PS 32-205 - Impact of natural enemy victim specialization and function response on enemy-victim dynamics and coexistence

Abstract:

Natural enemy mediated coexistence is one of the leading mechanisms that explains the coexistence of competing species. However, much of the existing theory on this mechanism assumes complete host specificity, while empirical work suggests otherwise. Most studies till date have also restricted their scope to only victim dynamics, and hence cannot account for the possibility of the enemy going extinct as a result of complete specialization of enemy’s preference. Existing models have also primarily focused on linear functional responses of the enemies to their victims’ density. These assumptions are often unrealistic and can lead to erroneous conclusions regarding the potential of natural enemies to mediate victim species coexistence.

In our study, we constructed a model of two competing victim species and two enemy species. Our model allows us to change the degree of specialization, as well as the functional responses of enemy-victim interactions in order to investigate the effect on community stability and persistence. We used a mix of analytical calculations and numerical simulations to explore the behavior of the system over varying degrees of victim specialization, as well as for saturating, linear, and accelerating functional responses of the enemy to the victim.

Our results show that enemy feasibility is hindered by specialization. We found that enemies with linear functional response do not contribute to stable coexistence of otherwise equivalent competitors, and generate neutral cycles away from the equilibrium. The coexistence equilibrium is stable under accelerating functional response of the enemies, with higher specialization leading to faster convergence to the equilibrium. Saturating functional responses generate limit cycles that start off stable and eventually become chaotic until the limit cycles collapse as the enemies become more effective in depleting their victims. Our results suggest that generalist enemies may play a more important role in coexistence of their victims than is currently recognized. We recommend that future work on enemy-mediated coexistence explicitly considers the population dynamics of enemies, including how their functional responses may emerge from interactions in a spatial and behavioral context.