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Biomes are the largest recognizable terrestrial components, or units, of the biosphere (the entire surface of the Earth that is occupied by life). They are distinguished from one another by vegetation and climate. The dominant vegetation, or flora, in each biome supports a collection of animal life, or fauna, characteristic of that particular biome. The biota (plants and animals) of each kind of biome have similar characteristics worldwide. Biomes comprise smaller units called habitats.
Climate is the primary feature that distinguishes one biome from another. For example, the climate in a tropical rain forest differs dramatically from that of coniferous forests of the U.S. West. The tropical rain forest always has ample water, and its daily temperatures are nearly perfect for plant growth. The coniferous forest typically must tolerate drought in summer and very cold weather in winter. Thus, different types of flora and fauna have evolved in each biome, eventually filling the ecological niches. For example, the climate of the northern tundra is frigid most of the year, and the growing season is cool and wet. The flora and fauna, however, have adapted to survive these rigorous conditions. The treeless tundra flora of the Arctic region is mostly low-growing perennials. The low stature of the plants is an adaptation to the harsh, drying winds that wear away the tall plants, as well as to such other conditions as soil moisture and nutrients.
Similar biomes often are found at the same latitude. This is apparent in the tundra and the northern coniferous boreal forest, or taiga. In addition to latitudinal similarities, altitudinal similarities exist as well. As one ascends a tall mountain the temperature becomes cooler, the air thinner, and the winters longer and more severe (even tall mountains near the equator are snow-capped). Therefore, several different climates often exist on one mountain. For example, Mount Washington in the White Mountains of New Hampshire has three different biomes, in order of increasing altitude: temperate deciduous forest biome mantles the foothills and lower slopes, boreal forest, and tundra.
A map of the world's major biomes would in general correspond fairly closely to one of the world's climatic zones, although the names of the climatic zones vary somewhat. Thus the "tropical rain forest" biome includes the "tropical wet" climatic zone along with the "monsoon" climates of the eastern coasts of India and Southeast Asia. The "tropical grassland and savanna" biome corresponds to the "tropical wet-dry" climate, and so on.
Bibliography: Brown, James H., and Lomolino, Mark V., 2d ed., Biogeography (1998); Cox, C. Barry, and Moore, Peter D., Biogeography, 5th ed. (1993); Pielou, E. C., Biogeography (1979; repr. 1992); Tivy, Joy, Biogeography: A Study of Plants in the Ecosphere, 3d ed. (1993); Whittaker, Robert H., Classification of Natural Communities (1977).