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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.


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-formsmost 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.)


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.


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.


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.


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.


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.


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.


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 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 moistas 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.


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.


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 pondsany 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.


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 productivitythe first level in the food webthe 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 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 thesepets, houseplants, and the likeare 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.


Biomes of the World (Web site). <>.

Earth Floor: Biomes (Web site). <>.

Habitats/Biomes (Web site). <>.

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). <>.

The World's Biomes (Web site). <>.



A measure of the degree to which an ecosystem possesses large numbers of particular species. An abundant ecosystem may or may not have a wide array of different species. Compare with complexity.


A term that refers to a biogeographic zone that includes mountain slopes above the timberline.


A type of plant that produces flowers during sexual reproduc tion.


Influenced by human activity.


The study of the geographic distribution of plants and animals, both today and over the course of extended periods.


The living components of an ecosystem.


A large ecosystem, characterized by its dominant life-forms. There are two basic varieties of biome: terrestrial, or land-based, and aquatic.


A combination of all flora and fauna (plant and animal life, respectively) in a region.


The upper portion or layer of the trees in a forest. A forest with a closed canopy is one so dense with vegetation that the sky is not visible from the ground.


A meat-eating organism, or an organism that eats only meat (as distinguished from an omnivore).


A theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession. In such a situation, the community is at equilibrium with environ mental conditions, and conditions are stable, such that the biota experiences little change thereafter.


The range of ecological niches within a biological community. The degree of complexity is the number of different species that theoretically could exist in a given biota, as opposed to its diversity, or actual range of existing species.


A type of tree that produces cones bearing seeds.


A term for a tree or other form of vegetation that sheds its leaves seasonally.


Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.


Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers; however, unlike decomposerswhich tend to be bacteria or fungidetritivores are relatively complex organisms, such as earthworms or maggots.


A measure of the number of different species within a biological community.


The study of the relation ships between organisms and their environments.


A community of inter dependent organisms along with the in organic components of their environment.






A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these consumes nutrients and passes them along to other organisms (or, in the case of the decomposer food web, to the soil and environment). The food web may be thought of as a bundle or network of food chains, but since the latter rarely exist separately, scientists prefer the concept of a food web to that of a food chain.


In general terms, a forest is simply any ecosystem dominated by tree-size woody plants. A number of other characteristics and parameters (for example, weather, altitude, and dominant species) further define types of forests, such as tropical rain forests.


A type of plant that reproduces sexually through the use of seeds that are exposed, not hidden in an ovary, as with an angiosperm.


A plant-eating organism.


An organism that eats both plants and other animals.


At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide.


The progressive replacement of earlier biological communities with others over time.




Layers of vegetation below the canopy in a forest.


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An ecosystem is a community of organisms that interact with each other and with the abiotic (chemical and physical) factors in their particular environment. A biome is the largest well-defined ecosystem. Biomes include vast grasslands, continent-wide deserts, and sweeps of arctic tundra. Biomes also include such well-defined ecosystems as coral reefs, lakes, and river systems.

Biomes are characterized by climate, by typical vegetation, and by the way organisms have adapted to that environment. Biomes are not permanent. Grasslands can be transformed into deserts; forests can be converted into grasslands. Climate change at the end of the last Ice Age dramatically altered the biomes of North America because of natural changes in climate and the movements of land masses. Since the Industrial Revolution, human activity has become an increasingly important factor in alteration of biomes.

Biomes are usually classified on the basis of average temperature and precipitation. This classification scheme results in many different biomes. Five typical biomes are:

  • Aquatic
  • Deserts
  • Forests
  • Grasslands
  • Tundra

Aquatic Ecosystems

Water covers about 75 percent of Earth's surface (including both freshwater and marine environments). While some ecologists reserve the term "biome" to refer to terrestrial ecosystems, aquatic ecosystems play a very important part in the ecology of Earth. Aquatic ecosystems can be freshwater (such as ponds and lakes, streams and rivers, and wetlands) or marine (such as oceans, coral reefs, mangrove swamps, salt marshes, and estuaries ).

Freshwater Regions.

Water is classified as fresh if it contains less than 1 percent salt (sodium and potassium chloride and other salts) in solution. Most freshwater plants and animals are unable to tolerate higher concentrations of salt in the water.

Ponds may be only a few square meters, whereas the largest lakes cover thousands of square kilometers. Ponds and lakes are scattered all over Earth. Many important ponds are seasonal and last for only one or two months at a time. These seasonal ponds may still be important resting places for migrating birds and other animals. Some large lakes have lasted for tens of thousands of years.

