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