Predicting impacts of a novel predator on prey communities using size-specific functional response models

Background/Question/Methods

Changes in species diversity via invasions and subsequent extinctions are increasingly widespread with dramatic ecological and socio-economic consequences. Therefore methodologies that enable ecologists to forecast the potential impacts of an invasive species are urgently needed. One common characteristic of successful invaders is that they are more efficient consumers than are native species.  Invasive species may be more efficient consumers as a result of reduced biotic resistance by native species that lack co-evolutionary history or because the invasive is released from their native consumers. 

We develop a method that uses key relationships that define consumer-resource interactions – the functional response—to predict the ecological impacts of an invasive consumer species. Specifically, we develop a null model for the effects of an invasive species into an assemblage based on an aggregate functional response derived from the functional responses for individual taxa.  Our approach allows us to parse out emergent properties that may propagate from complex interactions from those that stem from nonlinear but fundamentally predictable processes (such as size dependent foraging).

Results/Conclusions

The effects of invasive consumer species often stem directly from their per capita effects on native prey. We present a model that explicitly incorporates the effects of prey depletion and prey growth dynamics on per capita consumption rates over time. Specifically, we assume Holling type II functional responses with attack rates and handling times modeled as functions of prey size. Our approach differs from classical consumer-resource models because we explicitly incorporate growth dynamics, which allows us to capture nonlinear changes in the per capita effects of consumers through time as prey grow or are consumed. We show that neglecting such nonlinearities the per capita effects of consumers on prey, can bias our expectations of consumer impacts on native communities over time. We also show how prey community size structures and how variation in growth rates within and among species affects predicted impacts of an invading (novel or native) consumer. Species introductions and biodiversity loss are altering community assemblages worldwide. By using models that explicitly link the population dynamics of consumers and their prey, we provide a mechanistic framework for understanding the effects of consumer introductions (or extinctions) on communities and ecosystems.