The enemy of my enemy is my friend: Native pine marten recovery reverses the decline of the red squirrel by suppressing grey squirrel populations

Emma Sheehy, Chris Sutherland, Catherine O’Reilly, Xavier Lambin (Corresponding Author)

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Shared enemies may instigate or modify competitive interactions between species. The disequilibrium 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. In 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.
Original languageEnglish
Article number20172603
JournalProceedings of the Royal Society of London. B, Biological Sciences
Issue number1874
Early online date7 Mar 2018
Publication statusPublished - 14 Mar 2018



  • occupancy modelling
  • spatial capture recapture
  • apparent competition
  • predator mediated competition
  • pest-regulating ecosystem service
  • species interactions

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