A bioregion is an ecologically and geographically defined area that is smaller than an ecozone, but larger than an ecoregion or an ecosystem, in WWF classification scheme. There is also an attempt to use the term in a rank-less generalist sense, similar to the terms "biogeographic area" or "biogeographic unit".
The WWF scheme further subdivides the ecozones into bioregions, defined as "geographic clusters of ecoregions that may span several habitat types, but have strong biogeographic affinities, particularly at taxonomic levels higher than the species level (genus, family)." The WWF bioregions are as follows:
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^Miller, K. 1999. What is bioregional planning?. In: R. Crofts, E. Maltby, R. Smith and L. Maclean (eds). Integrated Planning: International Perspectives. Battleby, Scotland 7–9 April 1999: IUCN & Scottish Natural Heritage.
^Dinerstein, E., Olson, D. Graham, D.J. et al. (1995). A Conservation Assessment of the Terrestrial Ecoregions of Latin America and the Caribbean. World Bank, Washington DC., .