Freshwater Fish

Fish have been a major source of human food and of oil, fertilizer, and feed for domestic animals since the dawn of history. Efforts to propagate fish as a source of high-grade protein for human consumption have been more recent but still date to ancient China and the Roman Empire. In the twenty-first century, fish provide about 25 percent of the animal protein consumed by people in developing countries and as much as 75 percent in countries such as Bangladesh and the Philippines.

One reason for the long-standing popularity of fish as food is sheer numbers. More than 70 percent of the Earth's surface is covered with water, and well over twenty thousand different species of fish live in marine, fresh, and brackish waters, making them the most diverse of all the animals. Fish can live at temperatures ranging from below freezing in Antarctic waters to over 100^oF (40^oC) in hot springs. They range in size from the .5-inch (1.3-centimeter) dwarf goby of the Philippine Islands to the 45-foot (14-meter), 25-ton whale shark of the tropical oceans. The nutritional profile of fish is also outstanding. Rich in the essential omega-3 unsaturated fatty acids so lacking in other foods, fish are also high in protein yet low in calories, sodium, sugars, saturated fats, and cholesterol.

Biology

Scientifically speaking, fish are aquatic vertebrates with gills instead of lungs and fins instead of external limbs. In contrast to the higher animals, fish are also cold-blooded, that is, their body temperatures remain the same as that of the water. A few species, such as tuna, are able to maintain their body temperatures a degree or two higher than the water.

The chain of life leading to fish production begins with the microscopic diatoms and algae in lakes, rivers, and the ocean. These aquatic plants, collectively termed phytoplankton, use the energy in sunlight to convert carbon dioxide dissolved in the water into the organic matter that eventually becomes food for fish. Fish were the earliest animals with backbones to appear in the fossil record, evolving from more primitive forms over 500 million years ago. In turn, the terrestrial animals evolved from the fishes.

Biologists class the more than twenty thousand known species of fish into three main groups, the Agnatha (primitive jawless fishes, such as the blood-sucking lamprey), the Chondrichthyes (sharks, skates, and rays that have skeletons of cartilage instead of bone), and the Osteichthyes (fishes with a bony skeleton, such as salmon and trout). All of the fish important as food are members of this latter group.

A typical bony fish is torpedo-shaped with a head containing a brain and eyes, a trunk with a muscular wall, and a postanal tail. Fish generally propel themselves through the water by undulating movements of the muscular trunk, using their fins to control direction. All have skins covered with a layer of mucus that decreases friction with the water, and nearly all are covered with an external layer of scales (catfish are one exception). Fishes also have a system of sensory organs along their sides, called the lateral line, that can detect pressure changes in the water caused by sounds. Fish obtain oxygen and eliminate carbon dioxide (breathe) by sucking water into the mouth and pumping it out over the gills. Oxygen dissolved in the water thus diffuses into the bloodstream, and carbon dioxide diffuses out. A few species (such as the African lungfish) also have air-breathing lungs as an additional means of respiration.

Most fish live in either saltwater or freshwater, but some important food fish are physiologically capable of migrating from one to the other. For example, Pacific and Atlantic salmon are hatched and reared in freshwater but then migrate to the ocean to grow and mature, returning to their natal streams and lakes to spawn. The eel has the opposite life history pattern. Thus eel and salmon may be thought of as either freshwater or saltwater fishes depending on age and season.

Over the years, a number of other aquatic animals have been given common names that include the term "fish," such as shellfish, but these do not resemble and are not related to true fish. Furthermore, some animals that have adopted an aquatic way of life, such as whales, seals, and sea snakes, superficially resemble fish and may even be called fish. But they are air breathers, and their anatomical structure is that of land animals.

Preparation and Food Safety

Fish are a highly perishable food product, and historically they had to be marketed live or preserved (cured) by smoking, salting, pickling, or a combination of these methods. Fish to be cured by any method are first cleaned, scaled, and eviscerated. They are salted by packing them between layers of salt or by immersion in brine. Smoking preserves fish both by permeation of smoke ingredients and by partial drying due to heat penetration. Fish can also be dried per se by carefully controlling temperature, humidity, and air velocity. However, dried fish are relatively unappetizing, and rehydration is slow. With the exception of smoked fish, the ready availability of ice and modern freezing and canning facilities has largely supplanted curing as a method of fish preservation. Fish are routinely shipped around the world either fresh or frozen. Fresh fish are shipped on ice and have an acceptable shelf life of about ten days. Frozen fish packaged in oxygen-impermeable plastic wrap, such as Saran, may be stored frozen at –20^oF (–29^oC) for up to six months with no appreciable loss in quality.

Fresh fish are almost always marketed as either whole fish on ice (viscera removed), dressed fish (head, fins, and viscera removed), fillets (sides cut lengthwise away from the backbone), or steaks (cut longitudinally into sections). Due to consumer demand, boneless cuts are increasingly available in the United States and Europe.

Fish is a naturally tender protein food, free of tough fibers that need to be softened by prolonged cooking. Thus fish products are best cooked using high-temperature, short-time methods. They may be deep-fat fried (325–350^oF; 163–177^oC), pan fried (sautéed) in a small amount of butter, broiled, poached (simmered, never boiled), or baked (400–450^oF; 204–232^oC). Pan frying or sautéing is one of the most widely used methods of cooking thin fillets in general. Microwaving is especially well suited to the high-temperature, short-time method of cooking fish. The advent of individually quick-frozen fish fillets has enabled timesaving cooking techniques, such as brushing marinades directly on the frozen product and grilling or oven roasting without the necessity of defrosting. Fish is generally ready to eat when cooked to a temperature of 140^oF (60^oC) and the flesh has turned opaque and flakes easily. Fish is eaten raw by some ethnic groups (such as Asians). Other ethnic specialty preparations, such as blackened fish (Cajun) or gefilte fish (Yiddish), are also popular.

To ensure food safety, fresh fish should be clean smelling, and the flesh should be firm and resilient when pressed. Fish should be kept wrapped and refrigerated at 40^oF (4^oC) or less and eaten within two days. Frozen fish should be rock hard, free of ice crystals, and have no white spots, visible drying, or browning around the edges. In the home, fish should be stored frozen at 0^oF (–18^oC) or below and for no more than three months. It should be thawed in a refrigerator, never at room temperature.

Freshwater Fish Commonly Used as Food

Historically, the human race has used literally thousands of different species of fish in its continuing search for sustainable sources of food. In the twenty-first century, the most popular in North America and Europe include carp, catfish, crappie, eel, lake herring, mullet, muskellunge, yellow perch, yellow pike, pickerel, salmon, suckers, sunfish, tilapia, trout, lake trout, and whitefish. In Indonesia, the Philippines, and Taiwan, milkfish have been used for food for centuries. In Asia, carp, ayu, and eel are important freshwater food fish. Some of the most interesting of these freshwater fish are discussed in more detail below.

Ayu. The ayu (Plecoglossus altivelis), also known as sweet fish in Japan and aroma fish in China, is an extremely popular and economically important freshwater food fish in many Asian countries. Historically, it was caught by Japanese fishermen using trained cormorants with rings around their necks to prevent them from swallowing. In the twenty-first century, it is wild-caught in rivers by sport and commercial fishermen or raised commercially for both restaurant consumption and home use. Ayu are usually sold live, on ice in the round, or frozen. The food quality of wild-caught ayu is especially desirable, characterized by a sweet, delicate taste and an odor reminiscent of cucumber or watermelon.

Carp. Carp (Cyprinus carpio) are the largest members of the minnow family and can easily reach a weight of ten kilograms or more. Although greatly underutilized in North America, the common carp has always been a widely popular freshwater food fish in the rest of the world. History records that carp were grown in ponds for food in ancient China in the fifth century b.c.e. In Europe, carp were grown in monastery ponds as early as the sixth century C.E. so the monks would have something to eat during the many meatless fasting days prescribed by the church. By the late Middle Ages, carp had become a well-established food item for the general populace. In the twenty-first century, carp are wild-caught or grown for food in Russia, Ukraine, Hungary, Poland, India, China, Japan, Latin America, Egypt, Iran, Indonesia, and Israel, to name only the major consumer nations. The world's leading producer is China, where carp are often grown in rice paddies in rotation or even simultaneously.

The most common market forms of carp are fresh whole fish, dressed fish, or fillets. Gefilte fish, fish balls blended with egg and matzo meal and simmered in a vegetable broth, is an ethnic specialty item (Yiddish) traditionally made from carp.

Catfish. The channel catfish (Ictalurus punctatus), native to warm water lakes and rivers in North America, is a traditional food fish in the southern United States. Consumer demand has moved from regional to national and even international. In the United States, the per capita consumption of catfish is exceeded only by that of tuna, shrimp, pollack, and salmon. To satisfy American consumer demand, several hundred thousand metric tons of channel catfish are produced by aquaculture each year in the southern United States.

Imported catfish from Vietnam has been marketed aggressively to restaurant chains and food service companies with considerable success. Advertised as delta-raised catfish, it is actually a catfish relative raised in the delta of the Mekong River. Another catfish species, the walking catfish, is a popular food fish in tropical regions and even in some European countries, especially the Netherlands.

