Biome.

Biome. Biome, also known as life zones, all plants, animals, and other organisms, as well as the physical environment in a particular area. A biome is characterized by its plant life, the types of which are determined by a location's climatic conditions, latitude, and altitude. For example, northern coniferous forests exist in subarctic portions of North America and Asia, but further north, the conditions are simply too harsh and the season too brief for trees to grow. Instead of trees, the short vegetation of the tundra thrives in these areas. The same phenomenon occurs with altitude, as trees give way to short alpine vegetation in high mountainous regions. A biome is composed of many ecosystems--smaller communities of plants and animals and their habitats (the physical parts of their environment that affect them). Whereas the boundaries of a biome are determined by climate, the boundaries of ecosystems are physical features, such as ridges or riverbanks, that separate one community from another. The ecosystems of a particular biome tend to have plants with similar growth forms and animals with similar feeding habits. Major terrestrial biomes include tropical rain forest, northern coniferous forest, tundra, desert, grassland, savanna, and chaparral. Although not necessarily associated with marine environments, the term biome is sometimes used by ecologists to describe marine life zones such as the littoral zone, found in shallow water; the pelagic zone in the open ocean; and the benthic zone on the ocean floor. The tropical rain forest is the most complex biome in the world. This biome is found at low elevations in the tropics where it is perpetually warm and wet. Rain forests are characterized by a dense tree canopy--tree top branches and leaves that overlap with each other, creating a shaded forest interior. These canopies may reach up to 50 m (160 ft) high. The thick canopy allows little sunlight to penetrate, so rain forest floors have sparse ground cover. The soils are nutrient-poor, and most plants are able to store what few nutrients they can absorb. The northern coniferous forest, also known as the taiga in Russia, is found in a broad subarctic band across Alaska, Canada, Scandinavia, Russia, and China, where the winters are long and cold. Conifers, such as spruce, larch, and fir, are the dominant plants, but lichens and mosses are abundant too. Muskegs, or bogs, occur throughout the region. The tundra is the treeless plain that lies north of the northern coniferous forests and on the Antarctic Peninsula in the southern hemisphere. Trees cannot survive in this biome because of the cold temperatures, high winds, and heavy snowfall, as well as the permafrost, a layer of permanently frozen subsoil. Plant life tends to grow low to the ground. In the summer, large numbers of birds migrate to the tundra to feed on insects. Other animals found in this area include reindeer, wolves, fox, voles, and lemmings. A similar biome, called the alpine biome, is found in high mountain areas around the world. Similar harsh conditions cause vegetation to grow low to the ground in alpine meadows. Desert biomes are characterized by less than 254 mm (10 in) of annual precipitation and high temperatures. To combat this lack of moisture, desert plants have developed water-conserving features, such as leaves that are light-colored, small, thick, or waxy. Animals that live in the desert are often light-colored, blend in well with their surroundings, and are usually more active at night to avoid the blazing heat during the day. Grassland biomes are found on every continent except Antarctica, accounting for about one quarter of the Earth's land surface. Typically found on flat or rolling terrain, grasslands tend to occur in the interior of continents where precipitation is lower. Periodic droughts occur in most grasslands, accompanied by searing heat that scorches most vegetation in the area. Grasslands are covered with grasses, sedges, and other low-growing, perennial plants. Drought, fire, and grazing by herbivores, such as bison and deer, restrict tree growth. Most grasslands have been extensively cultivated and are now regions where major crops of wheat, corn, and other grains are grown. Tropical savannas are expansive grasslands dotted with trees. The world's largest and best-known savanna is the African savanna, which covers much of the continent south of the Sahara desert. In the African savanna, herds of animals graze on the tall grass, and giraffes browse on the trees. Other tropical savannas are found in South America, India, and Australia. The chaparral biome is dominated by dense thickets of mostly small-leafed evergreen shrubs. It is characterized by hot, dry summers and cool, wet winters. This biome can be found in the foothills of California and Mediterranean climate regions. Chaparral plants have adapted to the frequent fires that result from lightning and dry conditions. Contributed By: Joel P. Clement Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.