An ecoregion (ecological region) is an ecologically and geographically defined area that is smaller than a bioregion, which in turn is smaller than an ecozone. All three of these are larger than an ecosystem. Ecoregions cover relatively large areas of land or water, and contain characteristic, geographically distinct assemblages of natural communities and species. The biodiversity of flora, fauna and ecosystems that characterise an ecoregion tends to be distinct from that of other ecoregions. In theory, biodiversity or conservation ecoregions are relatively large areas of land or water where the probability of encounter of different species and communities at any given point remain relatively constant, within an acceptable range of variation (largely undefined at this point). Three caveats are appropriate for all biogeographic mapping approaches. Firstly, no single biogeographic framework is optimal for all taxa. Ecoregions reflect the best compromise for as many taxa as possible. Secondly, ecoregion boundaries rarely form abrupt edges; rather, ecotones and mosaic habitats bound them. Thirdly, most ecoregions contain habitats that differ from their assigned biome.