The impact of adaptive defence on top-down and bottom-up effects in systems with intraguild predation

摘要

Questions: How does adaptive prey defence alter the responses of a system with intraguild predation to the bottom-up effects of enrichment and the top-down effects of predator mortality? Is the cost of defence an important determinant of the answer to this first question? Is persistence and/or increase of the prey population following enrichment more likely when the prey have adaptive defence? Methods: General equilibrium analysis of a simple model of the three-species intraguild predation system is used to determine the signs of population responses to top-down and bottom-up perturbations. Simulations are used to explore systems with unstable dynamics. Key assumptions: The system consists of three species, and all consumer species have linear functional responses in the absence of flexible prey defence. Results: Adaptive defence allows the equilibrium prey abundance to increase with enrichment of the basal resource. Defence may allow prey persistence over all possible levels of enrichment when high levels of defence imply near-immunity from predation. Predicting the direction of change in prey abundance in response to top-down or bottom-up perturbations requires knowledge of the costs of prey defence. The direction of response of abundance to a perturbation often changes sign when the perturbation shifts the system from stable to cyclic dynamics. Given costly defence, intraguild predation systems and food chains exhibit qualitatively similar responses to top-down and bottom-up perturbations in stable systems.