A biome is made up of ecoregions or settlements at stable steady state and all associated transitional, disturbed, or degraded, vegetation, fauna and soils, but can often be identified by the climax vegetation type.
The biodiversity characteristic of each biome, especially the diversity of fauna and subdominant plant forms, is a function of abiotic factors and the biomass productivity of the dominant vegetation. Terrestrial biomes with higher net primary productivity, moisture availability, and temperature.
A fundamental classification of biomes is into:
- Terrestrial (land) biomes and
- Aquatic (water) biomes.
Biomes are often given local names. For example, a temperate grassland or shrubland biome is known commonly as steppe in central Asia, savanna or veld in southern Africa, prairie in North America, pampa in South America and outback or scrub in Australia. Sometimes an entire biome may be targeted for protection, especially under an individual nation's Biodiversity Action Plan.
Climate is a major factor determining the distribution of terrestrial biomes. Among the important climatic factors are:
- latitude: arctic, boreal, temperate, subtropical, tropical.
- humidity: humid, semi-humid, semi-arid, and arid.
- seasonal variation: rainfall may be distributed evenly throughout the year, or be marked by seasonal variations.
- dry summer, wet winter: most regions of the earth receive most of their rainfall during the summer months; Mediterranean climate regions receive their rainfall during the winter months.
- elevation: increasing elevation causes a distribution of habitat types similar to that of increasing latitude.
The most widely used systems of classifying biomes correspond to latitude (or temperature zoning) and humidity.
Robert G. Bailey developed a biogeographical classification system for the United States in a map published in 1975. Bailey subsequently expanded the system to include the rest of North America in 1981, and the world in 1989. The Bailey system is based on climate, and is divided into four domains (Polar, Humid Temperate, Dry, and Humid Tropical), with further divisions based on other climate characteristics (subarctic, warm temperate, hot temperate, and subtropical, marine and continental, lowland and mountain).
A team of biologists developed an ecological land classification system for the World Wide Fund for Nature (WWF) that identified 14 biomes, called major habitat types, and further divided the world's land area into 867 terrestrial ecoregions. This classification is used to define the Global 200 list of ecoregions identified by the (WWF) as priorities for conservation. The WWF major habitat types are as follows:
- Forests ('broadleaf' = Angiosperm trees)
- Tropical and subtropical moist broadleaf forests (tropical and subtropical, humid)
- Tropical and subtropical dry broadleaf forests (tropical and subtropical, semihumid)
- Tropical and subtropical coniferous forests (tropical and subtropical, semihumid)
- Temperate broadleaf and mixed forests (temperate, humid)
- Temperate coniferous forests (temperate, humid to semihumid)
- Boreal forests/taiga (subarctic, humid)
- Mediterranean forests, woodlands, and scrub or sclerophyll forests (temperate warm, semihumid to semiarid with winter rainfall)
- Tropical and subtropical grasslands, savannas, and shrublands (tropical and subtropical, semiarid)
- Temperate grasslands, savannas, and shrublands (temperate, semiarid)
- Flooded grasslands and savannas (temperate to tropical, fresh or brackish water inundated)
- Montane grasslands and shrublands: alpine (above the tree line) or montane (below the tree line, and hence with trees).
- Tundra (Arctic)
- Deserts and xeric shrublands (temperate to tropical, arid)
- Mangrove (subtropical and tropical, salt water inundated)
The Endolithic biome, consisting entirely of microscopic life in rock pores and cracks, kilometers beneath the surface, has only recently been discovered and does not fit well into most classification schemes.