|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.
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
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
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
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
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
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
which are defined by climate, vegetation, and terrain.