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neers need to increase their understanding of ecological function. With this background, they can help ecologists develop standardized methods to measure ecosystem resistance and resilience, and quantify ecosystem tolerance limits, such as thresholds (Khan, 1992; Schaeffer and Cox, 1991).
Certain laws provide explicit protections for the known and knowable aspects of the domain encompassed in the term "ecosystem" in the conservation-based philosophy. For example, the Endangered Species Act affords protection to the spotted owl by ensuring the sustainability of the old-growth forest that is its habitat. This implicitly also protects the unknowable relationships among the owl, other species, and abiotic habitat components. The Endangered Species Act does not require balancing economic costs against the ecological benefits resulting from protection of a species. If the Endangered Species Act is an idealistic implementation of the conservation-based philosophy, the National Environmental Policy Act requires a pragmatic balancing of the production-based and conservation-based philosophies. Thus, if threatened and endangered species are not at risk, the National Environmental Policy Act will allow for an ecosystem to be exploited for some purposes, provided that efforts are made to avoid, minimize, or mitigate effects on other ecosystem components.
Upper Mississippi River Navigation Studies
The UMR-IWW navigation system provides habitat for at least 485 species of birds, mammals, amphibians, reptiles, and fish, including many endangered or threatened species. It includes a national fish and wildlife refuge of more than 226,650 acres and provides drinking water, irrigation, and recreation services to hundreds of communities. To ensure that the natural resources and other services are not adversely affected by increases in barge traffic and the associated engineering efforts, the Corps has embarked on a $42 million program to assess the effects from increased navigation and recreational traffic. These studies will result in the preparation of a final environmental impact statement in 1999 by the Corps, with advice from state and federal natural resource agencies (U.S. Fish and Wildlife Service, U.S. Environmental Protection Agency, Illinois Department of Conservation, Iowa Department of Natural Resources, Minnesota Department of Natural Resources, Missouri Department of Conservation, and Wisconsin Department of Natural Resources) and other organizations that can influence legal acceptability through the Courts (e.g., Izaak Walton League, Sierra Club, and Minnesota-Wisconsin Boundary Area Commission). However, the differences in the perspectives and goals of these various agencies, the Corps, and other organizations have resulted in a decade-long conflict in setting the goals for the ecological studies (Schaeffer et al., 1992; U.S. Army Corps of Engineers, 1991). As discussed below, the conflict reflects a clash of philosophies.