Catfish is firm textured and has a mild, slightly nutty taste that complements a variety of flavors. It is a lean fish, and modern processing methods have eliminated bones. That, together with its lack of a fishy odor, gives it wide consumer appeal. Catfish were traditionally wild-caught and marketed as iced whole dressed fish. Modern farm-raised catfish are processed within minutes and shipped either on ice or as individually quick-frozen fillets, making it one of the freshest fish available. In addition to fresh or frozen fillets, steaks and nuggets (pieces) breaded or marinated with flavors and spices, such as Cajun spices or mesquite, are also common in seafood markets and restaurants and have even been introduced into school lunch programs. As Mark Twain once said, "The Catfish is a plenty good enough fish for anybody."

Eel. Although appreciated before the Civil War in North America, freshwater eels (primarily Anguilla anguilla and A. japonica ) are a widely popular food item in Asian countries, particularly Japan, Korea, China, and Taiwan. Eels are also an important delicacy in Europe, particularly Italy, where they must be produced commercially by aquaculture to satisfy consumer demand. Overall, however, China produces more than 70 percent of the eels sold in the world, and many rice paddies have been converted to eel production. Japan is the world's largest eel consumer, where kabayaki, eel fillets grilled with a sweet basting sauce, is practically a national dish.

As mentioned, eel consumption in North America is minor. However, freshwater eel unagi is common in Japanese restaurants in the United States, where it may be served grilled with teriyaki sauce or used in sushi or unadon (eel over rice). In addition, each year many tons of market-sized eels are wild-caught by U.S. fishermen and exported to Europe, where it is eaten roasted or even jellied and baked into pies.

Eels have an interesting life history in that they live in freshwater rivers and lakes, where they grow to adult size, then they migrate into the ocean, where they swim long distances to the Sargasso Sea to spawn and die. The newly hatched young eels may then ride the ocean currents for several years until they reach coastal waters and swim back to freshwater rivers. There they grow to adult size and can be harvested for food.

Eels under a kilogram in size are the most tender. The rich, sweet, firm flesh of eel must be refrigerated and eaten immediately, so the best restaurants keep live eels in aquarium tanks. The skin and outer layer of fat are removed by the chef, and the fillets are either grilled or roasted. Eel is also available frozen, smoked, or jellied in cans.

Asian folklore holds that eel consumption confers strength and vitality, particularly in hot summer weather. Eels are amazingly rich in vitamin E and in the omega-3 fatty acids (DHA) that are essential to brain functions involving mood.

Milkfish. Milkfish (Chanos chanos) have been an important food fish for people in Southeast Asia for many centuries. Although they are an oceanic fish, milkfish spawn in shallow coastal areas, where fry and fingerlings are collected in nets and carried to freshwater or brackish water ponds for rearing to market size. Milkfish have been raised in this fashion for at least seven hundred years in the Philippines and Indonesia. Taiwan is also a major producer. Milkfish (bangus) is sold in Asian markets and restaurants either fresh, smoked, marinated, as fish balls, as fish sausages, or as fish nuggets. It is also exported frozen to North America, where sinigang na bangus (milk-fish in sour broth) is a popular dish among ethnic Indonesians and Filipinos.

Tilapia. Although relatively new to North American fish markets, tilapia are actually a group of fish (cichlid) that traces its origins to North Africa and the Middle East. These mild, white, sweetly flavored fish have been wild-caught or pond-raised around the world for centuries. Called St. Peter's fish in many parts of the world, legend has it that the fish Jesus multiplied to feed the multitudes in the story of the seven loaves and fishes was tilapia (Matt. 17:24–27).

Because of their versatility, tilapia have been nicknamed "the aquatic chicken" and can be baked, broiled, fried, blackened, grilled, poached, or sautéed. Sautéing is one of the most popular methods of preparing thin fillets in general, and in most recipes tilapia can easily substitute for catfish or even sole and flounder.

Tilapia are grown in floating cages, ponds, or rice fields in temperate and tropical regions around the world. Only Chinese carp and salmon or trout exceed tilapia in total worldwide fish production. Although they are less popular in the United States, tilapia consumption has grown to rival trout among the commercially raised fish species. Since relatively modest numbers of tilapia are produced by U.S. aquaculture, large quantities of frozen fillets are imported from Indonesia, Taiwan, and Mexico to satisfy consumer demand. Many large U.S. cities report a significant demand for live tilapia delivered to ethnic Asian markets.

Trout. Many trout species have historically been used for food, but rainbow trout (Oncorhynchus mykiss) have been by far the most popular. Originally native to cold water environments in the north temperate zone, this prized food fish has been transplanted around the world and is well established in North and South America, Japan, China, Europe, Australia, New Zealand, and parts of Africa. Top trout-producing countries include Chile, Denmark, France, Italy, and the United States.

Most rainbow trout is marketed as head-on dressed fish, as fresh or frozen boneless fillets, or as smoked fish. Farmed trout are typically rich in the omega-3 fatty acids so essential to normal brain and eye function, while they are less expensive than most other fish products.

Walleye pike. Walleye (Stizostedion vitreum), a member of the perch family with an excellent reputation for its food quality, is a widely sought cool-water fish mostly caught by anglers for home use but also available in fish markets and restaurants in much of the northern United States and Canada. In the United States, a limited commercial harvest comes from the Great Lakes. However, most of the commercial harvest is from Canadian fishing on Lake Erie and the inland waters of Ontario and Saskatchewan. The walleye is Canada's most economically valuable freshwater fish. Only a few commercial growers produce food-size walleye, but because of its reputation for excellent food quality (aroma, flavor, and texture), its name recognition, and its high retail price, walleye has considerable aquaculture potential. Traditionally, walleye are sold as scaled, skin-on fillets. A two-pound fish yields about two eight-ounce dinner-size fillets.

Whitefish. Lake whitefish (Coregonus clupeaformis) native to the deep cold lakes of North America are popular food fish in the United States and Canada. They are widely sold in restaurants, and some believe their flaky, non-oily white meat is the best tasting of all the freshwater fish. Early settlers claimed they could eat nothing but whitefish for days at a time and never tire of it. A large commercial fishery for whitefish exists in Lake Superior and the other Great Lakes of the midwestern United States. In Canada, close to 600,000 kilograms of whitefish a year is caught and sold by tribal fishermen of the Great Slave Lake alone. Most whitefish is marketed frozen and sold in restaurants or supermarkets, but limited amounts are also available smoked or fresh. White-fish eggs, termed freshwater or golden caviar, are sometimes sold as a less-expensive substitute for sturgeon caviar.

See also Aquaculture; Fishing; Fish, Salted; Fish, Smoked.

BIBLIOGRAPHY

American Tilapia Association. Available at http://www.ag.arizona.edu. Hosted by the University of Arizona as a service to the industry.

Catfish Institute. Available at http://www.catfishinstitute.com.

Costa-Pierce, Barry A., and James E. Rakocy, eds. Tilapia Aqua-culture in the Americas. Baton Rouge, La.: World Aquaculture Society, 1997–2000.

Lagler, Karl F., John E. Bardach, Robert R. Miller, and Dora R. Miller Passino. Ichthyology. 2d ed. New York: John Wiley, 1981.

National Fisheries Institute. Available at http://www.nfi.org.

Nelson, Joseph S. Fishes of the World. 3d ed. New York: John Wiley, 1994.

Restaurants USA. 1992–1996. National Restaurant Association. Washington D.C., various issues.

Schweid, Richard. Consider the Eel. Chapel Hill: University of North Carolina Press, 2002.

Stickney, Robert E., ed. Encyclopedia of Aquaculture. New York: Wiley, 2000.

Tucker, Craig S., and Edwin H. Robinson. Channel Catfish Farming Handbook. New York: Van Nostrand Reinhold, 1990.

U.S. Trout Farmers Association. Available at http://www.ustfa.org.

Gary A. Wedemeyer

Biome

Major biomes and their characteristics

Terrestrial biomes

Freshwater biomes

Marine biomes

Resources

A biome is a major, geographically extensive ecosystem, structurally characterized by its dominant life forms.

Terrestrial biomes are usually distinguished on the basis of the major components of their mature or climax vegetation, while aquatic biomes, especially marine ones, are often characterized by their dominant animals. Most of the oceans are considered part of a single biome, although areas with particularly unusual or unique physical characteristics or inhabitants may be considered as separate biomes.

Similar biomes can occur in widely divergent places as long as the environmental conditions are appropriate for their development. Some environmental conditions affecting the location of biomes include climate, latitude, topography, and fire. Often, different species having similar, convergent growth forms will dominate at different places within the same biome. For example, the boreal coniferous forest occurs in suitable environments of northern North America and Eurasia. In northeastern North America this biome is dominated by stands of black spruce, while in the northwest white spruce is dominant. Norway spruce is most important in this biome in northwestern Europe, while in parts of Siberia species of pine and larch are dominant. Because biomes are described according to the structural characteristics of their dominant organisms (in this example, coniferous trees growing under a particular climatic regime) all of these different forest types are considered convergent ecosystems within the same biome, the boreal coniferous forest.

