habitat in surrounding areas; these circumstances lead to a large "floating" component of the population.
The paper concluded that risk assessment in higher vertebrate populations must often rely on analysis of samples of marked individuals. A robust theory exists for study design and the analysis of such data. Selection of appropriate models is critical for rigorous assessment of impacts. Analysis of capture-recapture data allows inferences about the separate processes of birth, death, emigration, and immigration. Risk to a population does not affect population size directly; rather, it acts on the fundamental processes of birth and death.
(Led by M. E. Kentula, U.S. Environmental Protection Agency, and O. L. Loucks, Miami University)
Dr. Kentula commented that the case study (like others in the workshop) focused on individuals and populations and thus took a bottom-up approach. An alternative, top-down approach is to conduct an ecosystem risk assessment from a landscape perspective. For example, Kentula stated that EPA's Wetlands Research Program is developing methods to assess impacts on landscape function due to cumulative wetlands loss (Abbruzzese et al., 1990). The method proceeds in two-stages: a landscape characterization map is used to classify and rank units of the landscape according to relative risk, and can also be used to set priorities for effort and allocation of resources; a response curve expresses the hypothesized relationship between stressors (such as loss or modification of wetlands) and reduction in landscape functions (e.g., maintenance of water quality, or life support). The system can be used both to identify areas at risk and to guide management decisions for landscapes that are already affected.
Dr. Loucks commented that the case study presents the consequences of the stress to one local owl population at one time. For assessment of risk to the regional or total population, one would need to construct a "dose-response" relationship, in which "dose" would be a measure of the degree of stress (e.g., the percentage of the old-growth forest that has been destroyed) and "response" would be the probability of extinction of the population within an appropriate period (e.g., 250 years). Calcula-