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

Annelid

Annelids, or true-segmented worms, are members of the animal phylum Annelida, the most complex of all wormlike groups of organisms. Annelids are commonly found in terrestrial, as well as marine, brackish, estuarine, and freshwater ecosystems worldwide. Most annelids are free-living, although several species have parasitic, mutualistic, or commensal relationships with other animals, and many species are commonly associated with aquatic and terrestrial plants.

Six major classes comprise this phylum: Polychaeta (polychaete, or many-bristled worms; primarily marine; more than 15,000 species [spp.]); Oligochaeta (oligochaete worms; freshwater, terrestrial, marine; more than 8,000 spp.), Hirudinea (leeches; freshwater, terrestrial, marine; more than 700 spp.), Branchiobdellida (crayfish worms; freshwater, live on crayfishes; more than 100 spp.), Aphanoneura (suction-feeding worms; freshwater; more than 30 spp.), and Acanthobdellida (bristle leech; parasitic on Arctic marine fishes; 1 sp.). As with any group of organisms, the phylogenetic relationships of the diverse groups within annelids, and of the phylum to others within the animal kingdom, is the subject of continuing debate. The marine invertebrate groups Echiura and Sipunculida recently were aligned with the annelids.

All annelids are bilaterally symmetrical , with an elongated, cylindrical body shape divided both externally and internally by a regular, linear series of segments. The highly developed digestive, circulatory, nervous, and excretory systems within the body cavity, or coelom, reflect external segmentation and generally are repeated serially; this is called metameric segmentation, and distinguishes annelids from all other wormlike groups. Annelids range in size from less than 0.7 millimeters (0.019 inch) to over 3 meters (9.8 feet) in length. The number of segments is relatively fixed in some groups (Branchiobdellida, Hirudinea), but indeterminate in others. External form of annelids is diverse, even within each group; the polychaetes may have distinct body regions, with limblike parapodia, chaetae (hairs), tentacles, and antennae, while others may appear similar to an earthworm, with few if any external appendages . Most oligochaete species have chaetae arranged in bundles on each segment. Several aquatic oligochaetes and many polychaetes have gills.

Leeches are usually flattened, with a posterior sucker and anterior suckerlike mouth; several species have jaws, others have an extendable proboscis. The branchiobdellidans have a posterior sucker and an anterior end with several fused segments and distinct teeth. Chaetae are absent in leeches and branchiobdellidans. The single species of Acanthobdellida is shaped like an elongate leech, with a few hooked chaetae located ventrally on a few anterior segments. Annelids are hermaphroditic; reproduction is commonly sexual, but many species reproduce asexually by budding or fragmentation. Annelids are important components of their respective habitats, whether it be the bottom of freshwater or marine environments, or the soil. The feeding habits of many species are important in the decomposition of organic matter and recycling of nutrients in terrestrial and aquatic environments. Many annelids feed on algae, insects, carrion, and other worms, and several leech species consume the blood of turtles, birds, fishes, and mammals.

see also Nematode; Platyhelminthes

Mark J. Wetzel

Bibliography

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, Inc., 1990.

Dindal, Daniel L., ed. Soil Biology Guide. New York: John Wiley & Sons, 1990.

Fauchald, Kristian. "Worms, Annelida." In Encyclopedia of Biodiversity, Vol. 5. Edited by S. A. Levin. San Diego, CA: Academic Press, 2001.

Ruppert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: Saunders College Publishing, 1994.

Thorp, James H., and Alan P. Covich. Ecology and Classification of North American Freshwater Invertebrates, 2nd ed. San Diego, CA: Academic Press, 2001.

Biome

Earth's major terrestrial, marine, and freshwater ecosystems are known as biomes. They are classified according to similarities in species composition of plants and animals, and by environmental attributes. These attributes include temperature, precipitation, and soil type in terrestrial biomes and temperature, depth, and salinity in aquatic biomes. There are no hard boundaries between biomes and there is much intermixing of species between them.