Major biomes and their characteristics

Most biomes are delineated on the basis of their naturally occurring communities of plants, animals, and microorganisms. Exceptions are the so-called anthropogenic biomes, which are strongly influenced by humans and their activities, as in the case of cities and agroecosystems.

Terrestrial biomes

Tundra

Tundra is a treeless biome occurring in areas with cold climates and a short growing season. Alpine tundra occurs at high altitudes on mountains, while arctic tundra occurs at high latitudes. Most tundras receive very small inputs of water as precipitation, but

nevertheless their soil may be moist or wet because there is little evaporation in such cold climates, and deep drainage may be prevented by frozen soil. The coldest, most northern, high-arctic tundras are very unproductive and dominated by long-lived but short-statured plants, typically less than 1.97-3.94 in (5-10 cm) tall. Low-arctic tundras are dominated by shrubs as tall as 3.28 ft (1 m), while wet sites develop relatively productive meadows of sedge, cottongrass, and grass. In North America, arctic tundras can support small densities of mammalian herbivores such as caribou and muskox (although during migration these animals can occur in locally large densities), and even smaller numbers of their predators, such as wolves.

Boreal coniferous forest

The boreal coniferous forest, or taiga, is an extensive northern biome occurring in moist climates with cold winters. The boreal forest is dominated by coniferous trees, especially species of fir, larch, pine, and spruce. Some broad-leaved, angiosperm trees are also important in the boreal forest, especially species of aspen, birch, poplar, and willow. Usually, particular stands of boreal forest are dominated by only one or several species of trees. Most regions of boreal forest are subject to periodic events of catastrophic disturbance, most commonly caused by wildfire and sometimes by insects, such as spruce budworm, that kill trees through intensive defoliation. Montane forests, also dominated by conifers and similar in structure to the boreal forest, can occur at sub-alpine altitudes on mountains in southerly latitudes.

Temperate deciduous forest

Forests dominated by species-rich mixtures of broad-leaved trees occur in relatively moist, temperate climates. Because these forests occur in places where the winters can be cold, the foliage of most species is seasonally deciduous, meaning that all leaves are shed each autumn and re-grown in the springtime. Common trees of this forest biome in North America are species of ash, basswood, birch, cherry, chestnut, dogwood, elm, hickory, magnolia, maple, oak, tulip-tree, and walnut, among others. These various tree species segregate into intergrading communities on the basis of site variations of soil moisture, fertility, and air temperature.

Temperate rainforest

Temperate rainforests develop under climatic regimes characterized by mild winters and an abundance of precipitation. Because these systems are too moist to support regular, catastrophic wildfires, they often develop into old-growth forests, dominated by coniferous trees of mixed age and species composition. Individual trees can be extremely large, and in extreme cases can be more than 1, 000 years old. Common trees of this biome are species of Douglas fir, hemlock, cedar, redwood, spruce, and yellow cypress. In North America, temperate rainforests are best developed on the humid west coast.

Temperate grassland

These grasslands occur under temperate climatic regimes that are intermediate to those that support forest and desert. In the temperate zones, grasslands typically occur where rainfall is 10-24 in (25-60 cm) per year. Grasslands in North America are called prairie (they are often called steppe in Eurasia), and this biome occupies vast regions in the interior. The prairie is often divided into three types according to height of the dominant vegetation—tall-grass, mixed-grass, and short-grass. The once extensive tall-grass prairie is dominated by various species of grasses and herbaceous broad-leaved plants such as sunflowers and blazing stars, some as tall as 10-13 ft (3-4 m). Fire was an important natural factor that prevented much of the tall-grass prairie from developing into an open forest. The tall-grass prairie is now an endangered natural ecosystem, because it has been almost entirely converted to agriculture. The mixed-grass prairie occurs where rainfall is less, and it supports shorter species of grasses and other herbaceous plants. The short-grass prairie has even less precipitation, and is subject to unpredictable years of severe drought.

Tropical grassland and savanna

Tropical grasslands can occur in regions with as much as 50 in (approximately 120 cm) of rainfall per year, but under highly seasonal conditions with a pronounced dry season. Savannas are dominated by grasses and other herbaceous plants, but they also have scattered shrub and tree-sized woody plants, which form a very open canopy. Tropical grasslands and savannas can support a great seasonal abundance of large, migratory mammals, as well as substantial populations of resident animals. This is especially true of savannas in Africa, where this biome supports a very diverse assemblage of large mammals, including gazelles and other antelopes, rhinos, elephant, hippopotamus, and buffalo, and some of their predators, such as lion, cheetah, wild dog, and hyena.

Chaparral

Chaparral is a temperate biome that develops in environments with seasonally extreme moisture gradients, characterized by a so-called Mediterranean climate with winter rains and summer drought. The chaparral biome is typically composed of dwarf forest and shrubs, and interspersed herbaceous vegetation. Chaparral is highly prone to events of catastrophic wildfire. In North America, chaparral is best developed in parts of the southwest, especially coastal southern California.

Desert

Desert is a temperate or tropical biome, commonly occurring in the center of continents, and in the rain shadows of mountains. The distribution of this biome is determined by the availability of water, generally occurring where there is less than 9.9 in (25 cm) of precipitation per year. Not surprisingly, the productivity of desert ecosystems is strongly influenced by the availability of water. The driest deserts support almost no plant productivity, while less-dry situations may support communities of herbaceous, succulent, and annual plants, and somewhat moister places will allow a shrub-dominated ecosystem to develop.

Semi-evergreen tropical forest

This type of tropical forest develops when there is a seasonality of water availability due to the occurrence of pronounced wet and drier seasons during the year. Because of this seasonality, most of the trees and shrubs of this biome are seasonally deciduous, meaning that they shed their foliage in anticipation of the drier season. This biome supports a great richness of plant and animal species, though somewhat less than in tropical rainforests.

Evergreen tropical rainforest

This biome occurs under tropical climates with abundant precipitation and no seasonal drought. Because wildfire and other types of catastrophic disturbance are uncommon in this sort of climate regime, tropical rainforests usually develop into old-growth forests. As such, they contain a diverse size range of trees, a great richness of species of trees and other plants, as well as an extraordinary diversity of animals and microorganisms. Many ecologists consider this biome to represent the peak of ecosystem development on land, because of the enormous variety of species that are supported under relatively benign climatic conditions in old-growth tropical rainforests.

Freshwater biomes

Ecosystems such as lakes and ponds occur in basins containing standing water. Depending on the rates of input and output of water, the flushing time of these ecosystems can range from days, in the case of small pools, to centuries, in the case of the largest lakes. The biological character of these freshwater ecosystems is strongly influenced by water chemistry, especially nutrient concentration and water transparency. Waters with a large nutrient supply are highly productive (eutrophic), while infertile waters are unproductive (oligotrophic). Commonly, shallow water bodies are much more productive than deeper water bodies of the same surface area. However, plant productivity is also influenced by how far light can penetrate into the water column. This factor is restricted in waters with large concentrations of turbidity associated with silt, or with a brown color caused by dissolved organic matter. In such cases, primary productivity is smaller than might be expected on the basis of nutrient supply. The ecosystems of lentic waters are usually highly zonal in two dimensions. Horizontal zonation is associated with variations of water depth, usually related to slope and length of the shoreline. Vertical zonation of deeper waters is determined by light availability, water temperature, and nutrient and oxygen concentrations. There are also distinct, benthic ecosystems in the sediment of lentic ecosystems.

Wetlands

Freshwater wetlands (or mires) occur in shallow waters, usually having pronounced seasonal variations of water depth, sometimes including dry periods during which water does not occur at the surface. The four major wetland types are: marsh, swamp forest, bog, and fen. Marshes are the most productive wetlands, and are typically dominated by relatively tall, emergent species of angiosperm plants such as reed, cattail, and bulrush, and by floating-leaved plants such as water lily and lotus. Swamps are forested wetlands, seasonally or permanently flooded, and in North America, dominated by tree-sized plants such as bald cypress or silver maple. Bogs are acidic, unproductive wetlands that develop in relatively cool but wet climates. Bogs depend on atmospheric inputs for their supply of nutrients, and are typically dominated by species of sphagnum moss. Fens also develop in cool and wet climates, but they have a better nutrient supply than bogs, and are consequently less acidic and more productive.

Marine biomes

Open ocean

The character of the open-water biome is determined by physical and chemical environmental factors, particularly waves, tides, currents, salinity, temperature, light intensity, and nutrient concentration. Primary productivity of this biome is small, and comparable to some of the least productive terrestrial biomes, such as deserts. Primary production in the open ocean is carried out by phytoplankton of diverse species, ranging in size from extremely small photo-synthetic bacteria, to larger but microscopic unicellular and colonial algae. The phytoplankton are grazed by small crustaceans known as zooplankton, and these are eaten in turn by small fish. At the top of the oceanic food web are very large predators such as bluefin tuna, sharks, squid, and whales. The deep benthic ecosystems of this biome are supported by a sparse rain of dead biomass from its surface waters. The benthic ecosystems are not well known, but they appear to be extremely stable, rich in species, and low in productivity.

