Type III functional response by omnivores and persistence of intraguild predation in a productive habitat
Intraguild predation (IGP) is a simplest kind of omnivory in which one species (IGpredator) in a predatory guild feeds on another predator species (IGprey) that shares and competes for a common prey. Classical mathematical models predicted that IGprey are excluded by IGpredators in highly productive environments but empirical results have not necessarily supported the prediction. Dynamics of IGP is known to depend on the functional responses of the IGpredators to two prey species; IGP with the type II functional responses can stabilize an otherwise unstable or chaotic food chain while IGP with the type I functional responses can create chaos when the indirect path from the common prey to the IGpredator through the IGprey is more efficient than the direct one. However, dynamics of IGP with the type III functional responses of the IGPredator is scarcely known. Here, I consider a dynamic model of IGP in which the IGPrey exhibits a type II functional response but the IGPredator’s responses to the IGPrey and common prey are of the type III. I both analytically and numerically examine the model and identify how the type III responses affect the conditions for coexistence by altering linear stability and nonlinear dynamics.
When the functional responses of the IGpredators to two prey species, IGPrey and common prey, are of the type III, the relative predation pressure on the fewer prey species becomes disproportionately lower when the relative abundance declines to zero, i.e. the IGPredators switch their diets depending on the relative abundance of the two prey species. Thus, the IGprey can invade and escape extinction when they are rare because of the reduced predation pressure even if the habitat is highly productive. This is the first mechanism of coexistence of the two predator species. When the functional responses are nonlinear, population oscillations appear both in the IGPrey-basal prey subsystem and in the IGPredator-basal prey subsystem. Even if the IGPrey-free steady state is attracting and the IGPrey cannot invade the state, they can increase when rare if the growth rate averaged over a period of IGPredator-basal prey oscillations is positive. If the type III functional response can release the predation pressure on the rare IGPrey even in non-equilibrium oscillations, the possibility of IGPrey persistence extends greatly. Therefore, the type III functional response is not only important for stabilizing a community but also for realizing species coexistence even in the oscillatory regime.