Classic theory shows that coexistence is predicted by niche overlap (the degree to which competitors share common resources and natural enemies) and species’ fitness differences (their relative ability to deplete shared resources and tolerate natural enemies). Coexistence theory has been applied to interactions involving competition and antagonism, demonstrating symmetrical and interactive effects that can either promote or limit diversity. Until now, however, coexistence theory has not explicitly considered mutualistic interactions. This is surprising given that mutualisms are fundamental and ubiquitous interactions supporting biodiversity in nature. Here, I apply coexistence theory to mutualism for the first time, thus uniting the three fundamental species interactions – competition, antagonism, and mutualism – under a single framework.
Results/Conclusions
The novel feature of the framework is that it explicitly incorporates competition for mutualistic commodities, such as plants competing for pollinators. There are three key results. First, mutualisms should be viewed symmetrically with competition and predation: competition for commodities is equally able to either promote or limit coexistence as are resources or natural enemies. Second, mutualisms can either reduce or increase niche overlap and either mitigate or accentuate fitness differences depending on the nature of the interactions. Third, competition for mutualistic commodities is distinct from competition for other resources because while consumers only deplete resources, mutualists also confer mutualistic benefits to the partners producing the commodities for which they compete. For example, a species may indirectly confer a competitive advantage to another species by rewarding partners that differentially supports that competitor. Importantly, considering mutualisms within the context of coexistence theory facilitates investigations of basic and applied questions such as the effects of partner extinctions or invasive mutualists on natural communities.