Abstract:
This map layer shows Omernik's Level III ecoregions, derived from a
1:7,500,000 map created by J.M. Omernik in 1987 and from refinements of
Omernik's framework that were made for other projects. Ecoregions
describe areas of general similarity in ecosystems and in the type,
quality, and quantity of environmental resources. Omernik's ecoregions
are based on the premise that a hierarchy of ecological regions can be
identified through the analysis of the patterns and the composition of
both living and nonliving phenomena, such as geology, physiography,
vegetation, climate, soils, land use, wildlife, and hydrology, that affect
or reflect differences in ecosystem quality and integrity. All the
characteristics are considered when determining ecoregions, but the
relative importance of each characteristic may vary from one ecoregion to
another. Level III is the most detailed level available nationally for
this system of ecoregions.
Purpose:
These data are intended for geographic display and analysis at the national level, and for large regional areas. The data should be displayed and analyzed at scales appropriate for 1:7,500,000-scale data. No responsibility is assumed by the National Atlas of the United States in the use of these data.
Supplemental_Information:
Detailed descriptions of the ecological regions represented in this map
layer are available from the U.S. Environmental Protection Agency (EPA),
Western Ecology Division at:
http://www.epa.gov/wed/pages/ecoregions/level_iii.htm. EPA also offers
data of Omernik's Level III Ecoregions on the same site; the file includes
the same ecoregions as are available through the National Atlas but uses
EPA shorelines and State boundaries.
A bibliography of ecoregion/biocriteria publications, called
ecoomr_Biblio.txt, is bundled with the downloadable data. Each reference
in the bibliography has a code; the codes are used for any citations in this
metadata file.
Ecoregions denote areas of general similarity in ecosystems and the type,
quantity, and quality of environmental resources; they are designed to
serve as a spatial framework for the research, assessment, management, and
monitoring of ecosystems and ecosystem components. By recognizing the
spatial differences in the capacities and potentials of ecosystems,
ecoregions stratify the environment by its probable response to
disturbance (BR-1-98). These general purpose regions are critical for
structuring and implementing ecosystem management strategies across
Federal agencies, State agencies, and nongovernmental organizations that
are responsible for different types of resources within the same
geographic areas (OM-1-00, OM-1-04).
The approach used to compile these ecoregions is based on the premise that
ecological regions are hierarchical and can be identified through the
analysis of the spatial patterns and composition of biotic and abiotic
phenomena that affect or reflect differences in ecosystem quality and
integrity (WI-1-86, OM-1-87, OM-1-95, OM-1-04). These phenomena include
geology, physiography, vegetation, climate, soils, land use, wildlife,
water quality, and hydrology. In the regionalization process, the full
range of characteristics is examined and integrated at each scale or level
of resolution. The relative importance of each characteristic varies from
one ecological region to another regardless of the hierarchical level.
Level I is the coarsest level of the hierarchy, dividing North America
into 15 ecological regions. Level II divides the continent into 52
regions (CE-1-97). At level III, the continental United States contains
104 ecoregions, with the conterminous United States having 84 (US-1-03).
More detailed level IV ecoregions have been delineated, or are in the
process of being mapped, for approximately 85 percent of the conterminous
United States.
Since 1994, when the National Interagency Technical Team (NITT) was formed
to develop a common framework of ecological regions (MM-1-01), efforts
have been made to include all of the major federal resource management
agencies in the level IV ecoregion mapping projects. The goal of each of
these collaborative projects has been to attain consensus across agencies
and ultimately to develop a consistent ecoregion framework for the entire
nation.
Because the collaborative projects to map level IV ecoregions often result
in refined level III boundaries, the map of level III ecoregions is
periodically revised. Likewise, much of the recent work to refine level
III boundaries and define level IV ecoregions has resulted in slight
differences between these two levels and the coarser level I and II
regions mapped earlier by the CEC.
Level IV ecoregions data and maps can be accessed through the
Environmental Protection Agency Level IV Ecoregions page at
http://www.epa.gov/wed/pages/ecoregions/level_iv.htm.
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