VIABILITY ANALYSIS IN BIOLOGICAL EVALUATIONS - CONCEPTS OF POPULATIONVIABILITY ANALYSIS, BIOLOGICAL POPULATION, AND ECOLOGICAL SCALE

Environmental protection strategies often rely on environmental impact assessments. As part of the assessment process biologists are routine ly asked to evaluate the effects of management actions on plants and a nimals. This evaluation often requires that biologists make judgments about the viability of affected populations. However, population viabi lity analyses that are analytically comprehensive require extensive ec ological data. Such data are usually unavailable and impossible for wi ldlife managers to collect given limitations of time and money. In thi s paper we present a conceptual framework to help managers assess popu lation viability given the reality of limited information and resource s. Our framework includes a series of steps that facilitate assessment of management impacts on population viability while stressing the imp ortance of reconciling disparities between the geographic scale of man agement 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., speci es viability) reduces the reliability of environmental analyses. Our f ramework stresses ''biological populations'' as the most appropriate l evel of ecological organization for conducting impact analyses. We arg ue that in most cases environmental analyses of local management actio ns should assess the ecological responses of populations rather than t he responses of entire species, as is now commonly the case. We also p resent ecological concepts that have been used effectively by biologis ts in making judgments about management effects and in developing cons ervation plans. Although not completely generalizable we believe these concepts, summarized from the conservation biology literature, can ai d in evaluating population viability: (1) connected habitats are bette r than disjointed habitats; (2) suitable habitats in close proximity t o one another are better than widely separated habitats; (3) late stag es of forest development are often better than younger stages; (4) lar ger habitat areas are better than smaller areas; (5) populations with higher reproductive rates are more secure than those with lower reprod uctive rates; and (6) environmental conditions that reduce carrying ca pacity or increase variance in the growth rates of populations decreas e persistence probabilities.