Ecoregions are relatively large areas of land or water that harbour a characteristic set of species, communities, dynamics and environmental conditions. An ecoregion is usually classified under one major habitat type, but may encompass multiple habitat types.
Biodiversity is not evenly distributed around the world. WWF has identified the most outstanding terrestrial, freshwater and marine ecoregions have been identified on which to focus conservation efforts. Using representation as the guiding principle, they identified the Global 200 -- truly outstanding examples of the major habitat types in each of the Earth's biogeographical realms and ocean basins, such as the Neotropics, the Paleoarctic, and the Indian Ocean. The selection criteria included species richness; levels of endemism; higher taxonomic uniqueness ([eg] unique genera or families, or primitive species lines); unusual ecological or evolutionary phenomena (such as large-scale migrations); and the global rarity of the major habitat type.