Prey profitability and prevalence of intraguild predation in productive environments

Background/Question/Methods
Intra-guild predation (IGP) occurs when one predator species, the intra-guild predator (IGpredator), preys on and competes with another predator species, the intra-guild prey (IGprey) that shares common basal prey with the IGpredator. Mathematical models predict that IGprey are excluded by IGpredators in highly productive environments; however, empirical results do not necessarily support the prediction. Among many factors influencing the dynamics of IGP, the importance of relative profitability (energy gained divided by prey handling time) of prey items in a predator’s diet choice is an accepted concept in optimal foraging theory. However, the effects of prey profitability on population dynamics of IGpredators and IGprey have not been fully analyzed in theoretical models of IGP. I examined how the profitability of basal prey and IGprey affected the outcome of competition between IGpredators and IGprey, and explained the mechanisms of the prevalence of IGP in highly productive environments. To investigate the effects of prey profitability, a dynamical system model of IGP, including the type II functional response to two prey items, is necessary. I numerically solved the model and identified the conditions required for the persistence of IGprey in a productive environment.

Results/Conclusions
Using numerical bifurcation analysis, I demonstrated that there is a stable equilibrium or a stable limit cycle in which IGpredators and IGprey coexist or one of them is excluded. Alternative stable states often appear when environmental productivity, defined by the carrying capacity of the basal common prey, is high. When both basal prey and IGprey are sufficiently profitable for the IGpredator, and the IGpredator can survive by feeding only on basal prey, IGprey goes to extinction in a highly productive environment. However, when basal prey is not sufficiently profitable for the survival of the IGpredator, the IGpredator cannot exclude IGprey and the two predators coexist. When basal prey is profitable but IGprey is not, two limit cycles, in which either the IGpredator or the IGprey is extinct, become stable. Moreover, when both basal prey and IGprey are unprofitable for the IGpredator, the IGpredator, rather than the IGprey, goes to extinction in a highly productive environment. My results demonstrate that the extinction of IGprey in a productive environment is not a theoretical norm, and whether IGprey or the IGpredator becomes extinct depends on the profitability of basal prey and IGprey for the IGpredator.