Biomes are divided into many kinds of ecosystems and habitats, according to local variations in species composition and physical environment (a cloud forest, mud flat, and meadow, to name a few). However, scientists generally recognize between twelve and fifteen major natural terrestrial biomes, including tropical rain forest, tropical deciduous forest, thorn woodland, tropical savanna , desert, sclerophyllous woodland, subtropical evergreen forest, temperate deciduous forest, temperate rain forest, temperate grassland, boreal forest, and tundra. Some scientists consider cultivated land to be a biome. There are seven major freshwater biomes: ice, spring, river, swamp, marsh, lake, and stream. There are six major marine biomes: coral reef; algal bed; estuary; upwelling zone; continental shelf ; and open ocean.

Significant changes in the global environment and climate are causing major shifts in some biomes, such as glacier movement and polar cap melting, and are threatening the survival of others, such as the deforestation of tropical and temperate rain forests.

see also Biodiversity; Desert; Estuaries; Forest, Temperate; Grassland; Habitat; Ocean Ecosystems; Tundra

Cristina G. Mittermeier and Russell A. Mittermeier

Bibliography

Brown, James H. and M. V. Lomolino. Biogeography, 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

biome A biological subdivision of the Earth's surface that reflects the ecological and physiognomic character of the vegetation. Biomes are the largest geographical biotic communities that it is convenient to recognize. They broadly correspond with climatic regions, although other environmental controls are sometimes important. They are equivalent to the concept of major plant formations in plant ecology, but are defined in terms of all living organisms and of their interactions with the environment (and not only with the dominant vegetation type). Typically, distinctive biomes are recognized for all the major climatic regions of the world, emphasizing the adaptation of living organisms to their environment, e.g. tropical rain forest biome, desert biome, tundra biome.

Biome

A biome is a particularly useful biogeographic unit that results from large-scale climatic patterns. A portion of a biome, such as the tropical rain forest found in the Amazon basin, may cover thousands of hectares. It will have recognizable vegetation features that impart a sameness that defines the unit. The fact that this region differs from others, such as the desert biome, is evident to even the most untrained observer. Frederic Clements (1874-1945) and Victor Shelford (1877-1968) observed in 1939 that not only was the vegetation characteristic, but that these were plant-animal formations with associated fauna. In the 1960s an ecological research effort, the International Biological Program, sought to characterize the structure and function of major biomes through intense study. American botanist and ecologist Robert Whittaker (1920-1980) summarized some characteristics of the biomes in his book Communities and Ecosystems, in which he arranged them graphically according to interacting gradients of mean annual temperature and moisture availability (see accompanying figure).

The Influence of Rainfall

You can bring this model to life though observation as you travel within the continental United States. The Interstate Highway System allows easy movement across the country along latitudes, parallel to the equator, or the longitudes, perpendicular at the equator. Travel on Interstate 80 from New York City to San Francisco during the summer growing season reveals an obvious gradient of change in vegetation. The deciduous forests of central New York State are lush green in most years, with trees predominating where human development has not occurred. In the Midwest, traveling from Illinois into Iowa and Nebraska, the trees are absent except along rivers, and grasses and broad-leaved nonwoody forbs cover the landscape like a carpet. Much of this region is now dominated by fields of grain such as corn and wheat, domesticated replacements of the native vegetation that once was predominant in the grasslands. (Remnants of the native prairie are rare.) In August, the green prairies begin to show some signs of the golden color of mature fields as the various annual grasses produce seeds that will ultimately produce plants for the following year. Meanwhile the perennials will die back to ground level by late fall, and then start growth again the following year.

The farther west one travels, the sparser the vegetation and the less luxuriant the crops, until dry short-grass prairies merge into deserts and badlands with more widely spaced plants and soil that is frequently bare. The predominant color throughout the year in the deserts tends toward earth tones of beige and brown with small green leaves being less evident. Two major north-south mountain barriers, the Rocky Mountains and the Sierra Nevada range in California, interrupt the gradual transition. In these areas, elevation-dependent differences produce a complex mosaic of vegetation that does not fit the overall pattern. Finally, on the West Coast, moisture-laden air deposits rain on the windward sides of the coastal mountains and abruptly changes the pattern. Lush temperate rain forest vegetation forms a margin along the western edges of California north of San Francisco, just as in coastal Oregon and Washington. Southward, the forests are replaced by coastal shrub lands because rainfall is not sufficient to support extensive forests in most places, and the inland deserts are even closer to the ocean shores. This east-west gradient across the country parallels a decline in moisture availability, the first of two major climatic factors that determine the character of biomes.

