Biomes organize the biological communities of the earth based on similarities in the dominant vegetation, climate, geographic location, and other characteristics. Aspects of the physical environment such as precipitation, temperature, and water depth, have a strong influence on the traits of species living in that natural environment, and thus biological communities experiencing similar environmental conditions often contain species that have evolved similar characteristics. There is no single classification of biomes that is agreed upon by all scientists because different scientists wish to emphasize different characteristics by their definition. Historically however, biomes have been identified and mapped based on general differences in vegetation type associated with regional variations in climate and terrain.
Terrestrial biomes characterize ecosystems on land, and are usually identified by the growth form of the dominant vegetation, climate, and/or where they are located on the earth. The major terrestrial biomes include the tundra biome, the forest biome, the grassland biome, and the desert biome. Note that forests and grasslands are defined based on the growth form of the dominant vegetation whereas deserts are classified based on the dominant climatic conditions. The geographic distribution of terrestrial biomes is mostly influenced by climatic conditions such as rainfall and temperature.
Some people prefer to subdivide these four broad biomes. For example, the forest biome can be subdivided into a temperate forest biome and tropical forest biome. The temperate forest biome and tropical forest biome can then be further divided based on the characteristics of the trees found in the biome. The World Wildlife Fund recommends the following classification scheme of terrestrial biomes.
- Tundra Biome
- Forest Biome
- Boreal Forest/Taiga Biome
- Temperate Coniferous Forest Biome
- Temperate Broadleaf and Mixed Forest Biome
- Mediterranean Forests, Woodlands and Scrub
- Tropical Subtropical Moist Broadleaf forests
- Tropical and subtropical Dry broadleaf forests
- Tropical and subtropical coniferous forests.
- Grassland Biome
- Temperate Grasslands, Savannas and Shrubland Biome
- Montane Grassland and Shrublands
- Tropical and subtropical grasslands, savannas and shrublands
- Flooded Grasslands and Savannas
- Desert Biome
- Deserts and Xeric Shrublands
Freshwater biomes are generally distinguished by characteristics such as water depth and whether the water is moving or standing. Major freshwater biomes include ponds and lakes, streams and rivers, and wetlands.
Marine biomes are generally distinguished by the depth of the water and whether there is a substrate on which organisms can attach. Important marine biomes include oceans, coral reefs, and estuaries. The ocean biome, the largest of all of the earth's biomes, can be divided into several zones including the shore/intertidal zone, the pelagic zone, the benthic zone, and the abyssal zone.
Humans have fundamentally altered global patterns of biodiversity and ecosystem processes. As a result, vegetation forms predicted by conventional biome systems are rarely observed across most of Earth's land surface. While not a replacement for existing biome systems, anthropogenic biomes provide an alternative view of the terrestrial biosphere based on global patterns of sustained direct human interaction with ecosystems, including agriculture, human settlements, urbanization, forestry and other uses of land. Anthropogenic biomes offer a new way forward in ecology and conservation by recognizing the irreversible coupling of human and ecological systems at global scales, and moving us toward an understanding how best to live in and manage our biosphere and the anthropogenic biomes we live in.
Other Ways of Classifying Habitats and Communities
Ecological communities can share characteristics for a number of reasons. Classifying communities into biomes attempts to highlight the role of the physical environment in determining characteristics of communities.
Communities can also be classified into biogeographical realms (e.g, Australasia, Antarctic, Afrotropic, Indo-Malayan, Neartic, Neotropic, Oceania, Paleartic). Classifying communities into biogeographic realms attempts to highlight the importance of a shared bioegoraphic/evolutionary history in determining the composition of biological communities. It is important to recognize that many different biomes can be found in the same biogeographic realm and that the same biome can be located in many different geographic realms.
The ecoregion, a relatively large unit of land that contains geographically distinct assemblages of natural communities, is a subset of a biome found within a biogeographic realm. The World Wildlife Fund has identified 825 terrestrial ecoregions, 450 freshwater ecoregions, and 229 marine ecoregions. Several distinct ecological communities may be found in a single ecoregion.