biome

(bī'ōm')
n.
A major regional or global biotic community, such as a grassland or desert, characterized chiefly by the dominant forms of plant life and the prevailing climate.

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Concept

On a political map of the world, Earth is divided into countries, of which there are almost 200. But nature, of course, knows no national boundaries, and therefore the natural divisions of the planet are quite different from those agreed upon by humans. While continents are a useful concept to geographers and earth scientists, in the worlds of biology, ecology, and biogeography, the concept of a biome makes much more sense. There are more than a dozen basic terrestrial and aquatic biomes or ecosystems, including boreal coniferous forests, deserts, tundra, and underwater environments. Each is a distinct "world" unto itself, with characteristic forms of plant life as well as animal species that congregate around the plants for food or shelter or both. Combined with these features of the biological community are aspects of the inorganic realm that likewise define a biome, for instance, climate and the availability of water.

How It Works

Ecosystems, Biomes, and Biological Communities

An ecosystem is a community of interdependent organisms along with the inorganic components of their environment, including water, soil, and air. Earth is the largest ecosystem, divided into biomes, large areas with similar climate and vegetation. A biome is a large ecosystem, extending over a wide geographic region, characterized by certain dominant life-forms—most notably, trees or the lack thereof. There are two basic varieties of biome: terrestrial, or land-based (of which there are six), and aquatic. The second of these types is divided further into marine and freshwater biomes.

Within a biome or ecosystem, the sum of all living organisms is referred to as the biological community. Sometimes the term biota, which refers to all flora and fauna (plants and animals) in a region, is used instead. Thus, biological community is a larger concept, since it includes microorganisms, which are vital to the functioning of the food web. The food web, which may be thought of as an interconnected network of food chains, is the means by which energy is transferred through a biological community. Without microorganisms known as decomposers, a key link in the food web would be missing. (See Food Webs for more on this subject.)

Succession and Climax

Over the course of time, ecosystems experience a process known as succession, the progressive replacement of one biological community by another. This is rather like the series of changes one might witness if one were to record the activity on a major city block over the space of a few decades, as stores come in and shut down and buildings are erected and demolished. In the case of biological succession, a process akin to natural selection (see Evolution) is occurring: the ecosystem becomes home, in turn, to a number of different biological communities until (in the absence of outside interference) the one that is most suited or adapted to local conditions finally takes root. (That is, until it is replaced, and the process of succession continues.)

This most suited or adapted biological community is described as a climax community, one that has reached a stable point as a result of ongoing succession. In such a situation, the community is at equilibrium with environmental conditions, and conditions are stable, such that the biota experiences little change thereafter. The most significant forms of climax vegetation are often the defining characteristics of terrestrial biomes. (See Succession and Climax for more about this subject.)

Defining Characteristics of a Biome

The boreal coniferous forest often is cited by biogeographers as a classic example of a biome, for a number of reasons. First, like most other terrestrial biomes, this one is defined by specific latitudinal positions: the term boreal means "northern," and these forests exist between 50 and 60 degrees north latitude. (Aside from the southernmost tip of South America and a few scattered islands, there is no significant landmass between 50 and 60 degrees south latitude.)

In North America the region between 50 and 60 degrees north latitude is the southerly band of Canadian provinces (Alberta and Saskatchewan, for example). The Eurasian equivalent of this region is a band encompassing the British Isles; an area of continental Europe that includes northern Germany, Poland, and southern Sweden; and a vast swath that spans the width of Russia from Saint Petersburg and Moscow in the west across the nation's wide expanse (10 time zones) to the Kamchatka peninsula north of Japan.

The boreal coniferous forest thus illustrates a key fact about biomes: they can occur in widely separated geographic regions as long as the environmental conditions are the same. In each of these locales average temperatures are low; summers are short, moist, and of moderate warmth; and winters are long, cold, and dry. Most precipitation is in the form of snow, and the A horizon of the soil, home of the organic material in which plants grow, is thin. Moreover, the soil is acidic and poor in nutrients. (See The Biosphere for more about soil.)

