Predicting effects of predator diversity on shared prey

Background/Question/Methods

Despite general appreciation for the importance of predators in ecosystems and recognition of global changes in predator species richness, we lack a general framework for predicting how compositional changes in predator communities influence prey populations.  Current statistical models for predicting the relationship between predator diversity and predator effects on prey often fails to predict even the combined effects of two predators on a single shared prey species. These models often to conflate nonlinear effects of prey size and density with context-dependent interactions in a way that makes emergent dynamics fundamentally unpredictable. In this study we develop a new framework for predicting the effects of multiple predators that incorporates density-dependent predation, prey depletion, prey growth and size-dependent predation. Using a combination of analytical and simulation approaches we explore multiple predator effects across model parameter space by varying size- and density-dependence of predation to understand their combined effects.  

Results/Conclusions

We show how non-linearity in the size- and density dependence of growth and predation risk interact and lead to deviations from current model predictions that are often assumed to reflect non-independence among predators (i.e., emergent multiple predator effects).  Because prey often grow rapidly during periods of predator exposure, and vulnerability to predators often changes nonlinearly with prey size and density our results suggest that the currently held consensus that context-dependence is nearly ubiquitous in multiple predator interactions needs to be reevaluated.  Our new general model provides a means to partition multiple predator effects into effects arising as a result of interaction modifications among predators and prey, and effects that arise as a result of nonlinearities in the functional forms of the predator and prey interactions. Further, it enables us to predict predator diversity effects through time and prey ontogeny. We argue this type of approach is needed to help move the study of the effects of predator diversity from a more qualitative descriptive approach toward one focusing on quantitative prediction.