Phenotypic variation among co-occurring organisms is commonly assumed to promote species diversity within communities by reducing interspecific competition relative to intraspecific competition. To test this assumption we must determine whether trait differences among interacting individuals reduce the negative effects of competition. The effect of traits on competition is often difficult to assess in natural communities because monospecific and heterospecific neighborhoods are rarely encountered in a single habitat. The Cape Floristic Region in South Africa is an ideal place to investigate competitive effects as the dominant plant species within a community often occur in both single-species and mixed-species stands with varying neighborhood densities. Species in the family Proteaceae are especially convenient focal taxa as they often represent the majority of shrubby species and total plant biomass in communities, and individuals retain their seedheads throughout their life providing an accessible index of relative fitness. We collected trait data for over 1,110 individuals representing four Proteaceae species occurring in 100 neighborhoods consisting of either a single species or species in mixture.
Results/Conclusions
We examined the fitness of each focal individual as a function of the average dissimilarity in five leaf traits and the identity and density of each neighbor species. If trait differences are important for mediating competition, there will be a positive relationship between trait dissimilarity and fecundity of the focal individual. If differences in traits we did not measure are also important, there will be effects of species identity and density. For four of the five leaf traits measured, we found a significant positive effect of neighborhood trait dissimilarity on fecundity of the focal species. This effect was observed both when all populations were analysed together and when monospecific and heterospecific stands were analysed separately. Detecting the effect in monospecific stands implies that trait differences among individuals reduce competition even in stands of a single species. We also identified a significant species effect, indicating that traits we did not measure also influence competition. The species effects were primarily driven by negative density-dependent impacts by conspecifics, suggesting that intraspecific competition is greater than interspecific competition. Our results show that individual trait differences both within and among species mediate competition and may contribute to species coexistence in this hyper-diverse region.