Background/Question/Methods
Individuals in a population often have more‐ or less‐consistent differences in their opportunities for survival and reproduction, as a result of various biological mechanisms. For instance, the survival of a plant seedling can depend on where the seed happens to germinate. An important question is how such individual heterogeneity affects the viability of a population. The expected time to extinction of a population is determined by the amount of demographic stochasticity in the population process, which is influenced by individual heterogeneity. However, whether the heterogeneity increases or decreases the demographic stochasticity has been an unresolved question, except for special cases. We have used a stochastic matrix population model to study the effects of individual heterogeneity on demographic stochasticity. This approach is more general than other methods which have previously been used to study this type of problem.

Results/Conclusions
We found that individual heterogeneity can increase, decrease, or have no effect on population viability, depending on the heterogeneity itself and on how it is maintained over time. This contrasts with some earlier results which have indicated that heterogeneity should always have a positive effect. Given certain assumptions, these can be shown to be special cases of our model. We can analyze several special cases, such as permanent heterogeneity and source-sink dynamics, as well as cases where individual heterogeneity is a function of age or stage. Thus, we provide a general theoretical framework for studying how individual heterogeneity, created by various biological mechanisms, affects the fluctuations of especially small populations.