Inherent stability of resource-mutualist-exploiter interactions

Background/Question/Methods

Despite the ubiquity and functional importance of mutualistic relationships in nature, the population dynamics of mutualism have long appeared to be unstable in two important ways: bounds to the growth of a mutually facilitating pair of species may not be immediately apparent, and the mutualism between them appears vulnerable to exploitation by a third, non-mutualistic consumer of the resource species. Previous approaches to the dynamics of communities with mutualists achieve stabilization using exogenous intra- or interspecific density dependence, implicit or explicit competitive asymmetries, or spatial or other competitive coexistence mechanisms. However, recent theoretical work focusing on resource demography, and including the effects of both consumers on resource demographic rates, shows that a mutualist and a consumer can coexist in the absence of other stabilizing mechanisms given exogenous regulation of their resource population. Here I investigate the potential for the consumers' coexistence itself to regulate the resource.

Results/Conclusions

Mathematical analysis reveals that a coexisting mutualist-exploiter pair can effectively regulate the population dynamics of their shared resource species. Thus, the three-species interaction is potentially self-stabilizing in a manner that cannot be identified from interacting pairs alone, or by approaches that neglect consumer-dependent resource demography. I show that the conditions necessary for this stabilization are biologically quite reasonable: a growing resource population must favor increase of the exploiter population, whereas a declining resource population must favor the mutualistic consumer. In systems where consumer species form long-term relationships with the resource species (such as ants competing for nesting space within plants), this can occur due to an acquisition-retention tradeoff between consumers that is very likely given the definition of an exploitative consumer. In systems where consumer-resource relationships are more short-lived (such as plant-pollinator-herbivore systems), the potential for stabilization depends on resource demographic structure. Because most species interact with multiple other species in nature, these results imply that rather than being destabilizing, mutualism can contribute to stabilization of multi-species communities through effects on resource species demography.