Most of the information conveyed in the preceding paragraph refers to the inorganic components of the boreal coniferous forest. (Organic does not necessarily mean "living," but it does refer to carbon-based chemical compounds other than carbonates, which are rocks, and carbon oxides, such as carbon dioxide.) As noted earlier, inorganic components of a biome include water and air, which in turn are involved in precipitation, weather, and climate. Although it does contain organic compounds from the decayed vegetable and animal matter that enriches it, soil, too, is largely inorganic, being formed from the weathering of rocks.

Flora

Biomes are differentiated most clearly, however, on the basis of their organic components. The second term in the phrase boreal coniferous refers to a type of plant that produces cones containing seeds. Thus, the dominant plant life in the boreal coniferous forest includes evergreen conifers that can tolerate cold weather: pine, fir, and spruce.

The varieties that dominate may differ between geographic regions, however. The boreal coniferous forests of northeastern North America, for instance, are dominated by black spruce, while those in the northwest are characterized by stands of white spruce. In northeast Europe, Norway spruce is dominant, while species of pine and larch occupy the key positions in the forests of Siberia. Despite these differences in dominant species, the conditions are much the same, not only in terms of inorganic environment but also with regard to flora and fauna. In most boreal coniferous forests, the canopy or upper layer is so thick that it allows little light through. The result is that the understory, or lower layers of vegetation, is very limited.

Fauna

As for animal life, species in the boreal coniferous forest include bear, moose, wolf, lynx, deer, weasels, rabbits, beavers, and chipmunks. With a few local variations, this roster of animal life is typical in most such biomes, whether in British Columbia or western Europe or Siberia.

A biome constitutes a complex network of interactions among plants, animals, and their surroundings, such that certain animals depend, either directly or indirectly, on certain plants for their sustenance. An obvious example is the beaver's use of coniferous tree limbs and even trunks for building shelter. Even more fundamental to the functioning of ecosystems is the role of plants as food.

Although few animals actually feed off the needles or bark of conifers, they do eat from these trees in more indirect ways. The woodpecker, for instance, consumes bugs that live in a tree's bark. Then there are the many insects that live off conifer seeds (see Ecosystems and Ecology for a discussion of conifer, or gymnosperm, reproduction), and these bugs, in turn, serve as food for birds, which are the prey of larger carnivores. Furthermore, though the understory in boreal coniferous forests is not dense, it provides enough vegetation to meet the needs of deer, rabbits, and other herbivores.

Classifying Biomes

Earlier it was stated that there are "almost" 200 countries on Earth. It might seem strange that something like the number of countries could be so inexact, when it would seem to be a matter of very exact quantities, like the number of states in the United States. But defining sovereign nations is a bit more challenging. Obviously, the United States, Switzerland, and Japan are sovereign nations, but many another political entity exists in a gray area.

If the number of independent nations on Earth is so open to question, it would stand to reason that the number of basic biomes is as well. After all, nations typically are delineated by such things as borders, seats at the United Nations, currency, and so forth, whereas the boundaries between biomes are much less exact. Therefore, it would be futile to attempt to say exactly how many biomes there are on Earth, since the number varies according to interpretation.

Terrestrial, Aquatic, and Other Categories

One of the more useful methods for classifying biomes is that of the American ecologist Eugene Pleasants Odum (1913-), introduced in his Fundamentals of Ecology (1953). The classification scheme that follows is based on that of Odum, who divided biomes into terrestrial and aquatic. In the present context, biomes have been grouped into five categories: forest, nonforest, freshwater, marine, and anthropogenic. The last of these categories refers to biomes strongly influenced by humans and their activities, though it should be noted that to some degree at least, human activities have influenced all of Earth's biomes. For example, many organisms carry in their fat cells trace amounts of human-manufactured contaminants, such as DDT. (See Food Webs for more on this subject.)

Biomes are organized here in such a way as to take into account their relative latitudes and corresponding climate. (Distinctions of latitude and climate are mostly relevant where terrestrial biomes are concerned.) As with biomes, there are many possible climate zones, particularly when rainfall patterns and other variables are considered. All climate zones, however, fall into one of three basic categories: tropical and subtropical, temperate, and polar and subpolar.