The Influence of Temperature

A similar trip along Interstate 75 in January from the Canadian border at Sault Ste. Marie, Michigan, to Florida provides evidence of the second major climatic factor gradient. In northern Michigan, it is likely that the ground will be snow covered most of the month. Ohio and Kentucky can present occasional periods with persistent snow cover; by the time you reach Georgia, however, snow generally disappears shortly after it falls, and Florida has only rain. This gradient is not so much a result of differences in volume of precipitation (the amount of rain or snowfall), but the prevailing temperatures. When the north to south temperature gradient overlays the east to west moisture gradient, the type of biological community that will be supported is determined. Similar interactions of temperature and moisture availability determine the presence of other biomes globally as indicated in the map of their distribution. Toward the poles, Canadian boreal forests change over to tundra and illustrate the results of extending the temperature gradient.

Tropical rain forests occur where the temperature is greater near the equator and daily tropical rains maintain high moisture levels. This tropical region along the equator forms a boundary with the temperature gradients in the Southern and Northern Hemispheres being mirror images of each other. In the tropical forest biomes, there is little seasonal change; the prevailing conditions are wet and warm. Seasonal temperature variation is minimal in regions adjacent to tropical forests, but ultimately at increasing distance from the equator, alternating wet and dry periods produce fluctuations in growth rates in subtropical seasonal forests. This moisture-dependent alternation is different from the temperate biomes covering the United States and Canada where the seasons are produced by the alternation of a cold winter and relatively warmer summer.

The classification of biogeographic areas into biomes is only one of several different methods of organizing terrestrial environments. Michael Barbour, in Terrestrial Ecosystems, gives a brief historical review of other classification systems, and Robert Bailey, in Ecosystem Geography, updates various ways of classifying the Earth's surface. Barbour uses the term vegetation types to classify landscape patterns and points out significant differences between the various subdivisions that can be identified, while Bailey prefers the term ecoregion as the unit of subdivision at the level of assemblages of landscapes.

Since the distribution of biomes is dependent on global climate patterns, one may question what impact global climatic change could have on these biogeographic units. A continuation of global warming would change the boundaries by moving warmer conditions closer to the poles.

In addition, the climatic changes do not just involve changes in temperature; they also involve changes in air movement patterns, alteration of temperatures (known as El Niño and La Niña) within the oceans, and storm distributions that change the annual growth of both plants and animals. This is not a new phenomenon on Earth. Cold periods have existed in the past, and biome boundaries have shifted in response. This is known from the pollen record left in lakes and bogs over many thousands of years. The challenge to plants and animals is not so much the extent or duration of expected temperature changes, but the rate of those changes. Vegetation can respond to changes that take hundreds or thousands of years to occur by gradually dispersing into new areas or evolving. It cannot move or reproduce fast enough to adapt to the same changes if they occur over tens or hundreds of years. The total ecologic impact of these potential rapid changes is not currently known.

see also Aquatic Ecosystems; Biodiversity; Chaparral; Clines and Ecotypes; Coastal Ecosystems; Coniferous Forests; Deciduous Forests; Deforestation; Desertification; Deserts; Ecology; Ecosystem; Global Warming; Grasslands; Plant Community Processes; Rain Forests; Savanna; Tundra.

W. Dean Cocking

Bibliography

Aber, John, and Jerry Melillo. Terrestrial Ecosystems. Philadelphia, PA: Saunders College Publishing, 1991.

Bailey, Robert G. Ecosystem Geography. New York: Springer-Verlag, 1996.

Barbour, Michael G., ed. Terrestrial Plant Ecology, 3rd ed. New York: Addison-Wesley, 1998.

Clements, Frederic E., and Victor Shelford. Bio-ecology. New York: John Wiley &Sons, 1939.