Continental shelf waters

Near-shore waters of the oceans are relatively shallow, because they overlie continental shelves. Compared with the open ocean, waters over continental shelves are relatively warm, and are well supplied with nutrients. The nutrients originate with inputs from rivers, and from occasional movements of deeper, more fertile waters to the surface, stirred from the bottom by turbulence caused by storms. Mostly as a result of the nutrient inputs, the phytoplankton of these waters are relatively productive, and they support a larger biomass of animals than occurs in the open ocean. Some of the world’s most important pelagic and benthic fisheries are supported by the continental shelf biome, for example, those in the North Sea and Barents Sea of western Europe, the Grand Banks and other shallow waters of northeastern North America, the Gulf of Mexico, and inshore waters of much of western North America.

Upwelling regions

In certain places or regions, oceanographic conditions favor upwellings to the surface of relatively deep, nutrient rich waters. Because of the enhanced nutrient supply, upwelling areas are relatively fertile, and they sustain this highly productive, open-ocean biome. Because of their large foundation of primary production, upwelling regions support sizable populations of animals, including large species of fish and shark, marine mammals, and seabirds. Some of Earth’s most productive fisheries occur in upwelling areas, such as that off the west coast of Peru and elsewhere off South America, and large regions of the Antarctic Ocean.

Estuaries

Estuaries are a complex group of coastal ecosystems that are semi-enclosed, but open to the sea. Estuaries display characteristics of both marine and freshwater biomes, because they typically have substantial inflows of fresh water from the nearby land, along with large fluctuations of salt water resulting from tidal cycles. Examples of estuaries include coastal bays, sounds, river mouths, salt marshes, and tropical mangrove forests. Because their large water-borne inputs of terrestrial nutrients are partially retained by their semi-enclosed water circulation, estuaries are highly productive ecosystems. Estuaries provide important habitat for juvenile stages of many commercially important species of fish, shellfish, and crustaceans, and they are often characterized as “nursery” habitat for these species.

Seashores

The seashore biome is a complex of ecosystems occurring at the interface of terrestrial and oceanic biomes. The local character of the seashore biome is determined by environmental factors, such as the intensity of wave action, the frequency of major events of disturbance, and bottom type. In temperate waters, hard-rock and cobble bottoms often develop ecosystems dominated by large species of macroalgae, broadly known as seaweeds, or kelp. In some cases, kelp forests can develop. These are highly productive ecosystems, which maintain large quantities of biomass, mostly of macroalgae. In situations characterized by softer bottoms of sand or mud, ecosystems are typically dominated by benthic invertebrates, especially mollusks, echinoderms, crustaceans, and marine worms.

Coral reefs

Coral reefs are a distinctive marine biome of tropical seas, occurring locally in shallow but relatively infertile areas close to land. The physical structure of coral reefs is provided by the calcium carbonate exoskeletons of dead coral polyps. This structure supports a species-rich population of living coral, crustose algae, invertebrates, and fish. This biome is dominated by corals, a diverse group of coelenterate animals, living in symbiosis with unicellular algae. Because this symbiosis is highly efficient in the acquisition and recycling of nutrients, coral reefs typically sustain a high productivity, even though they occur in nutrient-poor waters.

KEY TERMS

Anthropogenic— A situation that occurs because of, or is influenced by, the activities of humans.

Biome— A geographically extensive ecosystem, usually characterized by its dominant life forms.

Community— In ecology, a community is an assemblage of populations of different species that occur together in the same place and at the same time.

Convergence— An evolutionary pattern by which unrelated species that fill similar ecological niches tend to develop similar morphologies and behavior. Convergence occurs in response to similar selection pressures.

Monoculture— An ecosystem dominated by a single species.

Species richness— The number of species occurring in a community, a landscape, or some other defined area.

See also Continental shelf; Desert; Ecosystem; Forests; Grasslands; Lake; Ocean; Savanna; Tundra; Wetlands.

Resources

BOOKS

Allaby, Michael and Richard Garratt. Temperate Forests (Biomes of the Earth). New York: Chelsea House Publications, 2006.

VanCleave, Janice. Janice VanCleave’s Science Around the World: Activities on Biomes from Pole to Pole. New York: Jossey-Bass, 2004.

0 Woodward, Susan L. Biomes of Earth: Terrestrial, Aquatic, and Human-Dominated. Westport: Greenwood Press, 2003.

PERIODICALS

Clark, B.C. “Planetary Interchange of Bioactive Material: Probability Factors and Implications.” Origins of Life and Evolution of the Biosphere no. 31 (2001): 185-197.

Bill Freedman

Biome

A biome is a major, geographically extensive ecosystem , structurally characterized by its dominant life forms. Terrestrial biomes are usually distinguished on the basis of the major components of their mature or climax vegetation, while aquatic biomes, especially marine ones, are often characterized by their dominant animals. Most of the oceans are considered part of a single biome, although areas with particularly unusual or unique physical characteristics or inhabitants may be considered as separate biomes.

Similar biomes can occur in widely divergent places as long as the environmental conditions are appropriate for their development. Some environmental conditions affecting the location of biomes include climate, latitude, topography, and fire. Often, different species having similar, convergent growth forms will dominate at different places within the same biome. For example, the boreal coniferous forest occurs in suitable environments of northern North America and Eurasia. In northeastern North America this biome is dominated by stands of black spruce , while in the northwest white spruce is dominant. Norway spruce is most important in this biome in northwestern Europe , while in parts of Siberia species of pine and larch are dominant. Because biomes are described according to the structural characteristics of their dominant organisms (in this example, coniferous trees growing under a particular climatic regime) all of these different forest types are considered convergent ecosystems within the same biome, the boreal coniferous forest.

Major biomes and their characteristics

Most biomes are delineated on the basis of their naturally occurring communities of plants, animals, and microorganisms . Exceptions are the so-called anthropogenic biomes, which are strongly influenced by humans and their activities, as in the case of cities and agroecosystems. However, it should be remembered that to some degree, all of Earth's biomes have been influenced by human activities. All organisms, for example, contain trace contaminations of certain organochlorine chemicals of human manufacture, such as DDT and PCBs.

Ecologists have used a number of systems to divide the biosphere into major biomes. A classification of global biomes, modified from one proposed by E.P. Odum, is described below:

Terrestrial biomes

Tundra

Tundra is a treeless biome occurring in areas with cold climates and a short growing season. Alpine tundra occurs at high altitudes on mountains , while arctic tundra occurs at high latitudes. Most tundras receive very small inputs of water as precipitation , but nevertheless their soil may be moist or wet because there is little evaporation in such cold climates, and deep drainage may be prevented by frozen soil. The coldest, most northern, high-arctic tundras are very unproductive and dominated by long-lived but short-statured plants, typically less than 1.97-3.94 in (5-10 cm) tall. Low-arctic tundras are dominated by shrubs as tall as 3.28 ft (1 m), while wet sites develop relatively productive meadows of sedge, cotton-grass, and grass. In North America, arctic tundras can support small densities of mammalian herbivores such as caribou and muskox (although during migration these animals can occur in locally large densities), and even smaller numbers of their predators, such as wolves.

Boreal coniferous forest

The boreal coniferous forest, or taiga, is an extensive northern biome occurring in moist climates with cold winters. The boreal forest is dominated by coniferous trees, especially species of fir, larch, pine, and spruce. Some broad-leaved, angiosperm trees are also important in the boreal forest, especially species of aspen, birch, poplar, and willow. Usually, particular stands of boreal forest are dominated by only one or several species of trees. Most regions of boreal forest are subject to periodic events of catastrophic disturbance, most commonly caused by wildfire and sometimes by insects , such as spruce budworm, that kill trees through intensive defoliation. Montane forests , also dominated by conifers and similar in structure to the boreal forest, can occur at sub-alpine altitudes on mountains in southerly latitudes.

Temperate deciduous forest

Forests dominated by species-rich mixtures of broad-leaved trees occur in relatively moist, temperate climates. Because these forests occur in places where the winters can be cold, the foliage of most species is seasonally deciduous, meaning that all leaves are shed each autumn and re-grown in the springtime. Common trees of this forest biome in North America are species of ash, basswood , birch, cherry, chestnut , dogwood, elm , hickory, magnolia , maple, oak, tulip-tree, and walnut, among others. These various tree species segregate into intergrading communities on the basis of site variations of soil moisture, fertility, and air temperature .

Temperate rainforest

Temperate rainforests develop under climatic regimes characterized by mild winters and an abundance of precipitation. Because these systems are too moist to support regular, catastrophic wildfires, they often develop into old-growth forests , dominated by coniferous trees of mixed age and species composition. Individual trees can be extremely large, and in extreme cases can be more than 1,000 years old. Common trees of this biome are species of Douglas fir, hemlock, cedar, redwood, spruce, and yellow cypress. In North America, temperate rainforests are best developed on the humid west coast.

