Consumer–resource dynamics of indirect interactions in a mutualism–parasitism food web module

Abstract

Food web dynamics are well known to vary with indirect interactions, classic examples including apparent competition, intraguild predation, exploitative competition, and trophic cascades of food chains. Such food web mod- ules entailing predation and competition have been the focus of much theory, whereas modules involving mutualism have received far less attention. We examined an empirically common food web module involving mutualistic (N2) and parasitic (N3) consumers exploiting a resource of a basal mutualist (N1), as illustrated by plants, pollinators, and nectar robbers. This mutualism-parasitism food web mod- ule is structurally similar to exploitative competition, suggesting that the module of two consumers exploiting a resource is unstable. Rather than parasitic consumers destabilizing the module through (−,−) indirect interactions, two mechanisms associated with the mutualism can actually enhance the persistence of the module. First, the positive feedback of mutualism favors coexistence in stable limit cycles, whereby (+,−) indirect interactions emerge in which increases in N2 have positive effects on N3 and increases in N3 have negative effects on N2 .T his (+,−) indirect interaction arising from the saturatingpositivefeedbackof mutualism has broad feasibility across many types of food web modules entailing mutualism. Second, optimization of resource exploi- tation by the mutualistic consumer can lead to persistence of the food web module in a stable equilibrium. The mutualism- parasitism food web module is a basic unit of food webs in which mutualism favors its persistence simply through density-dependent population dynamics, rather than parasit- ism destabilizing the module.