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

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. 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.

共享天敌可能引发或调节物种间的竞争互作。外来物种引入所引发的生态失衡，揭示了这类间接互作的结果往往极具戏剧性。然而，尽管物种身处复杂的互作网络之中，当前天敌介导的竞争研究大多仅考量单一天敌的影响。本研究证实，英国本土的欧亚红松鼠与入侵性灰松鼠这两种陆生哺乳动物，原本仅通过资源竞争和疾病介导的表观竞争（apparent competition）产生关联，如今又因一种共享的本土天敌——历经历史迫害后种群正逐步恢复的欧洲松貂（European pine marten）——形成了第二种天敌介导的互作关系。研究结合空间捕获-重捕（spatial capture recapture）技术估算松貂种群密度，并结合松鼠的位点占用率（site occupancy）数据，发现捕食暴露的影响具有高度不对称性：入侵灰松鼠的栖息地占用率会因松貂的存在而受到显著的负面影响。与之相反，松貂的捕食压力则对红松鼠种群产生间接的正向作用。因此，松貂的捕食作用逆转了此前被广泛记录的红、灰松鼠间资源竞争与表观竞争的既定结果。