Lakes and larger ponds can be divided into three different "zones," determined by depth and distance from the shore. The littoral zone is the top layer of water near the shore. It may be warmer than the average temperature of the lake, but it also may have wider seasonal temperature variation. This is generally the most biologically diverse community in the lake, including algae (like diatoms), rooted and floating aquatic plants, grazing snails, clams, insects, crustaceans, fishes, and amphibians. Insect larvae may also inhabit this zone. The vegetation and animals living in the littoral zone provide food for other creatures such as turtles, snakes, and ducks.

The open water surface of the lake is known as the limnetic zone. Since this zone receives the most sunlight, it is a rich source of plankton. These plankton are the base of the food chain for the whole lake. Small fish also inhabit the limnetic zone, where they eat plankton or food that falls on the surface of the lake.

The bottom of the open water portion of the lake is known as the profundal zone. Since little light penetrates this region, its inhabitants are heterotrophs , which eat the small fish and other animals from the littoral and limnetic zones. Other inhabitants are detrivores, subsisting on material falling into the deep water from the surface.

The temperature of a pond or lake will vary seasonally. In colder climates, the surface of the lake may freeze solid. In large lakes the deep water will never freeze, remaining about 4°C (39° F) all winter. During spring and fall, substantial mixing of the lake water can occur (sometimes called "turning over").

A river is a long, narrow body of water flowing downhill. A river may start in the mountains and flow all the way to the ocean. Or it may start in a lake or spring and flow a short distance before joining another river or entering a lake. The start of the river will have a lower temperature, clearer water, and more oxygen. The middle part of the river will be wider and slower moving. The water will be warmer. There will be more species diversity, including plants and algae. Close to the mouth of the river, the water may be filled with sediment. Fish that can tolerate less oxygen and warmer temperatures, such as carp and catfish, will be found near the mouth.

Wetlands are areas of standing water that support aquatic plants. Marshes, swamps, and bogs are all wetlands. The water in wetlands flows slowly or is still. Wetlands can be seasonal or permanent. Since the water in wetlands is often low in oxygen, plant species often require special adaptations. Wetlands have the highest species diversity of all ecosystems, with many species of amphibians, reptiles, birds, and fur-bearing mammals living in them.

Marine Regions.

Marine ecosystems, which cover about three-fourths of Earth's surface, include oceans, seas, saltwater marshes, estuaries, and coral reefs. Because of their size, marine ecosystems are important parts of the atmospheric carbon dioxide cycle. Marine algae take in huge amounts of carbon dioxide from the atmosphere and release much of the world's oxygen supply. Marine ecosystems also supply most of the atmospheric water vapor that falls as precipitation on land.

Oceans are the largest of Earth's ecosystems. Like lakes, oceans are subdivided into separate zones: intertidal, pelagic, abyssal, and benthic . All four zones have a great diversity of species. The intertidal zone is the region along the shoreline between average low tide and average high tide. In other words, this region goes through cycles of submergence and being exposed to air. Animals in this zone must be able to survive the extended periods of exposure. The pelagic zone includes all the open ocean water. The abyssal zone is the deep ocean water, between 2,000 and 6,000 meters deep. The benthic ocean includes the deep ocean bottom inhabited by organisms.


Deserts cover about one-fifth of Earth's surface. The identifying characteristic of the desert biome is low annual rainfall. Some deserts receive large amounts of rain in the form of heavy thunderstorms, but these occur for a short time and run off rapidly. On average, any region that receives less than 50 centimeters of rainfall per year is considered a desert. The driest deserts receive less than 2 centimeters of rainfall per year. The Atacama in Chile may get no rainfall at all for years at a time.

Most of the world's deserts occur in a band between 10° and 40° of the Equator. Cold deserts, such as the basin and range area of Utah and Nevada and parts of western Asia, occur at higher latitudes. Most deserts have a wide variety of specialized vegetation, as well as many specialized animals. Because there is little rainfall, most desert soils have high nutrient levels. However, the humus (organic matter) in soils is very low.

Desert plants are typically low-growing shrubs and short woody trees. Leaves are of many different types, but all show various degrees of specialization because of the lack of rainfall. Many plants have small leaves that are covered with a waxy film to retain water. The thorns of cacti are highly modified leaves that shade and protect the plant. The thickened and often flattened stems of cacti have taken over the job of photosynthesis.

While mule deer, pronghorns (which resemble antelopes), desert bighorns (a type of wild sheep), and mountain lions are all found in the relatively humid deserts of North America, the dominant vertebrates of warmer and drier deserts are small reptiles and amphibians. Mammals are usually small, like the kangaroo mice of North American deserts. There are also insects, arachnids, scorpions, and birds. The animals stay inactive in protected hideaways during the hot day and come out to forage during twilight hours or at night.


