Niche overlap predicts the magnitude of the indirect effects of environmental change in a mechanistic resource competition model
Environmental change can affect species indirectly by changing the abundances of competitors and the intensity of competition. The unknown strength of competition-mediated indirect effects complicates attempts to predict the outcome of environmental changes such as global warming or N-deposition. Theory developed using phenomenological competition models predicts that competition-mediated indirect effects will be strong when niche overlap is large. However, in phenomenological competition models it is unclear how to simulate environmental change, which is likely to influence multiple aspects of species performance simultaneously. Our objective was to extend previous theory by testing the relationship between niche overlap, defined by differences in species resource use ratios, and indirect effects in a mechanistic two-resource model of species competition. We simulated environmental change as a small change in resource supply rates and quantified indirect effects as the resulting competition-dependent change in species equilibrium abundances.
The indirect effect of a simulated change in resource availability on species abundances increased non-linearly as the niche overlap between competitors increased. That is, indirect effects were strongest when resource use ratios were most similar. This result holds even though the two species respond to a change in resource supply in qualitatively different ways. In an essential resource model, the species limited at equilibrium by the perturbed resource will be affected directly by the change in resource supply and indirectly by the competitor’s response. The species not limited at equilibrium by the perturbed resource will only experience indirect effects. In both cases niche overlap determines the magnitude of the indirect effects, though in the latter case the sensitivity of the competitor to the change in resource supply also plays a role. Our results demonstrate the value of using niche overlap for predicting the magnitude of indirect effects of environmental change.