Prospective and retrospective perturbation analyses: Their roles in conservation biology

Demographic perturbation analysis explores how population statistics (espec ially population growth rate lambda) respond to changes in the vital rates (survival, growth, development, reproduction, and so on). Perturbation anal ysis is used in two logically distinct ways. Prospective analyses (sensitiv ity and elasticity) explore the functional dependence of lambda on the vita l rates. They predict the changes in lambda that would result from any spec ified change in the vital rates and are independent of previous patterns of variability of the vital rates. Retrospective analyses (life table respons e experiment [LTRE] methods and other kinds of variance decomposition) expr ess observed variation in lambda as a function of observed (co)variation in the vital rates. Their results are specific to the observed pattern of var iation. Sensitivity and elasticity analysis can be used to identify potenti al management targets because changes in vital rates with high sensitivity or elasticity will produce large changes in lambda. Sometimes that potentia l may not be realized because it is difficult or impossible to change those vital rates. Retrospective analyses cannot identify potential management t argets because they compare the contributions of past changes in vital rate s, not the effects of future changes. Just as the results of heritability a nalysis say nothing about the efficacy of clinical treatments, so the resul ts of retrospective analysis say nothing about the efficacy of potential ma nagement tactics. Perturbation analysis is a powerful tool with important i mplications for conservation. It is important that perturbation analyses be applied properly.