logical integrity have provided the supporting theory needed to develop standardized measurement techniques and criteria to determine whether efforts are complying with that goal. Biological integrity is now defined as ". . . the ability of an aquatic ecosystem to support and maintain a balanced, integrated, adaptive community of organisms having a species composition, diversity, and functional organization comparable to that of the natural habitats of a region" (Karr and Dudley, 1981). This is a workable definition that directly alludes to the measurable characteristics of biological community structure and function found in the least-impacted habitats of a region. This definition and its underlying ecological theory provide the basis for developing quantitative biological criteria based on conditions at regional reference sites. The EPA adopted a facsimile of this definition in their biological criteria national program guidance (United States Environmental Protection Agency, 1990).
The emerging issue of biodiversity should not be equated with biological integrity, even though the two concepts share many attributes (Karr, 1991). They differ in that biodiversity is primarily focused on ecosystem elements (i.e., genetic diversity, populations, bioreserves, etc.), whereas biological integrity includes these elements but also encompasses ecosystem processes (i.e., nutrient cycles, trophic interactions, speciation, etc.). The often-cited ecosystem approach to environmental management (e.g., Great Lakes Water Quality Initiative) can be even more restricted to dealing with elements that are not direct ecological parameters (i.e., chemical water-quality surrogates). Both the biodiversity and the ecosystem approaches would benefit by including the concept of biological integrity to improve the chances that each effort would succeed and assure that environmental problems are addressed from an ecological perspective.
A variety of quantitative indices for assessing biological data have been developed in the past 20 years. These indices represent significant advances because they use biological information for resource characterizations and for determining the attainment of environmental goals. Examples include the Index of Biotic Integrity, as originally developed by Karr (1981) and modified by many others (Leonard and Orth, 1986; Miller et al., 1988; Ohio Environmental Protection Agency, 1987b; Steedman, 1988); the Index of Well-Being (Gammon, 1976; Gammon et al., 1981); the Invertebrate Community Index (DeShon, 1995; Ohio Environmental Protection Agency, 1987b); the EPA Rapid Bioassessment Protocols for macroinvertebrate assemblages (Plafkin et al., 1989); and the Benthic Index of Biotic Integrity (Kerans and Karr, 1992).
Although quantitative biological indices have been criticized for potentially oversimplifying complex ecological processes (Suter, 1993), raw data must be distilled to be interpretable. Multimetric evaluation mechanisms extract ecologically relevant information from complex biological data while preserving the