Temperate grassland

These grasslands occur under temperate climatic regimes that are intermediate to those that support forest and desert . In the temperate zones, grasslands typically occur where rainfall is 9.9-24 in (25-60 cm) per year. Grasslands in North America are called prairie (they are often called steppe in Eurasia), and this biome occupies vast regions in the interior. The prairie is often divided into three types according to height of the dominant vegetation—tall-grass, mixed-grass, and short-grass. The once extensive tall-grass prairie is dominated by various species of grasses and herbaceous broad leaved plants such as sunflowers and blazing stars, some as tall as 9.8-13.1 ft (3-4 m). Fire was an important natural factor that prevented much of the tall-grass prairie from developing into an open forest. The tall-grass prairie is now an endangered natural ecosystem, because it has been almost entirely converted to agriculture. The mixed-grass prairie occurs where rainfall is less, and it supports shorter species of grasses and other herbaceous plants. The short-grass prairie has even less precipitation, and is subject to unpredictable years of severe drought .

Tropical grassland and savanna

Tropical grasslands can occur in regions with as much as 47.2 in (120 cm) of rainfall per year, but under highly seasonal conditions with a pronounced dry season. Savannas are dominated by grasses and other herbaceous plants, but they also have scattered shrub and tree-sized woody plants, which form a very open canopy. Tropical grasslands and savannas can support a great seasonal abundance of large, migratory mammals , as well as substantial populations of resident animals. This is especially true of savannas in Africa , where this biome supports a very diverse assemblage of large mammals, including gazelles and other antelopes, rhinos, elephant , hippopotamus, and buffalo, and some of their predators, such as lion, cheetah, wild dog, and hyena .

Chaparral

Chaparral is a temperate biome that develops in environments with seasonally extreme moisture gradients, characterized by a so-called Mediterranean climate with winter rains and summer drought. The chaparral biome is typically composed of dwarf forest and shrubs, and interspersed herbaceous vegetation. Chaparral is highly prone to events of catastrophic wildfire. In North America, chaparral is best developed in parts of the southwest, especially coastal southern California.

Desert

Desert is a temperate or tropical biome, commonly occurring in the center of continents, and in the rain shadows of mountains. The distribution of this biome is determined by the availability of water, generally occurring where there is less than 9.9 in (25 cm) of precipitation per year. Not surprisingly, the productivity of desert ecosystems is strongly influenced by the availability of water. The driest deserts support almost no plant productivity, while less-dry situations may support communities of herbaceous, succulent, and annual plants, and somewhat moister places will allow a shrub-dominated ecosystem to develop.

Semi-evergreen tropical forest

This type of tropical forest develops when there is a seasonality of water availability due to the occurrence of pronounced wet and drier seasons during the year. Because of this seasonality, most of the trees and shrubs of this biome are seasonally deciduous, meaning that they shed their foliage in anticipation of the drier season. This biome supports a great richness of plant and animal species, though somewhat less than in tropical rainforests.

Evergreen tropical rainforest

This biome occurs under tropical climates with abundant precipitation and no seasonal drought. Because wildfire and other types of catastrophic disturbance are uncommon in this sort of climate regime, tropical rainforests usually develop into old-growth forests. As such, they contain a diverse size range of trees, a great richness of species of trees and other plants, as well as an extraordinary diversity of animals and microorganisms. Many ecologists consider this biome to represent the epitome of ecosystem development on land, because of the enormous variety of species that are supported under relatively benign climatic conditions in old-growth tropical rainforests.

Freshwater biomes

Lentic ecosystems, such as lakes and ponds, occur in basins containing standing water. Depending on the rates of input and output of water, the flushing time of these ecosystems can range from days, in the case of small pools, to centuries, in the case of the largest lakes. The biological character of lentic ecosystems is strongly influenced by water chemistry , especially nutrient concentration and water transparency. Waters with a large nutrient supply are highly productive, or eutrophic, while infertile waters are unproductive, or oligotrophic. Commonly, shallow water bodies are much more productive than deeper water bodies of the same surface area. However, plant productivity is also influenced by how far light can penetrate into the water column. This factor is restricted in waters with large concentrations of turbidity associated with silt, or with a brown color caused by dissolved organic matter . In such cases, primary productivity is smaller than might be expected on the basis of nutrient supply. The ecosystems of lentic waters are usually highly zonal in two dimensions. Horizontal zonation is associated with variations of water depth, usually related to slope and length of the shoreline. Vertical zonation of deeper waters is determined by light availability, water temperature, and nutrient and oxygen concentrations. There are also distinct, benthic ecosystems in the sediment of lentic ecosystems.

Lotic

The character of this running-water biome is determined by physical factors, especially the quantity, velocity , and seasonal variations of water flow. These hydrologic characteristics influence other important characteristics of lotic ecosystems. For example, the bottom tends to be muddy in places with calm water flows where silt is deposited, and rocky in more vigorous places where fine particles are selectively eroded from the bottom. Similarly, turbidity is great during high water flows, and this interferes with the penetration of light, restricting plant productivity. Although they sustain some primary productivity of aquatic plants, the common lotic ecosystems such as rivers , streams, and brooks are not usually self-supporting in terms of fixed energy . These ecosystems typically rely on inputs of organic matter from their surrounding, terrestrial watershed , or from upstream lakes to support much of their productivity of aquatic invertebrates and fish .

Wetlands

Freshwater wetlands (or mires) occur in shallow waters, usually having pronounced seasonal variations of water depth, sometimes including dry periods during which water does not occur at the surface. The four major wetland types are: marsh, swamp forest, bog, and fen. Marshes are the most productive wetlands, and are typically dominated by relatively tall, emergent species of angiosperm plants such as reed, cat-tail, and bulrush, and by floating-leaved plants such as water lily and lotus. Swamps are forested wetlands, seasonally or permanently flooded, and in North America, dominated by tree-sized plants such as bald cypress or silvermaple. Bogs are acidic, unproductive wetlands that develop in relatively cool but wet climates. Bogs depend on atmospheric inputs for their supply of nutrients , and are typically dominated by species of sphagnum moss . Fens also develop in cool and wet climates, but they have a better nutrient supply than bogs, and are consequently less acidic and more productive.

Marine biomes

Open ocean

The character of the open-water, or pelagic oceanic biome is determined by physical and chemical environmental factors, particularly waves, tides , currents , salinity, temperature, light intensity, and nutrient concentration. Primary productivity of this biome is small, and comparable to some of the least productive terrestrial biomes, such as deserts. Primary production in the open ocean is carried out by phytoplankton of diverse species, ranging in size from extremely small photosynthetic bacteria , to larger but microscopic unicellular and colonial algae . The phytoplankton are grazed by small crustaceans known as zooplankton , and these are eaten in turn by small fish. At the top of the pelagic food web are very large predators such as bluefin tuna , sharks , squid , and whales. The deep benthic ecosystems of this biome are supported by a sparse rain of dead biomass from its surface waters. The benthic ecosystems are not well known, but they appear to be extremely stable, rich in species, and low in productivity.

Continental shelf waters

Near-shore waters of the oceans are relatively shallow, because they overlie continental shelves. Compared with the open ocean, waters over continental shelves are relatively warm, and are well supplied with nutrients. The nutrients originate with inputs from rivers, and from occasional movements of deeper, more fertile waters to the surface, stirred from the bottom by turbulence caused by storms. Mostly as a result of the nutrient inputs, the phytoplankton of these waters are relatively productive, and they support a larger biomass of animals than occurs in the open ocean. Some of the world's most important pelagic and benthic fisheries are supported by the continental shelf biome, for example, those in the North Sea and Barents Sea of western Europe, the Grand Banks and other shallow waters of northeastern North America, the Gulf of Mexico, and inshore waters of much of western North America.

Upwelling regions

In certain places or regions, oceanographic conditions favor upwellings to the surface of relatively deep, nutrient rich waters. Because of the enhanced nutrient supply, upwelling areas are relatively fertile, and they sustain this highly productive, open-ocean biome. Because of their large foundation of primary production, upwelling regions support sizable populations of animals, including large species of fish and shark, marine mammals, and seabirds. Some of Earth's most productive fisheries occur in upwelling areas, such as that off the west coast of Peru and elsewhere off South America , and large regions of the Antarctic Ocean.

Estuaries

Estuaries are a complex group of coastal ecosystems that are semi-enclosed, but open to the sea. Estuaries display characteristics of both marine and freshwater biomes, because they typically have substantial inflows of fresh water from the nearby land, along with large fluctuations of salt water resulting from tidal cycles. Examples of estuaries include coastal bays, sounds, river mouths, salt marshes, and tropical mangrove forests. Because their large water-borne inputs of terrestrial nutrients are partially retained by their semi-enclosed water circulation, estuaries are highly productive ecosystems. Estuaries provide important habitat for juvenile stages of many commercially important species of fish, shellfish, and crustaceans, and they are often characterized as "nursery" habitat for these species.

