Understanding the effects of invasive alien species (IAS) on native host-parasite relationships is of importance for enhancing ecological theory and IAS management. When IAS and their parasite(s) invade a guild, the effects of interspecific resource competition and/or parasite-mediated competition can alter existing native host-parasite relationships and the dependent biological traits such as native species' behaviour. We used a natural experiment of populations of native red squirrels Sciurus vulgaris that were colonized by the alien grey squirrel Sciurus carolinensis, comparing repeated measurements of red squirrel parasite infection and personality with those taken in sites where only the native species occurred. We explored two alternative hypotheses: (a) individual differences in personality traits (activity and/or sociability) of native red squirrel positively affect the probability of macroparasite spillover and thus the likelihood to acquire the alien's parasitic helminth Strongyloides robustus; (b) the combined effects of grey squirrel presence and parasite infection result in a reduction of costly personality traits (activity and/or exploration). Using data from 323 arena tests across three experimental (native species and IAS) and three control sites (only native species), we found negative correlations between native species' activity and infection with S. robustus in the sites invaded by the alien species. Activity was also negatively correlated with infection by its native helminth Trypanoxyuris sciuri but only when grey squirrels were present, while in the red-only sites there was no relationship of T. sciuri infection with any of the personality traits. Moreover, individuals that acquired S. robustus during the study reduced their activity after infection, while this was not the case for animals that remained uninfected. Our results show that parasite-mediated competition is costly, reducing activity in individuals of the native species, and altering the native host-native parasite relationships.

中文翻译：

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外来寄生虫的溢出减少了本地宿主物种中代价高昂的行为的表达

了解外来入侵物种 (IAS) 对本地宿主-寄生虫关系的影响对于加强生态理论和 IAS 管理具有重要意义。当 IAS 和它们的寄生虫入侵一个公会时，种间资源竞争和/或寄生虫介导的竞争的影响可以改变现有的本地宿主-寄生虫关系和依赖的生物特征，如本地物种的行为。我们对被外来灰松鼠 Sciurus carolinensis 定殖的本地红松鼠 Sciurus vulgaris 种群进行了一项自然实验，将红松鼠寄生虫感染和个性的重复测量与仅在本地物种发生的地点进行的测量进行了比较。我们探索了两种替代假设：(a) 本地红松鼠个性特征（活动和/或社交能力）的个体差异对宏观寄生虫外溢的可能性产生积极影响，从而对获得外来寄生蠕虫类圆线虫的可能性产生积极影响；(b) 灰松鼠的存在和寄生虫感染的综合影响导致代价高昂的人格特征（活动和/或探索）减少。使用来自三个实验（本地物种和 IAS）和三个控制地点（仅本地物种）的 323 个竞技场测试的数据，我们发现本地物种的活动与外来物种入侵的地点中的粗壮沙门氏菌感染之间存在负相关。活动也与其本地蠕虫锥虫的感染呈负相关，但仅当存在灰松鼠时，而在只有红色的站点中，T. sciuri 感染与任何个性特征都没有关系。此外，在研究期间获得粗壮沙门氏菌的个体在感染后减少了他们的活动，而未受感染的动物则并非如此。我们的结果表明，寄生虫介导的竞争代价高昂，会减少本地物种个体的活动，并改变本地宿主与本地寄生虫的关系。