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:
^Berg, P. and Dasmann, R. (1977). Reinhabiting California. The Ecologist 7 (10): 399-401.
^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.