What Are Biomes?

Biomes are large biogeographic units defined by the dominant form of their vegetation and by the climate that sustains it. A biome groups together communities that share a physical structure and a set of environmental constraints even when the species involved are unrelated: the shrublands of southern California, central Chile, the Cape of South Africa, and the Mediterranean basin are all assigned to the same biome because a wet winter and dry summer regime produces the same tough, small-leaved evergreen growth form on four continents. Biomes are the coarsest working division used in biogeography, sitting above ecoregions and ecosystems and below the biosphere as a whole.

The concept was formalized in the first half of the twentieth century as ecologists tried to explain why vegetation structure repeats across widely separated regions. It remains useful because it links a mapped unit on the ground to two measurable climate variables, which makes biomes a natural target for satellite classification and for the land surface components of Earth system models.

Climate Controls and Classification

Mean annual temperature and mean annual precipitation account for most of the global pattern, a relationship captured in the Whittaker climate diagram that plots the two axes and partitions the resulting space into biome fields. Related schemes formalize the same idea differently: the Köppen classification uses temperature and precipitation thresholds tied to plant tolerance, and Holdridge life zones add potential evapotranspiration as a third axis. Seasonality matters as much as annual totals, since the same rainfall delivered in a short monsoon rather than spread through the year yields savanna instead of forest. A survey of terrestrial biomes published by Nature Education works through the major types and the temperature and moisture envelopes that define each one, alongside the soil development and disturbance regimes, particularly fire and grazing, that keep some boundaries in place.

Terrestrial and Aquatic Divisions

Terrestrial classifications commonly recognize tropical rainforest, tropical seasonal forest, savanna, desert, temperate grassland, temperate deciduous forest, temperate rainforest, Mediterranean shrubland, boreal forest, and tundra, with alpine zones treated separately or folded into tundra. Published schemes divide the land surface into anywhere from about eight to eighteen classes depending on how finely the transitional types are split. Aquatic systems are organized on different criteria, using salinity, depth, light penetration, and flow: freshwater streams, lakes, and wetlands on one side, and estuaries, intertidal zones, coral reefs, continental shelves, pelagic open ocean, and deep benthic environments on the other. Finer-grained frameworks nest inside biomes, as in the global map presented in Terrestrial Ecoregions of the World, which subdivides fourteen biomes and eight biogeographic regions into 867 ecoregions carrying distinct species assemblages.

Mapping and Remote Sensing

Drawing biome boundaries from field survey alone is impractical at global scale, so mapping now relies on satellite observation. Multispectral instruments record surface reflectance from which vegetation indices, leaf area index, canopy phenology, and fractional tree cover are derived, and supervised classifiers assign each pixel to a class. The MODIS land cover type product distributed through NASA Earthdata supplies annual global maps at 500 meter resolution in several classification schemes, including the International Geosphere-Biosphere Programme legend and a plant functional type legend built for climate models. Radar and lidar sensors add vertical structure and biomass estimates that optical imagery cannot resolve.

Applications

Biomes have applications in a range of fields, including:

• Climate and Earth system modeling, where land surface schemes are parameterized by biome or plant functional type
• Conservation planning and protected area gap analysis
• Carbon stock accounting and forest inventory
• Agricultural suitability assessment and crop zoning
• Hydrological modeling of evapotranspiration and runoff
• Species distribution modeling and biodiversity assessment