Biomes

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Biomes

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

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

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

Aquatic
Deserts
Forests
Grasslands
Tundra

Aquatic Ecosystems

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

Freshwater Regions.

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

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

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

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

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

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

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

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

Marine Regions.

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

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

Deserts

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

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

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

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

Forests

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

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

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

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

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

Grasslands

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

Savanna.

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

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

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

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

Temperate Grassland.

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

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

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

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

Tundra

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

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

Arctic Tundra.

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

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

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

Alpine Tundra.

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