The first of these categories is a term comprising the region along the equator, extending north and south by about 30 degrees in either direction. In North America this would include southern Florida, Texas, and Louisiana. Temperate zones reach from about 30 to 60 degrees on either side of the equator, thus taking in most of the United States and southern Canada. Finally, subpolar and polar regions lie between 60 degrees and the poles, which are at 90 degrees.

Real-Life Applications

Forest Biomes

The term forest, as used in the realm of ecology, is one of those rare words that means the same thing within a scientific context as it does in the everyday world. An ecologist or biogeographer would define forest in more or less the same way that a nonscientist would: as any ecosystem dominated by tree-size woody plants. Of course, numerous other characteristics and parameters, such as weather, altitude, and dominant species, further characterize types of forests.

Boreal Coniferous Forests

Starting with the most northerly of forest biomes, there is the boreal coniferous forest, which we have discussed. Called taiga in Russia, boreal coniferous forests often are bordered on the north by tundra, discussed later in the context of nonforest ecosystems. An important subset of the boreal coniferous grouping is the montane forest, which also is dominated by conifers but which most often is found on mountains, at subalpine altitudes where the climate is cool and moist.

In addition to the dominant conifers, boreal coniferous forests also have important broad-leafed angiosperms (plants that flower during sexual reproduction), including aspen, birch, poplar, and willow species. These forests are typically subject to periodic catastrophes, which result in at least partial destruction of the dominant trees within stands if not across a given forest as a whole. Among these catastrophic events are wildfires as well as defoliation by such pests as the spruce budworm.

Temperate Deciduous Forests

Moving farther away from the poles, the next major forest biome is that of the temperate deciduous forest. The average American, especially on the East Coast, is likely to be more familiar with the temperate deciduous forest than with any other biome. This type of forest develops in a climate that is relatively moist, with winters that are fairly cold. The larger grouping of temperate deciduous forests is divided into smaller categories depending on the relative amount of annual rainfall.

The term deciduous refers to a tree that sheds its leaves seasonally, and these forests are dominated by such trees, broad-leafed species that include ash, basswood, birch, cherry, chestnut, dogwood, elm, hickory, magnolia, maple, oak, and walnut, among others. (Note that many of these species are angiosperms. Likewise, most coniferous trees are gymnosperms, or plants that reproduce sexually through exposed seeds as opposed to seeds hidden in a flower.) Among the varieties of animal life are squirrels, rabbits, skunks, opossums, deer, bobcat, timber wolves, foxes, and black bears.

Temperate Rain Forests

Temperate rain forests are not necessarily farther from the poles than temperate deciduous forests, but they are subject to milder winters. For example, the temperate rain forests of Washington State are north of many a temperate deciduous forest on the East Coast, but owing to differences in climate patterns, they are subject to milder winters than those typical of the deciduous forests to the east.

Characterized by abundant precipitation (most of it rain rather than snow, due to the milder temperatures), these systems are very moist—as the "rain forest" in their name implies. This, in turn, means that they are seldom subject to catastrophic wildfires, and therefore they often attain the climax stage of old-growth forests. In the temperate rain forest, coniferous trees are dominant, and many of these trees are extremely large and old. Among the tree species typical of this biome are Douglas fir, hemlock, cedar, redwood, spruce, and yellow cypress.

Tropical forests are discussed at some length in the essays Ecosystems and Ecology as well as The Biosphere. Among the two most basic varieties of this biome are semi-evergreen tropical forests and evergreen tropical rain forests. Most of the Amazon rain forest in South America, for instance, is an evergreen tropical forest, while surrounding biomes are semi-evergreen. Much the same is true of biomes in central and southern Africa, such as that surrounding the Congo River, with evergreen forests closest to the river and semi-evergreen ones in nearby areas.

