Consumer–resource dynamics of indirect interactions in a mutualism–parasitism food web module
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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 modules 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 (N 2) and parasitic (N 3) consumers exploiting a resource of a basal mutualist (N 1), as illustrated by plants, pollinators, and nectar robbers. This mutualism–parasitism food web module 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 N 2 have positive effects on N 3 and increases in N 3 have negative effects on N 2. This (+,−) indirect interaction arising from the saturating positive feedback of mutualism has broad feasibility across many types of food web modules entailing mutualism. Second, optimization of resource exploitation 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 parasitism destabilizing the module.
KeywordsCommunity Interaction strength Model Mutualism Parasitism Plant–pollinator–nectar robber
We thank W.F. Morris, S. Schreiber, and the anonymous reviewers for their comments which have improved this study. S.S. acknowledges support by the National Natural Science Foundation of China grant no. 31100277 and the Fundamental Research Funds for the Central Universities (lzujbky-2013-99). Y.W. acknowledges the support by NSKF of China grant no. 60736028, NSFC grant no. 11171355, and NSF of Guangdong Province S2012010010320. D.L.D. was supported by the US Geological Survey’s Southeastern Ecological Science Center. J.N.H. and D.L.D. acknowledge the support from NSF grants DEB-0814523 and DEB-1147630.
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