Viability analysis in biological evaluations: Concepts of population viability analysis, biological population, and ecological scale

Environmental protection strategies often rely on environmental impact assessments. As part of the assessment process biologists are routinely asked to evaluate the effects of management actions on plants and animals. This evaluation often requires that biologists make judgments about the viability of affected populations. However, population viability analyses that are analytically comprehensive require extensive ecological data. Such data are usually unavailable and impossible for wildlife managers to collect given limitations of time and money. In this paper we present a conceptual framework to help managers assess population viability given the reality of limited information and resources. Our framework includes a series of steps that facilitate assessment of management impacts on population viability while stressing the importance of reconciling disparities between the geographic scale of management actions and the scale of ecological responses. We argue that a gross mismatch of scale between local management actions (e.g., timber sales) and geographically extensive ecological responses (e.g., species viability) reduces the reliability of environmental analyses. Our framework stresses “biological populations” as the most appropriate level of ecological organization for conducting impact analyses. We argue that in most cases environmental analyses of local management actions should assess the ecological responses of populations rather than the responses of entire species, as is now commonly the case. We also present ecological concepts that have been used effectively by biologists in making judgments about management effects and in developing conservation plans. Although not completely generalizable we believe these concepts, summarized from the conservation biology literature, can aid in evaluating population viability: (1) connected habitats are better than disjointed habitats; (2) suitable habitats in close proximity to one another are better than widely separated habitats; (3) late stages of forest development are often better than younger stages; (4) larger habitat areas are better than smaller areas; (5) populations with higher reproductive rates are more secure than those with lower reproductive rates; and (6) environmental conditions that reduce carrying capacity or increase variance in the growth rates of populations decrease persistence probabilities.