PROSPECTIVE AND RETROSPECTIVE PERTURBATION ANALYSES: THEIR ROLES IN CONSERVATION BIOLOGY

Abstract

Demographic perturbation analysis explores how population statistics (especially population growth rate λ) respond to changes in the vital rates (survival, growth, development, reproduction, and so on). Perturbation analysis is used in two logically distinct ways. Prospective analyses (sensitivity and elasticity) explore the functional dependence of λ on the vital rates. They predict the changes in λ that would result from any specified change in the vital rates and are independent of previous patterns of variability of the vital rates. Retrospective analyses (life table response experiment [LTRE] methods and other kinds of variance decomposition) express observed variation in λ as a function of observed (co)variation in the vital rates. Their results are specific to the observed pattern of variation. Sensitivity and elasticity analysis can be used to identify potential management targets because changes in vital rates with high sensitivity or elasticity will produce large changes in λ. Sometimes that potential may not be realized because it is difficult or impossible to change those vital rates. Retrospective analyses cannot identify potential management targets because they compare the contributions of past changes in vital rates, not the effects of future changes. Just as the results of heritability analysis say nothing about the efficacy of clinical treatments, so the results of retrospective analysis say nothing about the efficacy of potential management tactics. Perturbation analysis is a powerful tool with important implications for conservation. It is important that perturbation analyses be applied properly.