Apparent competition is a type of negative indirect interaction between species that do not compete directly for resources. This phenomenon has been extensively studied as a predator-mediated effect between prey species, where an increase in the population size of one prey species has a negative effect on the population size of the other through the numerical response of a shared predator. A pathway through a higher trophic level has been the focus of most studies of apparent competition with little attention given to alternative pathways. For example, in a mutualistic network, competing resources (e.g., plants) can mediate a negative effect between the resources’ mutualistic and specialist partners (e.g., pollinators), even if the respective partners are not direct competitors. We demonstrate how apparent competition can proceed through both upper and lower trophic levels, then partition the indirect effect to examine its strength when mediated through different pathways and assess the consequences for coexistence.
Results/Conclusions
We use a Lotka-Volterra framework to model a simple five species system: two pairs of specialist mutualistic partners, which may be conceptualized as plants and pollinators, and a generalist predator. The inverse of the Jacobian matrix is used to determine net interaction coefficients, which, in the case of the two pollinators, is a taken as the strength of apparent competition since they do not interact directly. If the plants are direct competitors, apparent competition between the pollinators can proceed through their competing resources or through their shared predator. If one apparent competitor has a sufficient advantage, either mode of apparent competition alone or in concert can result in the exclusion of the weaker apparent competitor. When both pathways of apparent competition are acting simultaneously the predator acts to stabilize the system and allows the weaker apparent competitor to persist longer and over a wider range of parameter space than if the predator was absent. We show that when plant competition is strong, the predator can have a net positive effect on the weaker apparent competitor, even if it is the preferred prey. When partitioning the effect of apparent competition between the two alternative pathways, we find that the lower pathway contributes more to apparent competition as the strength of plant competition increases, though the fraction of the total effect due to predation increases near the exclusion point of the inferior apparent competitor.
