Print PageMutualistic interactions constitute key components of virtually every community. And yet, their dynamics are much less well understood than those of competitive or consumer-resource interactions. Here, we develop mathematical models of mutualistic interactions to determine the conditions under which ecologically stable interactions between generalists and specialists can arise from an initially obligate pairwise interaction. The novel features of our model that distinguish it from previous work include competition for benefits acquired from the mutualistic partner as opposed to competition for an implicit resource that is not part of the interaction, and a an explicit measure of the degree of dependence on the mutualistic partner, quantified in terms of the rate at which a species goes extinct in the absence of its partner.
Results/Conclusions
We report two key findings. First, the positive feedback due to the obligate mutualistic interaction leads to an Allee effect, the magnitude of which increases as a species becomes more dependent on, but receives less benefit from, its mutualistic partner. Second, a community of generalist and specialist mutualists can be assembled from an initially obligate pairwise interaction even when the invading mutualist is an inferior competitor for the shared benefits. Our findings illuminate the consequences of the indirect interactions (facilitative vs. competitive) that arise between two specialists when their mutualistic partner is a generalist. These results provide a framework for understanding the assembly of complex mutualistic networks, and in particular, why mutualistic communities exhibit nestedness as opposed to compartmentalization.
