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Ecoregions of the United States
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Environmental resources research, assessment, monitoring, and, ultimately, management usually require appropriate spatial structures. Ecoregion frameworks are well suited to these purposes. Ecoregions are defined as areas of relative homogeneity in ecological systems and their components. Factors associated with spatial differences in the quality and quantity of ecosystem components, including soils, vegetation, climate, geology, and physiography, are relatively homogeneous within an ecoregion. Ecoregions separate different patterns of human stresses on the environment and different patterns in the existing and attainable quality of environmental resources. They have proven to be an effective aid for inventorying and assessing national and regional environmental resources, for setting regional resource management goals, and for developing biological criteria and water quality standard.
The approach we have used to compile ecoregion maps is based on the premise that ecological regions can be identified by analyzing the patterns and composition of biotic and abiotic phenomena that affect or reflect differences in ecosystem quality and integrity. These phenomena include geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. We do not begin by treating any one phenomena with more weight than any other. Rather, we look for patterns of coincidence between geographic phenomena that cause or reflect differences in ecosystem characteristics.
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For more information on the maps above, visit the Commission for Environmental Cooperation (CEC) Ecological Regions of North America.
The relative importance of each factor 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. At level II, the continent is subdivided into 52 classes, and at level III, the continental United States contains 98 ecoregions. Level IV ecological regions are further subdivisions of level III units. The exact number of ecological regions at each hierarchical level is still changing slightly as the framework undergoes development at the international, national, and local levels.
Text from this page taken from: Level III and IV Ecoregions of Pennsylvania and the Blue Ridge Mountains, the Ridge and Valley, and the Central Appalachians of Virginia, West Virginia, and Maryland by Alan J. Woods1, James M. Omernik2, and Douglas D. Brown3. June, 1996 (Addenda May, 1999) U.S. Environmental Protection Agency National Health and Environmental Effects Research Laboratory 200 SW 35th Street Corvallis, Oregon 973331. Dynamac Corporation2, U.S. Environmental Protection Agency3, U.S. Forest Service.
Maps provided by Alan Woods and James Omernik.