Biomes are climatically and geographically defined similar climatic conditions communities of plants, animals, and soil organisms, and are often referred to as ecosystems. Biomes are defined by factors such as plant structures (such as trees, shrubs, and grasses), leaf types (such as broadleaf and needleleaf), plant spacing (forest, woodland, savanna), and climate. Unlike ecozones, biomes are not defined by genetic, taxonomic, or historical similarities. Biomes are often identified with particular patterns of ecological succession and climax vegetation (quasi-equilibrium state of the local ecosystem). An ecosystem has many biotopes and a biome is a major habitat type. A major habitat type, however, is a compromise, as it has an intrinsic inhomogeneity.

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. In terrestrial biomes, species diversity tends to correlate positively with net primary productivity, moisture availability, and temperature.

Ecoregions are grouped into both biomes and ecozones.

A fundamental classification of biomes is:

1. Terrestrial (land) biomes
2. Freshwater biomes
3. Marine biomes

Biomes are often known in English by local names. For example, a Temperate grassland or shrubland biome is known commonly as steppe in central Asia, prairie in North America, and pampas in South America. Tropical grasslands are known as savanna in Australia, whereas in Southern Africa it is known as veldt (from Afrikaans).

Sometimes an entire biome may be targeted for protection, especially under an individual nation's Biodiversity Action Plan.

Climate is a minor 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. Biodiversity generally increases away from the poles towards the equator and increases with humidity.