tion of the probability from the birth, death, and dispersal rates estimated in the case study would require stochastic population modeling that takes account of uncertainty and variability in the population parameters.
The Endangered Species Act is an example of preemptive risk management, in that a high probability of extinction of a single species is designated as unacceptable. A species-by-species approach, however, does not lead to quantitative assessment of the risk of impoverishment of an ecosystem. Where possible, ecological risk assessment should work across levels of organization and should assess risks of reduction in system utility.
CASE STUDY 5:Ecological Benefits and Risks Associated with the Introduction of Exotic Species for Biological Control of Agricultural Pests
R. I. Carruthers, USDA Agricultural Research Service
The accidental or deliberate introduction of exotic species into regions where they are not native can cause positive, negative, or no observable effects, depending on a wide variety of biological, sociological, economic, and other factors. About 40% of the major arthropod pests (Sailer, 1983) and 50-75% of weed species (Foy et al., 1983) in the United States are introduced species, and introduced pests also include vertebrates, mollusks, and disease organisms that affect animals and plants. Many countries have developed formal programs to limit the introduction and establishment of unwanted exotic organisms, and many have developed methods to assess benefits and risks associated with planned introductions. The United States has no federal statute or set of statutes that governs introductions; instead, it has cumbersome and sometimes conflicting regulations, protocols, and guidelines.
This paper addressed assessment of risks and benefits of "classical biological control" (CBC): the planned introduction of exotic enemies of an introduced pest collected from the pest's home range (DeBach, 1974). Classical biological control (either alone or integrated with other pest management methods) has frequently been successful in controlling