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Issues in Risk Assessment Understanding the Response Stressors have both direct and indirect effects, and both should be incorporated in the stress-response analysis. Timing of the stress (e.g., seasonality) can be critical in determining the response. A given stressor can have system-specific responses that vary geographically and with time. Other Considerations For the near term, some of the best data for assessing stress-response relationships in ecological systems are published empirical data on naturally or intentionally manipulated systems. For example, many whole-lake experiments have been conducted in which nutrient loading, acid deposition, and food-web structure have been manipulated (Schindler et al., 1985; Carpenter, 1989). Temporal and spatial scales of analysis should be appropriate to the stress and to the responses. For a point-source chemical release, the appropriate scale might be relatively small and short-term, depending on the dispersion pattern and degradation rate of the chemical and the life histories of the potentially affected organisms. For contamination or habitat change that affects large areas and threatens extinction of species, regional or global assessments that cover decades or centuries are appropriate. Additions to the 1983 Paradigm Needed for Ecological Risk Assessment The paradigm should include consideration of the resiliency of the ecological systems in question and the time to recovery. Both would vary with the type of system, geographic location, structure or process of interest, and type of stress. The paradigm should discuss the choice of end point(s), including the relation of an end point to other major technical and societal concerns.