An ecoregion (ecological region), sometimes called a bioregion, is the next smallest ecologically and geographically defined area beneath "realm" or "ecozone". Ecoregions cover relatively large area of land or water, and contain characteristic, geographically distinct natural communities and species. When defined by the World Wide Fund for Nature (WWF), the boundaries of an ecoregion approximate the original extent of the natural communities prior to any major recent disruptions or changes. The WWF has identified 825 terrestrial ecoregions, and approximately 450 freshwater ecoregions across the Earth.