Ecoregions of the United States-Level IV (EPA)

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. Designed to serve as a spatial framework for environmental resource management, ecoregions denote areas within which ecosystems (and the type, quality, and quantity of environmental resources) are generally similar. The most immediate needs are to develop regional biological criteria and water quality standards and to set management goals for nonpoint source pollution.

The approach used to compile this map is based on the premise that ecological regions can be identified through the analysis of the patterns and the composition of biotic and abiotic phenomena that affect or reflect differences in ecosystem quality and integrity (Wiken 1986; Omernik 1987, 1995). These phenomena include geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. The relative importance of each characteristic varies from one ecological region to another regardless of the hierarchical level. Because of possible confusion with other meanings of terms for different levels of ecological regions, a Roman numeral classification scheme has been adopted for this effort. Level I is the coarsest level, dividing North America into 15 ecological regions, whereas at Level II the continent is subdivided into 52 classes (CEC 1997). Level III is the hierarchical level shown on this map. For portions of the United States the ecoregions have been further subdivided to Level IV. The applications of the ecoregions are explained in Gallant et al. (1989) and in reports and publications from the state and regional projects.

The level IV state projects depict revisions and subdivisions of ecoregions, that were compiled at a relatively small scale (Omernik 1987). Compilation of the level IV maps, performed at the larger 1:250,000 scale, has been a part of collaborative projects between United States Environmental Protection Agency, National Health and Environmental Effects Laboratory (NHEERL)--Corvallis, Oregon, the U.S. Forest Service (USFS), Natural Resources Conservation Service (NRCS), and a variety of other state and federal resource agencies. The ecoregions and subregions are designed to serve as a spatial framework for environmental resource management. The most immediate needs by the states are for developing regional biological criteria and water resource standards, and for setting management goals for nonpoint-source pollution. Explanation of the methods used to delineate the ecoregions are given in Omernik (1995), Griffith et al. (1994), and Gallant et al. (1989). This series of maps has been produced as part of a regional interagency collaborative project aimed at obtaining consensus between the EPA, the NRCS, and the USFS regarding alignments of ecological regions.

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Environmental Protection Agency. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Environmental Protection Agency should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.



Omernik, J., & Griffith, G. (2008). Ecoregions of the United States-Level IV (EPA). Retrieved from


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