Whittaker, Robert. Communities and Ecosystems, 2nd ed. New York: Macmillan, 1975.

biome A biological subdivision that reflects the ecological and physiognomic character of the vegetation. Biomes are the largest geographical biotic communities that it is convenient to recognize. They broadly correspond with climatic regions, although other environmental controls are sometimes important. They are equivalent to the concept of major plant formations in plant ecology, but are defined in terms of all living organisms and of their interaction with the environment (and not only with the dominant vegetation type). Typically, distinctive biomes are recognized for all the major climatic regions of the world, emphasizing the adaptation of living organisms to their environment, e.g. tropical rain-forest biome, desert biome, tundra biome.

biome A biological subdivision of the Earth's surface that reflects the character of the vegetation. Biomes are the largest geographical biotic communities that it is convenient to recognize. They broadly correspond with climatic regions, although other environmental controls are sometimes important. They are equivalent to the concept of major plant formations in plant ecology, but are defined in terms of all living organisms and of their interactions with the environment (and not only with the dominant vegetation type). Typically, distinctive biomes are recognized for all the major climatic regions of the world, emphasizing the adaptation of living organisms to their environment, e.g. tropical-rainforest biome, desert biome, tundra biome.

freshwater fish Fish that live only in fresh water. Many families and orders of fish are considered to include only primary freshwater species (i.e. species that evolved without contact with a marine environment). Some families have a worldwide distribution (e.g. the Cyprinidae are found in freshwater basins all over the world, except for South America and the Australian region). Others (e.g. the Cobitidae, loaches) are found only in the Eurasian region. A few families have a very restricted distribution (e.g. the Amiidae (bowfin) are found only in the eastern half of the USA).

freshwater fish Fish that live only in fresh water. Many families and orders of fish are considered to include only primary freshwater species, i.e. species that evolved without contact with a marine environment. Some families have a worldwide distribution, e.g. the Cyprinidae are found in freshwater basins all over the world, except for S. America and the Australian region. Others, e.g. the Cobitidae (loaches), are found only in the Eurasian region. A few families have a very restricted distribution, e.g. the Amiidae (bowfin) are found only in the eastern half of the USA.

biome A major ecological community or complex of communities that extends over a large geographical area characterized by a dominant type of vegetation. The organisms of a biome are adapted to the climate conditions associated with the region. There are no distinct boundaries between adjacent biomes, which merge gradually with each other. Examples of biomes are tundra, tropical rainforest, taiga, chaparral, grassland (temperate and tropical), and desert.

annelid Any member of the Annelida phylum of segmented worms. All have encircling grooves usually corresponding to internal partitions of the body. A digestive tube, nerves, and blood vessels run through the entire body, but each segment has its own set of internal organs. The three main classes are: Polychaeta (marine worms), Oligochaeta (freshwater or terrestrial worms), and Hirudinea (leeches).

biome Extensive community of animals and plants whose make-up is determined by soil type and climate. There is generally distinctive, dominant vegetation, and characteristic climate and animal life in each biome. Ecologists divide the Earth (including the seas, lakes, and rivers) into ten biomes.

annelid (zool.) red-blooded worm. XIX. — F. annélide or modL. annelida, n. pl. f. F. annelés ‘ringed animals', pp. of anneler, f. OF. annel (mod. anneau) ring. See ANNULAR, -ID3.

bi·ome / ˈbīˌōm/ • n. Ecol. a large naturally occurring community of flora and fauna occupying a major habitat, e.g., forest or tundra.

an·ne·lid / ˈanlˌid/ • n. any segmented worm of the phylum Annelida, including earthworms, lugworms, and leeches. • adj. of or relating to annelids.

biome See COMMUNITY.

annelid •carangid • alleged • aged •frigid, rigid •turgid • naked • wicked • whizz-kid •orchid • crooked •pallid, valid •gelid • skidlid • eyelid •solid, squalid, stolid •Euclid • unsullied • annelid •chrysalid • Ozalid • desmid • timid •Fatimid •humid, tumid •pyramid • MacDiarmid • crannied •arachnid • Enid • hominid • honied •Leonid, Oceanid •salmonid • Achaemenid •unaccompanied • Sassanid • learned •winged •rapid, sapid, vapid •intrepid, tepid •insipid, lipid •limpid • poppied • torpid •Cupid, stupid •canopied

biome •brome, chrome, comb, Crome, dome, foam, gnome, holm, Holme, hom, home, Jerome, loam, Nome, ohm, om, roam, Rome, tome •Guillaume • biome • Beerbohm •radome • astrodome • Styrofoam •megohm • Stockholm • Bornholm •motorhome • backcomb • honeycomb •cockscomb, coxcomb •toothcomb • genome • gastronome •metronome • syndrome • palindrome •polychrome • Nichrome •monochrome • velodrome •hippodrome • aerodrome •cyclostome • rhizome