Ecoregions of the United States (CEC and Omernik)
This article is part of the Ecoregions Collection
Maps and descriptions for the ecological regions of the United States have been developed by the Commission for Environmental Cooperation Working Group (CEC), a joint United States, Mexico, and Canada collaboration, and by James Omernik and colleagues at the United States Environmental Protection Agency (EPA), along with a large team of collaborators at many federal, state and local agencies. Ecoregions are organized by four increasingly finer geographic scales. The Level I scale divides North America into 15 broad ecological regions. Fifty-two Level II ecological regions for North America provide a more detailed description of the large ecological areas nested within the Level I regions. About 200 Level III ecological regions are delineated that provide a more detailed description of the large ecological areas nested within the level II regions for the United States. Level IV ecoregions are defined for individual states.
Level I Ecoregions-North America
North America has been broken down into 15 broad, level I ecological regions. These highlight major ecological areas and provide the broad backdrop to the ecological mosaic of the continent, putting it in context at global or intercontinental scales:
Level II Ecoregions-North America
Fifty-two level II ecological regions are delineated that provide a more detailed description of the large ecological areas nested within the Level I regions. For example, the Tropical Humid Forests of level I is the region covering coastal portions of the United States and Mexico, and is composed of six level II regions. Level II ecological regions are useful for national and subcontinental overviews of physiography, wildlife, and land use.
A detailed classification and description of the Level II ecoregions is available here.
Level III Ecoregions-United States
About 200 level III ecological regions are delineated that provide a more detailed description of the large ecological areas nested within the level II regions. These smaller divisions are intended enhance regional environmental monitoring, assessment and reporting, as well as decision-making. Because level III regions are smaller, they allow locally defining characteristics to be identified, and more specifically oriented management strategies to be formulated.
A detailed classification and description of the Level III ecoregions is available here.
Level IV Ecoregions-United States
Ecoregions have been defined for all 50 states. The ecoregions shown here have been derived from Omernik (1987) and from refinements of Omernik's framework that have been made for other projects. These ongoing or recently completed projects, conducted in collaboration with the U.S. Environmental Protection Agency (EPA) regional offices, state resource management agencies, and with other federal agencies, involve refining ecoregions, defining subregions, and locating sets of reference sites.
A detailed classification and description of the Level IV ecoregions is available here.
- Commission for Environmental Cooperation, Ecological Regions of North America--Toward a Common Perspective, (Quebec, Canada, 1997), ISBN 2-922305-18-X
- Omernik, James M., 1987. Map Supplement: Ecoregions of the Conterminous United States, Annals of the Association of American Geographers, Vol. 77, No. 1 (March), pp. 118-125
- Omernik, James M., 1995. Ecoregions: A spatial framework for environmental management. In: Biological Assessment and Criteria: Tools for Water Resource Planning and Decision Making. Davis, W.S. and T.P. Simon (eds.) Lewis Publishers, Boca Raton, FL. Pp. 49-62. ISBN: 0873718941.
- United States Environmental Protection Agency, Western Ecology Division, Ecoregion Maps and GIS Resources, Accessed 1 May 2009.