Seashores

The seashore biome is a complex of ecosystems occurring at the interface of terrestrial and oceanic biomes. The local character of the seashore biome is determined by environmental factors, such as the intensity of wave action, the frequency of major events of disturbance, and bottom type. In temperate waters, hard-rock and cobble bottoms often develop ecosystems dominated by large species of macroalgae, broadly known as seaweeds, or kelp. In some cases, so-called kelp "forests" can develop. These are highly productive ecosystems, which maintain large quantities of biomass, mostly of macroalgae. In situations characterized by softer bottoms of sand or mud, ecosystems are typically dominated by benthic invertebrates, especially mollusks , echinoderms, crustaceans, and marine worms.

Coral reefs

Coral reefs are a distinctive marine biome of tropical seas, occurring locally in shallow but relatively infertile areas close to land. The physical structure of coral reefs is provided by the calcium carbonate exoskeletons of dead coral polyps. This structure supports a species-rich population of living coral, crustose algae, invertebrates, and fish. This biome is dominated by corals, a diverse group of coelenterate animals, living in symbiosis with unicellular algae. Because this symbiosis is highly efficient in the acquisition and recycling of nutrients, coral reefs typically sustain a high productivity, even though they occur in nutrient-poor waters.

Human-dominated biomes

Urban-industrial techno-ecosystems

This anthropogenic biome consists of large metropolitan districts, dominated by humans, human dwellings, businesses, factories, and other types of infrastructure. This biome supports many species in addition to humans, but, with few exceptions, these are non-native plants and animals that have been introduced from other places, and that cannot live independently outside of this biome, unless they are returned to their native biome.

Rural techno-ecosystems

This is another anthropogenic biome, occurring outside of intensively built-up areas, and consisting of certain components of the extensive technological infrastructure of civilization. This biome is comprised of transportation corridors (such as highways, railways, transmission corridors, and aqueducts), small towns, and industries involved in the extraction, processing, and manufacturing of products from natural resources. Typically, this biome supports mixtures of introduced species and those native species that are tolerant of the disturbances and other stresses associated with human activities.

Agroecosystems

This biome consists of ecosystems that are managed and harvested for human use. The components of this biome are uneven in their anthropogenic influence. The most intensively managed agroecosystems typically involve monocultures of non-native crop species of plants or animals in agriculture, aquaculture, or forestry , and are not favorable to native wildlife . The management objective is to cultivate the economically valuable species under conditions that ensure optimal growth. Less-intensively managed agroecosystems involve mixtures of species, or polycultures, and these may provide habitat for some native wildlife species.

See also Continental shelf; Desert; Ecosystem; Forests; Grasslands; Lake; Ocean; Savanna; Tundra; Wetlands.

Resources

books

Barbour, M.G., and W.D. Billings. North American TerrestrialVegetation. New York: Cambridge University Press, 1988.

Begon, M., J.L. Harper, and C.R. Townsend. Ecology. Individuals, Populations and Communities. 2nd ed. London: Blackwell Sci. Pub., 1990.

Odum, E.P. Ecology and Our Endangered Life-Support Systems. Sunderland, MA: Sinauer Assoc. Ltd., 1993.

Walter, H. Vegetation of the Earth. New York: Springer-Verlag, 1977.

periodicals

Clark, B.C. "Planetary Interchange of Bioactive Material: Probability Factors and Implications." Origins of Life and Evolution of the Biosphere no. 31 (2001): 185-197.

Bill Freedman

KEY TERMS

Anthropogenic

—A situation that occurs because of, or is influenced by, the activities of humans.

Biome

—A geographically extensive ecosystem, usually characterized by its dominant life forms.

Community

—In ecology, a community is an assemblage of populations of different species that occur together in the same place and at the same time.

Convergence

—An evolutionary pattern by which unrelated species that fill similar ecological niches tend to develop similar morphologies and behavior. Convergence occurs in response to similar selection pressures.

Monoculture

—An ecosystem dominated by a single species.

Species richness

—The number of species occurring in a community, a landscape, or some other defined area.

Biome

A biome is an ecosystem containing plant and animal species that are characteristic to a specific geographic region. (An ecosystem is the community of plants and animals in an area considered together with their environment.) The nature of a biome is determined primarily by climate, including a region's annual average temperature and amount of rainfall. Biomes are often named for the vegetation found within them. They can be classified as terrestrial (land), aquatic (water), or anthropogenic (dominated by humans). Some familiar examples of biomes include tundra, desert, chaparral, and open ocean. The accompanying drawing shows the variety of biomes that can be found along just two lines of longitude on Earth's surface.

A number of attempts have been made to classify the world's biomes. One of the best known was proposed by the Russian-born German climatologist Wladimir Koeppen (1846–1940). Some of the biomes described by Koeppen are described below.

Terrestrial biomes

Tundra. A tundra is a treeless region in a cold climate with a short growing season. Most tundras receive little precipitation. Still, their soil may be moist or wet because little evaporation occurs. Loss of water by seepage is also prevented because the soil is frozen. Very little vegetation grows and very few animals live in the coldest, most northern, high-arctic tundras. These tundras are dominated by long-lived but short-statured plants, typically less than 5 to 10 centimeters (2 to 4 inches) tall. Low-arctic tundras are dominated by shrubs as tall as 1 meter (3 feet), while on wet sites relatively productive meadows of sedge, cottongrass, and grass grow. In North America, arctic tundras can support small numbers of plant-eating mammals, such as caribou and musk oxen, and even smaller numbers of their predators, such as wolves.

Words to Know

Anthropogenic: Resulting from the influence of human action on nature.

Aquatic: Related to water.

Benthic: Referring to the deepest parts of the oceans.

Boreal: Located in a northern region.

Conifer: Plants whose seeds are stored in cones and that retain their leaves all year around.

Deciduous: Plants that lose their leaves at some season of the year, and then grow them back at another season.

Ecosystem: An ecological community, including plants, animals, and microorganisms, considered together with their environment.

Eutrophic: A productive aquatic region with a large nutrient supply.

Herbaceous: A type of plant that has little or no woody tissue and usually lives for only one growing season.

Lentic ecosystem: An ecosystem that contains standing water.

Lotic ecosystem: An ecosystem that consists of running water.

Monoculture: An ecosystem dominated by a single species.

Oligotrophic: An unproductive aquatic region with a relatively modest nutrient supply.

Pelagic: Referring to the open oceans.

Polyculture: An ecosystem that consists of a wide variety of species.

Temperate: Mild or moderate.

Tropical: Characteristic of a region or climate that is frost free with temperatures high enough to support—with adequate precipitation—plant growth year round.

Upwelling: The process by which lower, nutrient-rich waters rise upward to the ocean's surface.

Wetlands: Areas that are wet or covered with water for at least part of the year.

Boreal coniferous forest. The boreal coniferous forest, or taiga, is an extensive northern biome occurring in moist climates with cold winters. The boreal forest is dominated by coniferous (cone-bearing) trees, especially species of fir, larch, pine, and spruce. Some broad-leaved trees are also present in the boreal forest, especially species of aspen, birch, poplar, and willow. Most boreal forests are subject to periodic catastrophic disturbances, such as wildfires and attacks by insects.

Temperate deciduous forest. Temperate deciduous forests are dominated by a large variety of broad-leaved trees in relatively moist, temperate (mild or moderate) climates. Because these forests occur in places where the winters can be cold, the foliage of most species is seasonally deciduous, meaning that trees shed their leaves each autumn and then regrow them in the springtime. Common trees of the temperate deciduous forest biome in North America are ash, basswood, birch, cherry, chestnut, dogwood, elm, hickory, magnolia, maple, oak, tulip-tree, and walnut.

Temperate rain forest. Temperate rain forests are characterized by mild winters and an abundance of rain. These systems are too moist to support wildfires. As a result, they often develop into old-growth forests, dominated by coniferous trees of mixed age and various species. Individual trees can be very large and, in extreme cases, can be more than 1,000 years old. Common trees of this biome are species of Douglas-fir, hemlock, cedar, redwood, spruce, and yellow cypress. In North America, temperate rain forests are most commonly found on the humid west coast.

Temperate grassland. Temperate grasslands occur under climatic conditions that are between those that produce forests and those that produce deserts. In temperate zones, grasslands typically occur in regions where rainfall is 25 to 60 centimeters (10 to 24 inches) per year. Grasslands in North America are called prairies and in Eurasia they are often called steppes. This biome occupies vast regions of the interior of these continents.

The prairie is often divided into three types according to height of the dominant vegetation: tall grass, mixed grass, and short grass. The once-extensive tall grass prairie is dominated by various species of grasses and broad-leaved, herbaceous plants such as sunflowers and blazing stars, some as tall as 3 to 4 meters (10 to 13 feet). Fire played a key role in preventing much of the tall grass prairie from developing into open forest. The tall grass prairie is now an endangered natural ecosystem because it has been almost entirely converted for agricultural use.

The mixed grass prairie occurs where rainfall is less plentiful, and it supports shorter species of grasses and other herbaceous plants. The short grass prairie develops when there is even less precipitation, and it is subject to unpredictable years of severe drought.

Tropical grassland and savanna. Tropical grasslands are present in regions with as much as 120 centimeters (47 inches) of rainfall per year, but under highly seasonal conditions with a pronounced dry season. Savannas are dominated by grasses and other herbaceous plants. However, they also have scattered shrubs and tree-sized woody plants that form a very open canopy (a layer of spreading branches).

