Indirect effects of primary prey population dynamics on alternative prey

TL;DR

This paper develops a theoretical framework to understand how primary prey population dynamics, including mean abundance and variability, indirectly affect alternative prey through predator interactions, extending classic apparent competition theory.

Contribution

It introduces a generalized theory of predator-prey interactions that accounts for prey variability and predator responses, highlighting asymmetries and the importance of predator dynamics.

Findings

Predator effects on alternative prey decrease with higher primary prey abundance.

Increased primary prey variability generally increases impact on alternative prey.

Strong predator aggregative responses can reverse typical effects.

Abstract

We develop a theory of generalist predation showing how alternative prey species are affected by changes in both mean abundance and variability (coefficient of variation) of their predator's primary prey. The theory is motivated by the indirect effects of cyclic rodent populations on ground-breeding birds, and developed through progressive analytic simplifications of an empirically-based model. It applies nonetheless to many other systems where primary prey have fast life-histories and can become locally superabundant, which facilitates impact on alternative prey species. In contrast to classic apparent competition theory based on symmetric interactions, our results suggest that predator effects on alternative prey should generally decrease with mean primary prey abundance, and increase with primary prey variability (low to high CV) - unless predators have strong aggregative responses, in which case these results can be reversed. Approximations of models including predator dynamics (general numerical response with possible delays) confirm these results but further suggest that negative temporal correlation between predator and primary prey is harmful to alternative prey. We find in general that predator numerical responses are crucial to predict the response of ecosystems to changes in key prey species exhibiting outbreaks, and extend the apparent competition/mutualism theory to asymmetric interactions.