Forest biomes are ecological communities dominated by trees. There are three major types of forests distinguished by latitude: tropical, temperate, and boreal forests (taiga).

Tropical forests include the greatest diversity of species of any of the world's biomes. By definition, tropical forests occur in the tropics, within 23.5° of the equator. Instead of the four seasons typical of temperate regions, most tropical forests have two seasons, a dry season and a wet season. The temperature and amount of daylight are fairly constant. However, because of the heavy forest canopy, little sunlight reaches the forest floor.

The annual rainfall in tropical forests is generally 200 centimeters or more. This high amount of rainfall results in soils that have lost most of the nutrients. A square kilometer of tropical forest may contain 100 species of trees and thousands of species of insects. Orchids, bromeliads, ferns, mosses, and palms grow in the trees and on the forest floor. Birds, bats, small mammals, and insects live in and around the trees.

Temperate forests are characterized by four distinct seasons, including a winter season with extended periods below freezing. The tree canopy allows light to penetrate to the forest floor, so there is an understory of shade-tolerant trees and shrubs. This results in a stratification of the flora and fauna of the temperate forest, with distinct sets of plants and animals present in the treetops, the mid-levels, and on the forest floor. Most trees are broad-leaved deciduous trees that drop their leaves each fall; mixed in are some evergreen conifers. Typical tree species include oak, hickory, maple, elm, willow, and dogwood. Animals include rabbits, squirrels, skunks, many species of birds, deer, mountain lion, bobcat, timber wolf, fox, and black bear. Reptiles and amphibians inhabit the forest floor.

Boreal forests are cold forests of northern latitudes that experience many months of snow cover and below-freezing temperatures. Extensive boreal forests occur north of 50° north latitude to beyond the Arctic Circle. Boreal forests are found in a broad belt extending from Siberia across Alaska, into Canada and parts of the northern United States, and also in Scandinavia. These forests may also be found farther south at higher altitudes. Seasons are divided into short, moist, moderately warm summers and long cold, dry winters. The length of the growing season in boreal forests is very short, around 100 days. Precipitation is primarily in the form of snow, the equivalent of 40 to 100 centimeters of rainfall annually. Soil is generally thin, nutrient-poor, and acidic . The dense canopy permits little light to reach the forest floor, so the forest understory is limited. The trees are mostly evergreen conifers such as spruce, pine, and fir. Animals include woodpeckers, hawks, owls, many other bird species, moose, bear, weasel, fox, lynx, and deer.


Grasslands are lands dominated by grasses. Large shrubs or trees are restricted to stream banks and to isolated stands. There are two main divisions of grasslands: tropical grasslands, called savannas, and temperate grasslands.


A savanna is grassland with widely scattered individual trees or small groups of trees. Savannas of one sort or another cover most of central Africa and large areas of Australia, South America, and India. A savanna is not as arid as a desert but receives roughly the same amount of rainfall as a temperate forest. The annual rainfall ranges from 50 to 130 centimeters per year. However, the rainfall occurs in a "wet" season of four to six months. The wet season is followed by a dry season. During the dry season, drought and fires can occur. The drought and fires suppress the growth of trees and shrubs. This cycle of wet and dry seasons, fires, and drought maintains the savanna conditions.

Human activity can inadvertently convert a forest into a savanna. If the trees are removed and grass grows rapidly enough to cover the bare ground before the trees can recolonize, then fires may become the dominant force, and the savanna will become more or less permanent. The relatively infertile soil of savannas has discouraged the conversion of savannas to farming of wheat and other grasslike crops. When farming is instituted, the organic material is quickly exhausted and the soil then requires the addition of chemical fertilizer to remain productive.

Fire is essential to the life of a savanna. Most animals killed by the fires are insects and small animals. Large animals are able to escape the fire. Birds and other opportunistic feeders move in quickly to eat the killed animals or to prey on the animals fleeing the fire. The grass quickly sprouts from the extensive root structure out of reach of the fire. Many shrubs and woody plants are killed by the fire.

The shrubs and trees that survive have special adaptations that allow them to survive the fire or to sprout quickly when the rains come. The life cycle of animals that live on the savanna is tied to this same cycle. Calves are born soon after the start of the rainy season, when plenty of new grass is available. Other animals that live completely or partly on the savannas around the world (not all in the same place) include giraffes, zebras, buffaloes, kangaroos, mice, moles, gophers, ground squirrels, snakes, worms, termites, beetles, lions, leopards, hyenas, and elephants.