Tropical grasslands and savannas can support a great seasonal abundance of large, migratory animals as well as substantial populations of resident animals. This is especially true of Africa, where on the savanna range—among other animals—gazelles and other antelopes, rhinos, elephants, hippopotamuses, and buffalo, and various predators of these, such as lions, cheetahs, wild dogs, and hyenas.

Chaparral. Chaparral is a temperate biome that develops in environments where precipitation varies widely from season to season. A common chaparral pattern involves winter rains and summer drought, the socalled Mediterranean climate. Chaparral is characterized by dwarf forests, shrubs, and herbaceous vegetation. This biome is highly prone to wildfire. In North America, chaparral is best developed in parts of the southwest, especially coastal southern California.

Desert. Deserts occur in either temperate or tropical climates. They commonly are found in the centers of continents and in rain shadows of

mountains (a dry region on the side of a mountain sheltered from rain). The most prominent characteristic of a desert is the limited amount of water available. In most cases, less than 25 centimeters (10 inches) of rain fall each year. Not surprisingly, the plant life found in a desert ecosystem is strongly influenced by the availability of water: the driest deserts support almost no plant life, while less-dry deserts may support communities of herbaceous, succulent (having fleshy tissues that conserve water), and annual (returning year after year) plants. In somewhat moister places, a shrub-dominated ecosystem is able to develop.

Semi-evergreen tropical forest. A semi-evergreen tropical forest is a type of tropical forest that develops when a region experiences both wet and dry seasons during the year. Because of this pattern, most trees and shrubs of this biome are seasonally deciduous, meaning that they shed their foliage in anticipation of the drier season. This biome supports a great richness of species of plants and animals, though somewhat less than in tropical rain forests.

Evergreen tropical rain forest. Evergreen tropical rain forests occur in tropical climates with abundant precipitation and no seasonal drought. Because wildfires and other types of catastrophic disturbances are uncommon in this sort of climate regime, tropical rain forests usually develop into old-growth forests. As such, they contain a great richness of species of trees and other plants, a great size range of trees, and an extraordinary diversity of animals and microorganisms. Many ecologists consider the old-growth tropical rain forests the ideal ecosystem on land because of the enormous variety of species that are supported under relatively favorable climatic conditions.

Freshwater biomes

Freshwater biomes can be divided into three general categories: lentic, lotic, and wetlands.

Lentic. A lentic ecosystem is one such as a lake or pond that contains standing water. In lentic systems, water generally flows into and out of the lake or pond on a regular basis. The rates at which inflow and outflow occur vary greatly and can range from days, in the case of small pools, to centuries, in the case of the largest lakes.

The types of organisms that inhabit lentic biomes are strongly influenced by water properties, especially nutrient concentration and water transparency and depth. Waters with a large nutrient supply are highly productive, or eutrophic, while infertile waters are unproductive, or oligotrophic. Commonly, shallow bodies of water are much more productive than deeper bodies of water of the same surface area, primarily because plant growth is influenced by the ability of light to penetrate into the water. Water that becomes cloudy because of the accumulation of silt or dissolved organic matter is likely to have low productivity.

Lotic. A lotic biome is one that consists of running water, as in streams or rivers. The organisms found in a lotic biome depend on factors such as the amount of water in the system, the rate at which it flows, and seasonal changes in the flow rate. Consider a stream in which flooding is common in the spring. Rapidly moving water churns up clay, silt, sand, and other materials from the streambed. The water then becomes cloudy and murky, and light is thus prevented from penetrating it. In this case the stream will not be able to support many kinds of life-forms.

In general, the common lotic ecosystems such as rivers, streams, and brooks are not usually self-supporting in terms of the organisms that live within them. Instead, they typically rely on organic matter carried into them from the land around them or from upstream lakes to support fish and other organisms that live in the biome.

Wetlands. Wetlands are areas that are wet or covered with water for at least part of the year. Some examples of wetlands are marshes, swamps, bogs, sloughs, and fens. Marshes are the most productive wetlands, and are typically dominated by relatively tall plants such as reeds, cat-tails, and bulrushes and by floating-leaved plants such as water lilies and lotus. Swamps are forested wetlands that are seasonally or permanently flooded. In North America, swamps are dominated by tree-sized plants such as bald cypress or silver maple.

Bogs are wetlands that develop in relatively cool but wet climates. They tend to be acidic and, therefore, biologically unproductive. Bogs depend on nutrients obtained from the atmosphere, and are typically dominated by species of sphagnum moss. Fens also develop in cool and wet climates, but they have a better nutrient supply than bogs. Consequently, they are less acidic and more productive than bogs.

Marine biomes

Open ocean. The character of the open-water, or pelagic, oceanic biome is determined by factors such as waves, tides, currents, salinity (salt content), temperature, amount of light, and nutrient concentration. The number of organisms supported by these factors is small and can be compared to some of the least productive terrestrial biomes, such as deserts. The lowest level of food webs in the ocean are occupied by tiny organisms known as phytoplankton. Various species of phytoplankton range in size from extremely small bacteria to larger algae that consist of a single cell and may or may not live in large colonies.

The phytoplankton are grazed upon by small crustaceans known as zooplankton. Zooplankton, in turn, are eaten by small fish. At the top of the pelagic food web are very large predators such as bluefin tuna, sharks, squid, and whales.

The deepest levels of the ocean make up the benthic biome. Organisms in this biome are supported by a meager rain of dead organisms from its surface waters. The benthic ecosystems are not well known, but they appear to be extremely stable, rich in species, and low in nutrient productivity.

Continental shelf waters. Continental shelf waters are areas of ocean water that lie relatively near a coastline. Compared with the open ocean, waters over continental shelves are relatively warm and are well supplied with nutrients from rivers flowing into them. A secondary source of nutrients is water brought to the surface from deeper, more fertile waters that were stirred up by turbulence caused by storms.

Because of the nutrients found in the continental shelf waters, phytoplankton here are relatively abundant and support the larger animals present in the open ocean. Some of the world's most important commercial fisheries are on the continental shelves, including the North and Barents Seas of western Europe, the Grand Banks and other shallow waters of northeastern North America, the Gulf of Mexico, and inshore waters of much of western North America.

Upwelling regions. In certain regions of the ocean, conditions make possible upwellings to the surface of relatively deep, nutrient rich waters. Because of the increased nutrient supply, upwelling areas are relatively fertile, and they support sizeable populations of animals, including large species of fishes and sharks, marine mammals, and seabirds. Some of Earth's most productive fisheries occur in upwelling areas, such as those off the west coast of Peru and other parts of South America and large regions of the Antarctic Ocean.

Estuaries. An estuary is a region along a coastline where a river empties into the ocean. Estuaries display characteristics of both marine and freshwater biomes. They typically have substantial inflows of freshwater from the nearby land, along with large fluctuations of saltwater resulting from tidal cycles. Examples of estuaries include coastal bays, sounds, river mouths, salt marshes, and tropical mangrove forests.

Because the nutrients carried into them by rivers, estuaries are highly productive ecosystems. They provide important habitats for juvenile stages of many species of fish, shellfish, and crustaceans that are later harvested for food. Indeed, estuaries are sometimes called "nursery" habitats.

Seashores. The seashore biome is formed where the land meets the ocean. The specific character of any given seashore biome is determined by factors such as the intensity of wave action, the frequency of major disturbances, and bottom type. In temperate waters, biomes are often characterized by large species of algae, broadly known as seaweed or kelp. In some cases, so-called kelp "forests" can develop, abundant with marine life. In ecosystems characterized by softer bottoms of sand or mud, invertebrates such as mollusks, echinoderms, crustaceans, and marine worms dominate.

Coral reefs. Coral reefs are marine biomes that are unique to tropical seas. They grow in shallow but relatively infertile areas close to land. Corals are small, tropical marine animals that attach themselves to the seabed and form extensive reefs. The physical structure of the reef is provided by the calcium carbonate skeletons of dead corals. Corals live in symbiosis (in union) with algae, and together create a highly efficient system of obtaining and recycling nutrients. For this reason, coral reefs are

highly productive, even though they occur in nutrient-poor waters, and support a great variety of species, including living corals, algae, invertebrates, and fishes.

Human-dominated biomes

Urban-industrial techno-ecosystems. The urban-industrial techno-ecosystem consists of a large metropolitan district that is dominated by humans, human dwellings, businesses, factories, other types of buildings, and roads. This biome supports many species in addition to humans. With few exceptions, however, these species are nonnative plants and animals that have been introduced from other places. These organisms typically cannot live independently outside of this biome, unless they are returned to their native biome.

Rural techno-ecosystems. This anthropogenic biome occurs out-side of intensively built-up areas. This biome is made up of transportation corridors (such as highways, railways, electric power line transmission corridors, and aqueducts), small towns, and industries involved in the extraction, processing, and manufacturing of products from natural resources (such as mining). Typically, this biome supports mixtures of introduced species and those native species that are tolerant of the disturbances and other stress associated with human activities.

