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Map Layer Info

Omernik's Level III Ecoregions of the Continental United States

What this map layer shows:

Ecoregions as areas with generally similar ecosystems and with similar types, qualities, and quantities of environmental resources. Ecoregion boundaries were determined by examining patterns of vegetation, animal life, geology, soils, water quality, climate, and human land use, as well as other living and non-living ecosystem components.
opens the U.S. Environmental Protection Agency home page
Background Information
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A large area that includes generally similar ecosystems and that has similar types, qualities, and quantities of environmental resources is known as an ecoregion. The purpose of ecological land classification is to provide information for research, assessment, monitoring, and management of ecosystems and ecosystem components. Federal agencies, State agencies, and nongovernmental organizations responsible for different types of resources within the same area use this information to estimate ecosystem productivity, to determine probable responses to land management practices and other ecosystem disturbances, and to address environmental issues over large areas, such as air pollution, forest disease, or threats to biodiversity.

The Omernik ecoregion system is hierarchical and considers the spatial patterns of both the living and non-living components of the region, such as geology, physiography, vegetation, climate, soils, land use, wildlife, water quality, and hydrology. The patterns affect or reflect differences in ecosystem quality and integrity. All the components are considered when determining the location of ecoregion boundaries. The relative importance of each component may vary from one ecoregion to another, regardless of the level of the hierarchy. For example, for one ecoregion, geology may be the primary characteristic that determines the ecoregion boundaries. For another, a combination of soils and climate may be what defines the ecoregion. This is true whether the ecoregion is continental in scale, local in scale, or falls somewhere in between.

There are four levels in the Omernik ecosystem hierarchy: Level I divides North America into 15 broad ecoregions appropriate for analysis at a global or intercontinental scale. Level I ecoregions were mapped and described by the North American Commission for Environmental Cooperation (CEC) in 1997. The Level I North American ecoregions are subdivided into 52 Level II ecoregions which are useful for national and subcontinental overviews of physiography, wildlife, and land use. Level III represents a further subdivision, with 194 ecoregions to describe North America, of which 104 apply to the continental United States; this level is appropriate for regional analysis and decision-making. Level III is the most detailed level available nationally for this system of ecoregions and is what is included in the National Atlas. Work to define Level IV ecoregions, a scale that provides useful information for local analysis, is underway or complete for most of the United States.

In the mid-1990s, the National Interagency Technical Team (NITT) was formed to develop a common framework of ecological regions for the nation. The intention is that the framework will foster an ecological understanding of the landscape, rather than an understanding based on a single resource, single discipline, or single agency perspective. Participants include representatives from eight Federal agencies: the Natural Resources Conservation Service, the Forest Service, the Agricultural Research Service, the Bureau of Land Management, the National Park Service, the U.S. Fish and Wildlife Service, the U.S. Geological Survey, and the U.S. Environmental Protection Agency. This team works to develop national standards and procedures for detailed ecoregion mapping, and helps coordinate with State and regional agencies to ensure consistency and quality, with the goal of attaining consensus across agencies in the delineation of ecoregions. The mapping of Level IV ecoregions, based on the work of the NITT, may result in future refinements to Level III boundaries, and eventually to refinements in the Level II and Level I boundaries.

The Omernik's Level III Ecoregions of the Continental United States map layer shows ecoregion delineation based on common patterns of geology, physiography, vegetation, climate, soils, land use, wildlife, water quality, and hydrology. This map layer was compiled by the U.S. Environmental Protection Agency (EPA). Further information on ecoregions is available from the EPA Western Ecology Division, Level III Ecoregions page. A combined data set in Arc/INFO Export format, with Level I, Level II, and Level III ecoregions for all of North America, is available from the EPA Ecoregions of North America download page.

The National Atlas also includes a map layer showing Bailey's Ecoregions, which are defined by climate, vegetation, and terrain.


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Ecoregions - Omernik