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Ecoregions are identified by analyzing the patterns and composition of biotic and abiotic phenomena that affect or reflect differences in ecosystem quality and integrity (Omernik 1987, 1995). These phenomena include geology, landforms, soils, vegetation, climate, land use, wildlife, and hydrology. The relative importance of each characteristic varies from one ecological region to another regardless of the hierarchical level. A Roman numeral classification scheme has been adopted for different hierarchical levels of ecoregions, ranging from general regions to more detailed:

Level I – 12 ecoregions in the continental U.S.
Level II – 25 ecoregions in the continental U.S.
Level III -105 ecoregions in the continental U.S.
Level IV – 967 ecoregions in the conterminous U.S.
Explanations of the methods used to define these multi-agency ecoregions are given in Omernik and Griffith (2014), Omernik (1995, 2004), and CEC (1997).