Variation among Individuals, Demographic Stochasticity, and Extinction: Response to Kendall and Fox

Abstract

Kendall and Fox (2002) examined the effect of varia-tion among individuals on expected survival in rela-tion to the strength of demographic stochasticity.They concluded that if such variation among individu-als exists, then existing population viability analysis(PVA) models may overestimate the variance due todemographic stochasticity in survival. This paper isone of the first to address this issue within a conserva-tion framework. As emphasized by the authors, variationamong individuals may have a considerable influenceon extinction risk and is therefore of crucial impor-tance in conservation biology. Although we agree withmuch of the interpretation of Kendall and Fox, we do notcompletely agree with their general conclusion thatvariation among individuals is likely to reduce extinc-tion risk. Particularly, the absence of randomness inthe variation among individuals in their analysis is anassumption that makes their results accurate only un-der a constrained scenario.We developed a simple, individual-based model to il-lustrate the influence of unstructured variation in sur-vival and fecundity among individuals, uncovering itspotential deleterious effect and its interaction with pop-ulation growth rate.