Conditions under which plants help herbivores and benefit from predators through apparent competition.

Leaf domatia are tiny structures in leaf vein axils that are typically inhabited by predatory and fungivorous mites. A recent article reported plant domatia specifically suited for herbivorous mites, which seems paradoxical, since the plant is thus supporting a natural enemy that can harm itself. The authors claimed that domatia are created to promote herbivorous mites as "fodder" for predatory mites that attack another herbivorous mite damaging the plant, and that the relationship among the plant, the fodder mite, and the predatory mite constitute a multiway mutualism because all three species benefit from the interaction. I formulate this system using two simple mathematical models of apparent competition, which differ in how domatia are modeled, and then assess when it is advantageous for the plant to create such space for a natural enemy. As a necessary condition for mutualism, the product of reproductive efficiency and nutritious value of the fodder prey should exceed that of the pest prey. This condition is also sufficient, if the direct costs for making the structure of domatia are negligible. If there are significant costs, however, the condition is broader for predators with lower reproductive efficiency and higher mortality, and for non-fodder prey with high consumption rate and low predation rate. I suggest that creating domatia is more effective when predators are less prolific and non-fodder prey are more severe as pests. Finally, I discuss how this mathematical model can apply to a wider range of tritrophic mutualistic relationships such as those among plants, aphids, and ants.