The consequences of variation among individuals for species coexistence

Background/Question/Methods

Recent theoretical and empirical studies suggest that variation among individuals promotes the maintenance of species diversity. Individual variation can create niche axes where none would exist otherwise, enhance the stabilizing effects of existing species-level niche differences, or equalize species-level fitness differences. However, while the idea that individual variation only promotes coexistence is appealing, these conclusions rarely account for the findings of population demographic theory. This theory demonstrates that variation among individuals combined with finite population size tends to reduce population growth and persistence of small populations. Therefore, because coexistence relies on the ability of species’ populations to increase when rare, we predict that variation among individuals may have negative consequences for the maintenance of diversity. We test this prediction by adding variation among individuals to a range of demographic variables in a commonly used theoretical competition model. We determine the consequences for coexistence of variation among individuals in populations of both residents and invaders across a range of community sizes.

Results/Conclusions

Our analysis suggests that, as a first approximation, variation among individuals can hinder stabilized coexistence or have no effect. Individual variation within populations of resident species has little consequence for coexistence if community size is sufficiently large. In contrast, variation among individuals of the invader tends to reduce the likelihood of coexistence, although this result depends on the demographic trait in which variation occurs. Variation in survival has no effect on coexistence times but variation in fecundity and interaction strengths tends to reduce coexistence times. Our results show that variation among individuals in demographic rates will often reduce the probability of coexistence such that any benefits of individual variation must overcome this effect. Therefore, a comprehensive understanding of the effects of individual variation on coexistence should take into account both the positive and negative consequences of that variation.