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 ]