Species intraspecific trait variation has been confirmed empirically to have important ecological effects. This has inspired many efforts in the past decade to build a theoretical understanding of when this variation plays a role and what its effects are. In particular, studies have recently considered the effects of intraspecific variation on competitive coexistence. Many of the reported effects are negative. Unless there is an interspecific mean-variance tradeoff, intraspecific variation increases the dominance of superior competitors and lessens niche differentiation. However, positive effects have been found on the robustness of coexistence when the variation is heritable, which has typically been ignored. Here we consider intraspecific variation in the context of another factor ignored in these studies, and often in competition theory more broadly, namely immigration. We hypothesize that when stochasticity and immigration are considered intraspecific variation will raise species richness in some regimes because it could slow rates of exclusion of species not stabilized in the deterministic model. We study a stochastic community dynamics model with competition driven by trait diļ¬erences and immigration from a regional pool. We used this model to consider the effects of intraspecific variation on species richness, niche clustering pattern, and metrics of the species abundance distribution.
Results/Conclusions
Our simulations suggested intraspecific variation can indeed increase species richness in competitive communities with immigration. This occurs when there are more species immigrating than can stably coexist through trait differences. It is coupled with a weakening of clustering pattern, and an increase in the evenness of species abundance. Negative effects of intraspecific variation on species richness are mainly apparent under strong niche differentiation, i.e. when competition declines quickly with trait differences and most species stably coexist. This effect is coupled with a reduction in the number of clusters, and a decline in evenness. In either regime (weak or strong niche differentiation), intraspecific variation typically shifts community dynamic toward what occurs in neutral case. More complex effects occur when species with extreme trait values can dominate by avoiding competition. These edge effects are enhanced by high intraspecific variation, lowering richness compared with the neutral case, and decreasing evenness of species abundance. Our results highlight that the expected effects of intraspecific trait variation on competitive coexistence depend strongly on how niche differentiated species are according to their trait means. Further, developing a complete understanding of the effects of intraspecific trait variation requires consideration of the influence of immigration on community dynamics.