A bioregion is a geographical area defined not by administrative boundaries, but by distinct characteristics such as plant and animal species, ecological systems, soils and landforms, human settlements, and topographic features such as drainage basins. A bioregion can be on land or at sea. The idea of bioregions was adopted and popularized in the mid-1970s by a school of philosophy called bioregionalism, which includes the concept that human culture can influence bioregional definitions due to its effect on non-cultural factors. Bioregions are part of a nested series of ecological scales, generally starting with local watersheds, growing into larger river systems, then Level III or IV ecoregions, bioregions, then biogeographical realm, followed by the continental-scale and ultimately the biosphere.