Unstructured Individual Variation and Demographic Stochasticity

The term demographic stochasticity is plagued by vague and contradictory definitions.It appears to have been coined by Robert May, who used it to describe the "stochastic features [that arise] because the population variable is fundamentally a discrete one" (May 1973:33).This definition has been used in influential papers in conservation biology ( e.g., "chance events in the survival and reproductive success of a finite number of individuals," Shaffer [1981:131]) and textbooks (e.g., "individuals do not usually produce the average number of offspring, but rather, may have no offspring, fewer than the average, or more than the average," Primack [2000: 132] ).These definitions do not clearly distinguish between variation in individual traits and variation in individual fates .Indeed, a major ecology textbook explicitly includes both in the definition: "random variations in the number of individuals that are born male or female, or in the number that happen to die or reproduce in a given year or in the quality ( genotypic/phenotypic ) of the individuals" ( Begon et al. 1996:927 ).In contrast, theoretical ecologists (e.g., Roughgarden 1975) rapidly linked the concept to existing statistical models of variation in the fate of individuals all sharing the same demographic traits.The latter definition, assuming that all individuals share the same traits, has been used in all quantitative applications in conservation biology.Is trait variation important?We have shown that structured variation among individuals in their mortality risk can reduce the population-level variance associated with demographic stochasticity, suggesting that such structure might reduce extinction risk (Kendall & Fox 2002).By structure we mean that an individual's demographic trait is not independent of the traits of the other individuals in the population or of the identity of the individual.This can be because there are correlations among individuals at a given time ( e.g., first-hatched chicks have higher rates of survival than last-hatched chicks; "demo-