Temperate Grassland.

Temperate grasslands also have grasses as the dominant vegetation. Trees and large shrubs are restricted to the banks of streams. Temperatures vary widely from summer to winter. The amount of rainfall is less in temperate grasslands than in savannas. The plains and prairies of central North America were typical temperate savannas before the land was converted to farming. As in savannas, the cycle of seasonal drought and fire is essential to maintaining the biodiversity of temperate grasslands.

The soil of the temperate grasslands contains a thick layer rich in organic matter from the growth and decay of grass roots. The organic material holds the soil together and provides nutrients for new growth. Seasonal drought, fire, and grazing by large mammals all prevent woody shrubs and trees from invading and becoming established. Some trees, such as cottonwoods, oaks, and willows, grow along streams and in river valleys. Many annual plants, such as wildflowers, grow among the grasses.

Temperate grasslands also experience a wet season and a dry season. Most precipitation occurs in the late spring and early summer. The rest of the year is relatively dry, with rainfall averaging 50 to 90 centimeters per year. The temperature range of temperate grassland is very wide. Summer temperatures can be well over 40°C (104°F), while winter temperatures can drop as low as-40°C (-40°F).

Animals found in temperate grasslands in different parts of the world include gazelles, zebras, rhinoceroses, wild horses, lions, wolves, prairie dogs, jackrabbits, deer, mice, coyotes, foxes, skunks, badgers, blackbirds, grouse, meadowlarks, quail, sparrows, hawks, owls, snakes, grasshoppers, leafhoppers, and spiders.


Tundra is the coldest of all the biomes. "Tundra" is derived from the Finnish word tunturia, which means "treeless plain." Extremely low temperatures, little precipitation, poor nutrients, and short growing seasons characterize tundra. Dead organic material functions as a nutrient pool for nitrogen and phosphorus. Because of the constant freezing and thawing of the ground, the tundra is strangely humped into low mounds, often with a pentagonal or hexagonal shape.

In addition to the cold climate, tundra typically has low biological diversity, with only a few plants and animals present in any region. The vegetation is small and low to the ground. Drainage is very poor because there is frequently a layer of permafrost (permanently frozen soil) just below the surface. In the short summer after the surface ice melts, the tundra is covered with pools of water that provide breeding opportunities for millions of mosquitoes, blackflies, and other biting insects. Animals that live on the tundra often show large oscillations of population.

Arctic Tundra.

Arctic tundra is found in the Northern Hemisphere between the taiga and the Arctic Ocean. The growing season is around fifty to sixty days. The average winter temperature is-34°C (-29°F), and the average summer temperature is around 10°C (50°F). Total precipitation varies widely but is typically quite low. The rainfall equivalent may be around 15 to 25 centimeters a year, about the same as a desert. The soil layer is thin. Soil forms very slowly and is somewhat acidic. Below the soil is a layer of frozen gravel and silt. During the short growing season, water saturates the upper surface, forming bogs and ponds. A wide variety of plants are nevertheless able to survive these odd conditions, including small willows only a few centimeters tall, sedges, reindeer mosses, liverworts, and grasses. More than 400 varieties of flowers take advantage of the short growing season.

All the plants have adapted to fierce winds and cold temperatures by growing close together and close to the ground. The tallest plants in the tundra are only a few centimeters tall. In the winter a protective blanket of snow covers the plants, which have evolved the ability to conduct photosynthesis with very little light and at low temperatures. Because of the short growing season, most plants reproduce by budding and division rather than by flowering. Arctic herbivores include caribou, arctic hares, squirrels, lemmings, and voles. These are preyed on by bears, wolves, and foxes. The birds are migratory, and include ravens, snow buntings, falcons, loons, sand-pipers, terns, and various species of gulls. Insects include lots of mosquitoes, blackflies, moths, and grasshoppers.

The animals all migrate south in the winter or hibernate. The mammals have developed the ability to breed and raise young quickly in the short summer. Almost no reptiles or amphibians live in the arctic tundra.

Alpine Tundra.

Alpine tundra is generally similar to arctic tundra. However, alpine tundra has a longer growing season. Alpine tundra is found at high altitudes above the tree line throughout the world. The closer to the equator, the higher the altitude required. Even on the equator, there are still mountains high enough to have some areas of alpine tundra. Alpine tundra soils are generally well drained, but rocky subsoils serve the same function as arctic permafrost. Alpine tundra has a unique community of organisms different from arctic tundra. Animals include mountain goats, pikas, and marmots. Typical plants include heather, short bunchgrasses, and small trees.

see also Ecosystem; Habitat.

Elliot Richmond


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