The tropics, in general, are characterized not by the four seasons of the more temperate climate zones, but by a dry season and a wet season. The environment of a semi-evergreen tropical forest is one that is subject to great extremes of wet and dry, meaning that water is not available in abundance year-round. This means that most trees and shrubs in the biome are seasonally deciduous, shedding their leaves in anticipation of the drier season. In an evergreen tropical rain forest, on the other hand, rainfall is frequent and regular, so there is no seasonal drought. Deciduous trees may drop their leaves at various times of year, depending on the species, but with a wet climate and a wide range of trees, there is always something in bloom.

As with the temperate rain forest, the tropical variety experiences little in the way of wildfire or other catastrophic disturbances, and therefore an old-growth, climax community often develops in this biome. For this reason, tropical rain forests usually contain a wide diversity of trees, an enormous richness of species, and an extraordinary range of animals and microorganisms. Though biogeographers and ecologists often use the boreal coniferous forest as an example when examining biomes, those northerly forests are hardly examples of biological diversity. On the other hand, the tropical rain forest represents such diversity to its greatest extent.

Nonforest Biomes

Making an abrupt shift from the lush world of the tropical rain forest, let us look now at the tundra: a cold, treeless biome in the arctic and subarctic regions. (The Arctic Circle lies at approximately 66.5 degrees north latitude. Lands north of that line include northern Alaska and Canada, most of Greenland, extreme northern Scandinavia, and a northern strip of Russia and Siberia. The subarctic region, less clearly defined, comprises simply those lands that lie directly below the Arctic Circle.) Characterized by a short growing season, the tundra experiences very little precipitation in the form of liquid water. Yet the soil may well be marshy because temperatures are too low for significant evaporation and because the ground is usually frozen solid, preventing drainage.

In the most northerly tundras the dominant plants are small, hardy species that grow no more than 2-4 in. (5-10 cm) tall. In subarctic regions the dominant shrub species may grow as tall as 3.28 ft. (1 m), and the marshiest subarctic tundra may be home to sedge and cotton grass meadows. Among the larger forms of animal life on the tundra are the caribou and musk ox as well as the wolf, one of the larger predatory species.

Grasslands and Chaparral

Temperate grasslands are known as prairies in North America and steppes in Eurasia, and these grasslands often are divided into smaller subgroups depending on the height of the dominant grasses. Fire, aided by the dry climate, acts as a curb to prevent the tall grass from giving way to larger trees and forests. In the United States, however, so much prairie has been converted to agricultural or other anthropogenic purposes that it constitutes an endangered biome.

Much further south are the tropical grassland and savanna biomes that appear primarily in Africa. Although they do have scattered trees and shrubs, these biomes are dominated by grasses and other plants. In any case, the plant life is not what best characterizes this biome in the minds of most people. Rather, it is the extraordinarily abundant and diverse animal life, which includes such large mammals as the rhinoceros, elephant, hippopotamus, buffalo, cheetah, gazelle and other antelope, wild dog, and hyena. Then, of course, there is the lion, "king of beasts," sometimes incorrectly portrayed (for instance, in many Tarzan movies) as a jungle creature.

A biome typical of coastal southern California, the chaparral is distinguished by what often is described as a Mediterranean climate: dry, rainy in winter, and prone to drought in summers. The characteristic plant in a chaparral region has thick, leathery leaves that help it preserve moisture during the dry seasons. As with most other nonforest biomes, wildfire is a major controlling factor.

Desert

Deserts, discussed in more detail within The Biosphere, constitute a biome that may be temperate or tropical and which usually appears near the center of a continent. Such is particularly the case with the Gobi and Taklimakan deserts in northwestern and southwestern China, respectively; both deserts are located almost as far away from ocean as it is possible to be on Earth. Deserts also may occur in "rain shadows," areas separated from oceans by high mountains.

The unavailability of water is the chief defining feature of the desert, a biome that receives less than 9.9 in. (25 cm) of precipitation per year. Extremely dry deserts support virtually no plant productivity and therefore little, if any, animal life either. Such is the case, for instance, with the extraordinarily forbidding desert known as Rub' al Khali ("The Empty Quarter"), which occupies the lower third of the Arabian peninsula. On the other hand, less dry deserts may support relatively diverse plant life, as is the case, for instance, in Arizona.

