limits of distribution

Surrounding the areas of a species' distribution, whether this is considered on a geographical, habitat, or microhabitat scale, are areas where the species cannot maintain a population because physical conditions or lack of food resources are too extreme to permit survival. These areas can be viewed as barriersthat must be crossed by the species if it is to disperse to other favourable., but as yet uncolonized, places—much as the European settlers had to cross ocean barriers to colonize North America or Australia. Any climatic or topographic factor, or combination of factors, may provide a barrier to the distribution of an organism. For example, the problems of locomotion or of obtaining oxygen and food are quite different in water and air. As a result, organisms which are adapted for life on land are unable to cross oceans: their eventual death will be due, in varying proportions, to drowning, to starvation, to exhaustion and to lack of fresh water to drink. Similarly, land is a barrier to organisms which are adapted to life in sea or fresh water, because they require supplies of oxygen dissolved in water rather than as an atmospheric gas, and because they desiccate rapidly in air. Mountain ranges, too, form effective barriers to dispersal because they present extremes of cold too great for many organisms. The amount of rainfall, the rate of evaporation of water from the soil surface, and light intensity are all critical factors limiting the distribution of most green rooted plants. But in all these cases, and in most others, the ultimate barriers are not the hostile factors of the environment but the species' own physiology, which has become adapted to a limited range of environmental conditions. In its distribution a species is therefore the prisoner of its own evolutionary history.

At the habitat level, the microhabitats of organisms are surrounded by areas of small-scale variation of physical conditions, or microclimates —similar, but on a much smaller scale, to geographical variations in climate—and of food distribution. These form barriers restricting species to their microhabitats. The insects that live in rotting logs, for instance, are adapted by their evolution to a microhabitat with a high water content, and relatively constant temperatures. The logs provide the soft woody materials and micro-organisms they need for food, and also give good protection from predators. Around the logs are areas with fewer, or none, of these desirable qualities, and attempts by the animals to leave their microhabitat would result for many of them in death by desiccation, starvation, or predation.

Overcoming the barriers
A few inhabitants of rotting logs do occasionally make the dangerous journey from one log to another, and this shows that very few environmental factors are absolute barriers to the dispersal of organisms and that they vary greatly in their effectiveness. Most habitats and microhabitats have only limited resources, and the organisms living in them must have mechanisms enabling them to find new habitats and resources when the old ones become exhausted. These mechanisms often take the form of seeds, resistant stages, or—as in the case of the insects of the rotting-log microhabitat—flying adults with a fairly high resistance to desiccation.

Relatively new volcanic islands, such as the Galapagos have been eventually colonised as a result of dispersion by air and sea Human transport of species across the world either deliberately or inadvertantly has overcome many former continental barriers to species.