Competition between partial consumers: Interactions between inducible defense and consumer-dependent demography
In many biological systems, individual organisms can survive attack by consumers. This frequently occurs in herbivory, especially when herbivores attack only specific plant parts. Recent theoretical work has shown that, in such cases of partial consumption, the consumer-specific responses of the demographic rates (survival, growth, and reproduction) of the plant resource can have major consequences for the outcome of competition between consumers. This previous work does not address inducible defenses, which are a prominent plant response to partial consumption. Several previous theoretical studies have demonstrated that inducible defense affects consumer-resource dynamics and competition between consumers. We aim to determine the effect on herbivore competition when both inducible defense and consumer effects on resource demography are present, and to identify any potential for interaction between these two aspects of partial consumption. Setting aside specificity of induction to the identity of consumer species, and also specificity of consumer responses to induction, we ask how nonspecific induced defenses can affect demographic pathways of interaction between consumers.
Building on our previous work, we present models using a one-plant, two-herbivore consumer-resource framework that explicitly includes plant demographic structure, and herbivore population dynamics that are fast relative to plant demography. We consider induction that is either fast compared to plant demography, or on similar or slower temporal scales relative to demography, and we also distinguish cases in which induction affects herbivore performance from cases where it affects herbivore foraging behavior (avoidance of induced resource individuals). We show that when induction affects herbivore performance, it does not interact with demographic pathways of indirect interaction between consumers unless it is fast compared to plant demography. Induction certainly can, however, affect resource demography and the population dynamics of all three species. In contrast, when induction protects plants against further herbivore attack by changing herbivore foraging behavior, it can attenuate the demographic effects of consumers on their resource, and thereby largely neutralize the potential for consumer-dependent resource demography to affect competitive outcomes. As a result, we predict that the importance of demographic responses to herbivores should be inversely proportional to herbivore behavioral responses to induced plant defenses. We also discuss important directions for future work.