Apparent Competition or Apparent Mutualism? Shared Predation When Populations Cycle

Abstract

We use simple mathematical models to explore the indirect interactions between two prey species that share a predator when all three species undergo population cycles. The results are compared to analogous findings for systems that reach a stable equilibrium point. It is common for removal of one prey from a cycling system to result in a decrease in the mean density of the remaining prey species, contrary to the usual logic of apparent competition. Even when apparent competition between prey exists, its magnitude is usually reduced by population cycles. This effect occurs when the predator has a concave-down relationship between prey abundance and its own per capita growth rate. Such relationships can occur because of a saturating functional or numerical response. We investigate how prey density dependence and the shape of the predator’s functional and numerical responses affect the sign and magnitude of this indirect interaction between prey species. There may also be (+, −) interactions between prey that differ significantly in their susceptibility to the common predator.