Freshwater Biomes

Now we make another abrupt shift, in this case from the dry desert to aquatic biomes, beginning with the freshwater variety. Among these are lentic biomes, which appear in the area of lakes and ponds—any place where water is still. (This would include even a vast body of water such as Lake Superior, which, though it looks like a sea from the edge, and experiences waves and heavy swells, is nonetheless a freshwater body where water is not flowing. Hence, it is by definition a lentic environment.) The water in these bodies may take a few days to flush; on the other hand, it may take centuries. By sitting for such long periods of time, the water may accumulate large amounts of nutrients, and this is one of the variables whereby subgroups of the lentic biome are classified.

In a lotic biome, such as that of a river or stream, water is flowing. A lotic biome may be as small as a babbling brook that runs for less than a mile, or as great as the Mississippi River, which drains much of the continental United States. Where lotic biomes are concerned, the greatest variables involve the strength of the flow, including its quantity, velocity, and seasonal variations. These characteristics influence other aspects of the ecosystem: for example, if water flows calmly and slowly, the bottom tends to gather silt. This provides a habitat for certain small species, such as the crustacean, known variously as a crayfish, crawfish, or crawdaddy.

A lotic environment is not fully self-sustaining, and therefore it may not qualify as a true biome. Though they support some plant life, these ecosystems do not have a full complement of autotrophs, or life-forms (usually plants) that depend only on the Sun and the atmosphere, rather than other organisms, for sustenance. Usually, the lotic ecosystem relies on the input of organic matter from the nearby terrestrial environment or from lakes upstream or both. These other biomes provide the lotic biome with the nutrients necessary to feed its fish and other aquatic species.

Wetlands

Yet another freshwater biome is that of the wetlands. Just as rain forests were known as jungles until ecology and environmentalism entered the mainstream in the 1970s, so the term wetlands replaced a more blunt-sounding word: swamp. That word and several other old-fashioned ones are preserved in the terms for the four major wetland types: marsh, swamp forest, bog, and fen. Regardless of the name, this is a biome found in shallow waters, often in regions known for their pronounced seasonal variations of water depth.

The most biologically productive wetlands, marshes typically are dominated by relatively tall angiosperm varieties, such as the reed, cattail, and bulrush, as well as by floating flowers, such as the water lily and lotus. Swamp forests, such as the Okeefenokee on the Georgia-Florida border, are heavily populated with trees that include bald cypress and silver maple and may be flooded either seasonally or permanently. The Okeefenokee is notable for its animal life: not just heron and other bird species but also some of the more terrifying reptilian forms, including alligators and water moccasins, for which swamps are notorious.

Much less biologically productive than either marshes or swamps are bogs, which generally have acidic soil that supports only a limited range of vegetation. Characterized by a cool, wet climate (many of them are found in England), bogs often are dominated by sphagnum moss of one species or another. Later, when the sphagnum moss dies and the remains of several generations are compacted together with other plant debris, this becomes the basis for peat, which provides fuel for some homes in the British Isles and Europe.

Finally, there is the fen, another wetland found throughout the British Isles. Fens resemble bogs in several ways, including the fact that the local climate is usually cold (unlike the swamp forest, where the climate is generally hot). The fen has a better nutrient supply than the bog, however, and consequently the soil is less acidic, meaning that the biome as a whole is more productive.

Marine Biomes

The largest biome, geographically, is that of the open ocean, sometimes called a pelagic oceanic biome. Yet in terms of primary productivity—the first level in the food web—the ocean might as well be a desert. Tossed by waves and tides and heavily affected by the powerful salt content in its chemistry, the open ocean depends for its primary productivity not on plants but on phytoplankton, microscopic organisms that include a range of bacteria and algae.

