Positive, negative, or neutral effects of individual variation on coexistence?

Background/Question/Methods

It is clear that variation among individuals can have profound consequences for species coexistence. Nevertheless, existing theoretical and empirical work creates a confusing picture about the nature of these effects. Theoretical studies have demonstrated positive, negative, and neutral effects of individual variation on species diversity and empirical investigations have done little to reconcile these conflicting results. On the one hand it is not surprising that individual variation can have contrasting effects on coexistence because variation among individuals takes qualitatively different forms (e.g. genetically determined vs. environmentally determined variation, morphological vs. demographic variation, etc.). Nevertheless, there is a danger that because of this complexity the study of these effects will deteriorate into the study of a series of special cases. Our aim is to reduce the complexity (and apparent contingency) of the problem by identifying a limited number of scenarios for the effects of individual variation on coexistence using a common analytical framework. To illustrate our approach we use stochastic simulations of competitive population dynamics to demonstrate broad similarities in the effects of two qualitatively different types of individual variation on species coexistence.

Results/Conclusions

We demonstrate that correlated variation among individuals generally has positive effects on coexistence irrespective of the cause of that variation. For example, heritable genetic variation in competitive ability can increase average population competitive ability via selection. Such an effect increases the likelihood that a rare species can invade an established assemblage, thus promoting species diversity. When competitive ability is determined by environmental heterogeneity and dispersal is limited, coexistence is promoted in a similar way. Despite the many qualitative differences between these two scenarios our analytical approach demonstrates that a common underlying intergenerational correlation in individual variation ensures similar effects on coexistence. We hope that such an analytical foundation has substantial promise for unifying approaches to the study of individual variation on coexistence.