Agroecosystems. Agroecosystems are biomes consisting of regions that are managed and harvested for human use. Farms and ranches are examples of agroecosystems. Many agroecosystems are monocultural, consisting of single types of crops, such as corn or wheat, and are not favorable to native wildlife. The objective in such systems is to manage the species in such a way as to produce a maximum dollar profit. Competing species (weeds and insect pests) are destroyed or prevented from growing or surviving. Less-intensively managed agroecosystems may contain mixtures of species, a form of land management known as polyculture. Polyculture systems may provide habitat for some native wildlife species.

[See also Corals; Desert; Ecosystem; Forests; Lake; Ocean; Rain forest ]

Biome

A biome is a large geographical area characterized by a distinct climate and soil as well as particular kinds of plants and animals. Biologists have divided the globe into six main biomes—which they also call major life zones—and have named them after the dominant type of vegetation that grows there. These six terrestrial (or land-based) biomes are the rain forest, tundra, taiga, temperate deciduous forest, desert, and grasslands. There are also three other biomes for the aquatic environment: freshwater, saltwater, and estuaries.

Biomes can be described as environments that have a lot of things in common, such as climate, vegetation, and animal life. Grouping the world's environments into separate biomes helps biologists to better understand them and to organize what has been discovered about them. Each biome is home to certain groups or communities of plants and animals that have adapted to its particular temperature and precipitation (amount of rainfall). Since a biome is necessarily a large geographic area, it usually supports a wide variety of life within the same major zone. A biome can be made up of different habitats or places where organisms live; altogether, the biomes of Earth make up the biosphere, which is the region of Earth that support life.

RAIN FOREST BIOMES

Among the six major land biomes, the biome known as the rain forest supports more species than any other, containing about half of the world's plant and animal species. Rain forests are commonly found in tropical areas close to the equator in areas of very heavy rainfall and constantly warm temperatures. They are considered one of the most biologically diverse ecosystems (an area in which living things interact with each other and the environment) on Earth. In a typical rain forest, almost every plant has another plant growing on it and the tallest trees create a canopy or rooflike effect. Actually, there are four other separate layers or levels or vegetation beneath this canopy (including lianas) that can be as thin as a rope or as thick as a tree trunk. Other plants called epiphytes (like ferns and bromeliads) grow in clumps on tree branches, and at ground level there is little vegetation since little sunlight can penetrate to the forest floor. High temperatures, constant moisture, and high humidity cause anything dead to rot very quickly. There are few ground-dwelling animals other than ants and termites; and most birds, mammals, reptiles, and amphibians live somewhere in the multilevel canopy. Fifty-seven percent of all tropical rain forests are located in South America where they are steadily being destroyed. Increasing population pressure has resulted in vast expanses of these forests being cleared for farming, only to prove unsuitable for agriculture. Constant rains quickly wash away the soil's nutrients (which were contained in a thin layer of humus), and heavy fertilizers must be added regularly. When the habitat is destroyed in this fashion, the variety of life that used to thrive in these areas disappears as well.

TUNDRA BIOMES

No environment on Earth is more different from a rain forest than the biome known as tundra. It is the coldest of all terrestrial biomes. Characterized by extremely long, severe winters and short, cool summers, tundra has been described as a polar desert. What makes it unique is the existence of "permafrost"—a condition in which the subsoil, or Earth below the top few feet of soil, remains permanently frozen. The small amount of precipitation this area does receive is trapped at the surface by the permafrost below and forms bogs in the summer. Since the subsoil never thaws, no trees or shrubs can survive, and the vegetation that manages to live there is dominated by mosses, lichens, and other plants that do not grow very large and which can reproduce very quickly during the short growing season. Few animals live on the tundra, although the Snowy owl and Arctic hare are exceptions. There are, however, vast swarms of flies and mosquitoes that appear when the few herbs and grasses flower and attract migratory birds. Although the tundra is a relatively simple environment, it is easily damaged and very slow to recover. The northern parts of Canada are composed primarily of tundra.

TAIGA BIOMES

Taiga, also called the coniferous or evergreen forest biome, supports a diverse and complex community of organisms. Although its winters can be very cold, its summers are longer than those of the tundra, allowing its soil to thaw completely. The taiga is primarily an environment where evergreen trees flourish since they are specially adapted to survive the long, cold winters. The evergreens' needle-shaped leaves prevent water loss during the harsh winter months and their flexible branches allow heavy snow to slide off without breaking. The ground below them has little vegetation and is home to mice, squirrels, porcupines, grouse, warbler birds, wolves, and moose. This biome is particularly common in southern Canada and is particularly susceptible to the effects of acid rain.

TEMPERATE DECIDUOUS FOREST BIOMES

The temperate deciduous forest biome has what we consider to be normal, well-defined seasons and is named after its most common feature—deciduous trees or trees that drop their leaves in the autumn (like oak, maple, hickory, and ash). Most of the eastern part of the United States is typical of this biome, and it is characterized by a moderate climate and fairly high rainfall. Although it is a single biome, temperate deciduous forests can be quite different according to their specific location. All support a wide variety of animal life. Insects are often abundant and some birds live there year-round, despite its winters. In this biome, leaves that fall from deciduous trees decay and result in a deep, rich humus layer that provides nutrients and conserves water. While many different mammals were once present in great numbers in this biome, today they have been reduced substantially by humans. Still, there are many raccoons, deer, moose, bobcats, a large variety of birds, and invertebrates.

DESERT BIOMES

In those parts of the world where rainfall is very low and irregular, a desert biome can be found. A desert is not only very dry, with a total rainfall between 1 to 10 inches (2.54 to 25.38 centimeters) a year, but it is also usually very hot. While most would guess that the average temperature of a desert is extremely high, it actually fluctuates or changes dramatically. At night the land cools rapidly because there is seldom any cloud cover to insulate and keep the desert heat from radiating off into space. In a desert like the Kalahari in Africa, this fluctuation can go from well over 100 °F (37.78 °C) to just above freezing. That is why a desert biome is characterized primarily by its low rainfall and not its temperature. Plants that grow in a desert are highly specialized and able to gather and store water in many different ways. For example, the leaves on a cactus became spines, while other cactus have extremely deep and spreading root systems. Despite this harsh environment, many other plants and animals live in deserts, although the desert reptiles, small mammals, and spiders and scorpions usually spend the day in burrows or shaded areas and become active at night (meaning they are nocturnal). Some animals, like the camel, have also adapted to the lack of moisture in interesting ways. Contrary to what most people believe however, the camel does not store water in its hump. Rather, it stores energy in the form of fat, and by concentrating all of its fat in one part of its body, it is allows its body heat to escape more easily from the rest of its body. Camels also can allow their body temperature to fluctuate more when water is scarce.

GRASSLAND BIOMES

Certain parts of the world are too rainy for a desert but not wet enough to support a real forest. These areas are called a grassland biome. Grasslands (also called savannahs) have hot summers and cold winters and receive between 15 and 30 inches (38.1 and 76.2 centimeters) of rain a year. Abundant grasses replace trees (that seldom grow in these fertile places). If the soil is deep and rich, tallgrass grows, while shortgrass grows in thinner soil. In the United States, grasslands are called prairies; in South America they are the pampas; in Asia they are called steppes; and in South Africa they are known as veldt. Whatever the actual name, these flatlands are covered by fast-growing grasses that are enormously efficient at converting sunlight into plant tissue. Animals common to grasslands are the prairie dog, coyote, grasshopper, rattlesnake, pronghorn antelope, and meadowlark. Grasslands often catch fire during dry times, but the roots usually remain alive allowing grass to regrow there. Grasslands are amazingly fertile and can support regular farming if treated properly, but they also turn into dustbowls with wrong management.

FRESHWATER, SALTWATER, AND ESTUARY BIOMES

Earth's water systems have their own types of biomes divided simply between freshwater, saltwater, and estuary biomes. Freshwater biomes either move (streams and rivers) or stand (lakes and ponds), and the types of fish and other organisms that live in them are determined by temperature and other variables like oxygen and minerals. The marine, or saltwater, biome is the largest of all the world's biomes, covering nearly two-thirds of the planet's surface. With a salt content of between 3.0 and 3.7 percent, saltwater biomes support fish and other organisms that have adapted to this specialized environment. An estuary is a partially closedoff part of the sea that is fed by fresh water. These include bays and tidal marsh inlets. Since the salt content varies greatly, relatively few species can tolerate estuaries but those that do are extremely productive. Because estuaries are fairly shallow, sunlight can penetrate them completely making them ideal habitats for algae, grasses, oysters, and barnacles.

Human influence has altered the world's biomes. Some biomes no longer have the diversity of life that they once contained. The quantity and diversity of mammals found in North America today can in no way compare to those that existed there some five hundred years ago. Nearly all of the original forest trees are also gone. Human activity is thought to result in a global temperature rise, known as the greenhouse effect. This rise in temperature could also alter the nature of each biome causing the extinction of plants and animals.

[See alsoChaparral; Desert; Ecosystem; Forests; Grasslands; Ocean; Rain Forest; Taiga; Tundra; Wetlands ]