Small crustaceans known as zooplankton eat the phytoplankton, only to be consumed, in turn, by small fish. Thus it goes up the trophic levels of the oceanic food web to the largest predators: bluefin tuna, sharks, squid, and whales. At the bottom of the ocean are other ecosystems, which depend on the slow rain of dead organic matter, or biomass, from the surface. Little is known about the deep-ocean biomes, but they appear to be diverse, if low in productivity (i.e., they have a relatively wide range of species but a small number of individuals).

Upwelling regions are relatively deep, nutrient-rich areas that sustain highly productive biomes. Among the large variety of species included in such biomes are fish and shark, marine mammals, and birds such as gulls. Upwellings off the west coast of South America and in the Antarctic Ocean provide some of the human world's most abundant fisheries.

Closer to Shore

Closer to the major landmasses are the biomes of the continental shelves. There the water is warm compared with that of the open ocean, and the nutrient supply is relatively high. This flow of nutrients is fed partly by rivers that empty into the seas but also by the occasional rising of deeper, richer waters to the surface. Not surprisingly, then, phytoplankton and animal life are highly productive here, and continental shelf regions such as those of the Grand Banks off northeastern North America offer highly abundant and commercially important fisheries.

Another ocean biome closer to shore is that of the estuary, an ecosystem that is enclosed by land on several sides but is still open to the sea. Because they typically experience substantial inflows of river water from the nearby land, estuaries feature characteristics of both marine and freshwater biomes and offer highly productive ecosystems. Many a commercially important species of fish, shellfish, and crustacean makes its early home in an estuary before moving on to deeper waters after reaching maturity.

Seashores constitute a variety of oceanic biome or, indeed, several varieties. Environmental factors such as the intensity of wave motion determine the characteristics of the seashore biome, as do latitude. For example, temperate seashore ecosystems can develop kelp "forests." (See Succession and Climax for more about the interrelations of species in the kelp forest.) In other areas, where the bottoms are soft and covered with sand or mud, dominant species include mollusks, crustaceans, and marine worms.

As with seashores, coral reefs are those rare oceanic biomes affected by latitude; in fact, this type of biome appears only in tropical regions. The principal characteristic of the coral reef is its substrate of calcium carbonate, formed from the exoskeletons of dead coral polyps and other creatures. On this structure is built a highly productive biome in which coral, algae, fish, and invertebrates (animals without a backbone) thrive.

Anthropogenic Biomes

Finally, there are anthropogenic biomes, such as the urban-industrial techno-ecosystem found in many a large metropolitan area. Such an ecosystem may include many species in addition to humans, but these—pets, houseplants, and the like—are not always native to the region, and probably would not flourish unless returned to their native biomes. New York City would be perhaps the ultimate example of an urban-industrial techno-ecosystem. Though it is far from a natural environment, it teems with life, from the oaks and elms in Central Park to the rats of the sewers, and from the pigeons that peck at crumbs on the sidewalks to the houseplants on the balconies and fire escapes of apartment buildings.

Another anthropogenic biome is the rural techno-ecosystem, which is not as removed from human civilization as the "rural" in its name would imply. This type of biome appears in regions around transportation and transmission corridors, including highways, railways, canals, and aqueducts, as well as alongside power and telephone lines. Small towns are a characteristic area for such a biome, as are the regions around coal mines and other industrial plants devoted to the extraction, processing, or manufacture of products from natural resources. This biome usually supports a mixture of introduced species and native species, the latter being those varieties that can survive the disturbances, pollution, and other stresses associated with the human presence. For example, in the woods along an interstate highway, there are bound to be omnivorous creatures such as raccoons, which thrive on litter thrown out of passing cars. Such creatures must be agile enough to survive the threat of becoming "road kill," as well as other hazards associated with the environment. As with the city biome, the rural techno-ecosystem includes plenty of life-forms that have been introduced artificially, an example being the wildflowers planted on a median by state highway workers.

Agro-ecosystems are ecosystems that are managed and harvested for human use: farms, orchards, fisheries, commercial forests, and other agricultural concerns. Here the defining characteristic is the level of management, or the degree of anthropogenic influence. Very heavily managed agro-ecosystems involve the planting of non-native crop species and the introducing of non-native plants, often to the exclusion of native species. The "crop" may be a herd of animals, as when western ranchers introduced non-native cattle, and in the process killed off native predators such as coyotes. On the other hand, there are less heavily managed agro-ecosystems that do allow native wildlife species to thrive alongside those species introduced for commercial purposes.

