A good definition is provided by the World Resources Institute:
“A bioregion is a land and water territory whose limits are defined not by political boundaries, but by the geographical limits of human communities and ecological systems. Such an area must be large enough to maintain the integrity of the region’s biological communities, habitats, and ecosystems; to support important ecological processes, such as nutrient and waste cycling, migration, and stream flow. It must be large enough to meet the habitat requirements of keystone and indicator species; and to include the human communities involved in the management, use, and understanding of biological resources. It must be small enough for local residents to consider it home.
A bioregion would typically embrace thousands to hundreds of thousands of hectares. It may be no bigger than a small watershed or as large as a small state or province. In special cases, a bioregion might span the borders of two or more countries.
A bioregion is also defined by its people. It must have a unique cultural identity and be a place in which local residents have the primary right to determine their own development. This primary right does not, however, imply an absolute right. Rather, it means that the livelihoods, claims, and interests of local communities should be both the starting point and the criteria for regional development and conservation. Within that framework many other state, investor, and other economic interests must be accommodated.
Within a bioregion lies a mosaic of land or aquatic uses. Each patch provides habitats in which different species survive and flourish, and each has its own particular relationship to the region’s human population. All the elements of the mosaic are interactive; the management of a watershed affects riverine habitats, farms, estuaries, fisheries, and coral reefs. The components are also dynamic; each changes over time as rivers change course, fallow fields regenerate, storms batter coasts, and fires ravage forests. This dynamism gives a well-managed bioregion the resilience and flexibility to adapt to natural evolution and human-induced activity—be it changing climate or changing markets.
Within this ecological and social framework, governmental, community, corporate, and other private interests share responsibility for coordinating land-use planning for both public and private land and for defining and implementing development options that will ensure that human needs are met in a sustainable way. Innovative forms of institutional integration and social cooperation are needed to meet these needs. Dialogue among all interests, participatory planning, and great institutional flexibility are essential. A wide range of conservation tools and technologies must also be brought to bear—among them, protected-areas management, ex situ technologies, landscape restoration, and sustainable management of such resources as forests, fisheries, and croplands.”