Ecoregion frameworks are useful for 1) inventorying and assessing national and regional environmental resources, 2) setting regional resource management goals, 3) establishing geographical research frameworks, and 4) developing regional biological criteria and water quality standards (Arkansas Department of Pollution Control and Ecology, 1988; Bazata, 1991; Environment Canada, 1989; Gallant and others, 1989; Heiskary and Wilson, 1989; Hughes, 1989b; Hughes and others, 1987, 1990, 1994; Larsen and others, 1986; Lyons, 1989; Ohio Environmental Protection Agency, 1988; Plotnikoff, 1992; Rohm and others, 1987; U.S. Environmental Protection Agency, Science Advisory Board, 1991; Warry and Hanau, 1993; Whittier and others, 1988).
Ecoregion frameworks have been developed for several countries, including the United States, Canada, New Zealand, and the Netherlands (Bailey, 1976, 1983, 1995; Bailey and others, 1985, 1994; Biggs and others, 1990; Ecological Stratification Working Group, 1995; Klijn, 1994; Omernik, 1987, 1995a; Omernik and Gallant, 1990; U.S. Environmental Protection Agency, 2005; Wiken, 1986). The first compilation of ecoregions in the conterminous United States by the U.S. Environmental Protection Agency (U.S. EPA) was performed at a relatively cursory scale (1:3,168,000), and was published at a smaller scale (1:7,500,000) (Omernik, 1987). Subsequently, this ecoregion framework was expanded to include Alaska and all of North America, revised, and made hierarchical (Gallant and others, 1995; Omernik, 1995b; U.S. Environmental Protection Agency, 2005).
Level I is the coarsest level in the ecoregion hierarchy; it divides North America into 15 ecological regions. Level II divides the continent into 50 regions. At level III, the continental United States contains 104 ecoregions, whereas the conterminous United States has 84. Level IV ecological regions are further subdivisions of level III ecoregions. The exact number of ecological regions at each hierarchical level is still changing slightly as the framework undergoes development at the international, national, and state levels.
Detail resolution on Omernik’s (1987) ecoregion map of the conterminous United States was necessarily limited by its rather small scale of 1:7,500,000. Subsequently, many larger scale, collaborative, state projects refined Omernik’s original ecoregion map, and subdivided its level III ecoregions into level IV ecoregions. Completed level IV ecoregion projects cover Alabama, Arkansas, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Mississippi, Missouri, Montana, Nebraska, Nevada, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Virginia, Washington, West Virginia, Wisconsin, and Wyoming. Other state level IV ecoregion projects are still in progress including Minnesota, New Jersey, and New Mexico. These level IV ecoregion projects have involved state agencies, U.S. EPA regional offices, and the U.S. EPANational Health and Environmental Effects Research Laboratory, Western Ecology Division in Corvallis, Oregon. Projects have included participation by the U.S. Department of AgricultureNatural Resources Conservation Service and the U.S. Department of AgricultureForest Service as part of an interagency effort to develop a common framework of ecological regions.
In this paper we refine the level III ecoregions of Illinois, subdivide them into more detailed level IV subdivisions, and provide descriptions and summary information for each ecoregion in the state. Illinois has 6 level III and 25 level IV ecoregions; all but 8 continue into ecologically similar parts of adjacent states (Chapman and others, 2001, 2002; Griffith, 1994; Omernik and others, 2000; Woods and others, 1998, 2002).
The work in Illinois was done in cooperation with U.S. EPA Region 5. The Illinois ecoregionalization project involved personnel from several state agencies, federal agencies, and universities, including the Illinois Environmental Protection Agency (Illinois EPA), Illinois Natural History Survey, Illinois State Geological Survey, Illinois State Water Survey, U.S. Department of AgricultureNatural Resources Conservation Service (NRCS), and Eastern Illinois University. This project was partly conducted under the auspices of the Program for Cooperative Research on Aquatic Indicators Award # CR-831682-01.
The procedures used to define Illinois ecoregions are consistent with those that were used in preceding U.S. EPA ecoregion studies of neighboring states (Chapman and others, 2001, 2002; Griffith, 1994; Omernik and others, 2000; Woods and others, 1998, 2002). They are based on the premise that ecological regions can be identified through the analysis of biotic and abiotic characteristics that affect or reflect differences in ecosystem quality and integrity (Omernik, 1987, 1995a; Wiken, 1986). Spatial pattern, composition, and spatial correspondence of physiography, natural vegetation, soil, surficial and bedrock geology, climate, land use, land cover, wildlife, and fish were considered as part of the process. The relative importance of each biotic and abiotic characteristic varied from one ecological region to another regardless of the ecoregion hierarchical level. Expert judgment was employed throughout the selection, analysis, and classification of data to define the ecoregions. Information from the literature and input from state and regional experts were very important to this project, as well as the earlier ones in neighboring states. Ecoregion lines were compiled at 1:250,000-scale onto 1:250,000-scale topographic base maps. More detailed explanations about the methods, materials, rationale, and philosophy for the ecoregionalization process can be found in Gallant and others (1989), Omernik (1995a), and Omernik and Gallant (1990).
Evaluation of the ecoregion framework presented in this paper is a necessary future step. U.S. EPA ecoregions have been evaluated extensively in the past, and the most meaningful of these efforts have involved the use of measures of water quality and indices of biotic integrity (IBI’s) (Hughes, 1989a; Larsen and others, 1986, 1988; Whittier and others, 1987; Yoder and Rankin, 1995). A better tool would be a more encompassing index of ecological integrity (IEI) (Omernik, 1995a, 1995b); although an IEI is not available yet, there is considerable interest in at least two states to begin its development. Verification of ecoregions cannot be done by considering individual ecosystem components; this is because the ecoregion framework was not intended to show regional patterns specific to either the flora or fauna of terrestrial ecosystems, nor was it intended to reflect distributions of fish or aquatic macroinvertebrates.
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PRINCIPAL AUTHORS: Alan J. Woods (Oregon State University), James M. Omernik (U.S. Geological Survey), Charles L. Pederson (Department of Biological Sciences, Eastern Illinois University), and Brian C. Moran (Indus Corporation).
COLLABORATORS AND CONTRIBUTORS: Candice Bauer (U.S. Environmental Protection Agency), Bill Ettinger (Illinois Environmental Protection Agency), Gregg Good (Illinois Environmental Protection Agency), Ed Hammer (U.S. Environmental Protection Agency), Mark Joseph (Illinois Environmental Protection Agency), Don Keefer (Illinois State Geological Survey), Laura Keefer (Illinois State Water Survey, Andrew Phillips (Illinois State Geological Survey), Don Pitts (U.S. Department of AgricultureNatural Resources Conservation Service), Mike Retzer (Illinois Natural History Survey), and Roy Smoger (Illinois Environmental Protection Agency).
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