Where to Learn More

Biomes of the World (Web site). <http://www.snowcrest.net/geography/slides/biomes/>.

Earth Floor: Biomes (Web site). <http://www.cotf.edu/ete/modules/msese/earthsysflr/biomes.html>.

Habitats/Biomes (Web site). <http://www.enchantedlearning.com/biomes/>.

Johnson, Rebecca L. A Walk in the Boreal Forest. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001.

——. A Walk in the Desert. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001.

——A Walk in the Rain Forest. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001.

——. A Walk in the Tundra. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001.

Major Biomes of the World (Web site). <http://www.runet.edu/~swoodwar/CLASSES/GEOG235/biomes/main.html>.

The World's Biomes (Web site). <http://www.ucmp.berkeley.edu/glossary/gloss5/biome/>.

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A major community of plants and animals having similar life forms or morphological features and existing under similar environmental conditions. The biome, which may be used at the scale of entire continents, is the largest useful biological community unit. In Europe the equivalent term for biome is major life zone, and throughout the world, if only plants are considered, the term used is formation. See also Ecological communities.

Each biome may contain several different types of ecosystems. For example, the grassland biome may contain the dense tallgrass prairie with deep, rich soil, while the desert grassland has a sparse plant canopy and a thin soil. However, both ecosystems have grasses as the predominant plant life form, grazers as the principal animals, and a climate with at least one dry season. Additionally, each biome may contain several successional stages. A forest successional sequence may include grass dominants at an early stage, but some forest animals may require the grass stage for their habitat, and all successional stages constitute the climax forest biome. See also Desert; Ecological succession; Ecosystem; Grassland ecosystem.

Distributions of animals are more difficult to map than those of plants. The life form of vegetation reflects major features of the climate and determines the structural nature of habitats for animals. Therefore, the life form of vegetation provides a sound basis for ecologically classifying biological communities. Terrestrial biomes are usually identified by the dominant plant component, such as the temperate deciduous forest. Marine biomes are mostly named for physical features, for example, for marine upwelling, and for relative locations, such as littoral. Many biome classifications have been proposed, but a typical one might include several terrestrial biomes such as desert, tundra, grassland, savanna, coniferous forest, deciduous forest, and tropical forest. Aquatic biome examples are fresh-water lotic (streams and rivers), fresh-water lentic (lakes and ponds), and marine littoral, neritic, upwelling, coral reef, and pelagic. See also Fresh-water ecosystem; Marine ecology; Plants, life forms of.

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A naturally occurring community characterized by distinctive life forms which are adapted to the broad climatic type. Major biomes are tundra, coniferous (boreal) forest, temperate (deciduous) forest, tropical rain forest (selva), tropical grassland (savanna), temperate grassland (steppe), and hot deserts. A biome is an idealized type; local variations within a biome are sometimes more significant than variations between biomes. The present-day biomes have evolved in the last 10 000 years.

Smaller biomes are recognized, such as rocky coast biomes or coral reef biomes. In this way, the term is not synonymous with formation.

[Ge]

An ecological community of plants and animals established over a wide area.

It is a plant and animal community that covers a large geographical area. Complex interactions of climate, geology, soil types, water resources, and latitude all determine the kinds of plants and animals that thrive in different places. Fourteen major ecological zones, called "biomes," exist over five major climatic regions and eight zoogeographical regions. Important land biomes include tundra, coniferous forests, deciduous forests, grasslands, savannas, deserts, chaparral, and tropical rainforests.

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The world's major communities classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.

A large, distinct, easily differentiated community of organisms in a major ecological region.

For a list of words related to biome, see:

• Environment, Ecology, and Animal Behavior - biome: discrete community and region, such as desert or tropical rain forest, characterized by same major life forms and climactic conditions

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See words rhyming with "biome."

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