Abstract
Shared enemies may instigate or modify competitive interactions between species. The dis-equilibrium caused by non-native species introductions has revealed that the outcome of such indirect interactions can often be dramatic. However, studies of enemy-mediated competition mostly consider the impact of a single enemy, despite species being embedded in complex networks of interactions. Here, we demonstrate that native red and invasive grey squirrels in Britain, two terrestrial species linked by resource and disease-mediated apparent competition, are also now linked by a second enemy-mediated relationship involving a shared native predator recovering from historical persecution, the European pine marten. Through combining spatial capture–recapture techniques to estimate pine marten density, and squirrel site-occupancy data, we find that the impact of exposure to predation is highly asymmetrical, with non-native grey squirrel occupancy strongly negatively affected by exposure to pine martens. By contrast, exposure to pine marten predation has an indirect positive effect on red squirrel populations. Pine marten predation thus reverses the well-documented outcome of resource and apparent competition between red and grey squirrels.
Highlights
Understanding the mechanisms through which species interact and the consequences of perturbations to those interactions is a fundamental goal of ecology
We modelled betweenregion variation in detection using a site level categorical variable, REGION
Central to the design was the stratified sampling in three regions, each with a contrasting time since recolonization by pine martens, and differing lengths of time for which squirrel populations have been exposed to pine marten predation
Summary
Understanding the mechanisms through which species interact and the consequences of perturbations to those interactions is a fundamental goal of ecology. Two-species interactions may be mediated, in part or wholly, by the presence of a third species, as in predatoror pathogen-mediated apparent competition [1]. The potential outcomes of such indirect interactions include complete competitor exclusion and fugitive coexistence where inferior competitors thrive temporarily in the absence of a shared enemy [2]. Which outcomes emerge depends on the nature of trophic interactions, including how the natural enemy affects, and responds to, the competing prey/host species, and on the ability of prey to occupy refuges that are temporarily or permanently unoccupied by their enemies. The introduction of non-native species has provided some dramatic examples of indirect interactions leading to species extirpation. Pathogen-mediated apparent competition between invasive grey squirrels, Sciurus carolinensis, and native red squirrels, Sciurus vulgaris, in Britain where the process of replacement via exploitative competition is vastly expedited in
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