Invading parasites: spillover of an alien nematode reduces survival in a native species

Abstract It is widely assumed that spillover of alien parasites to native host species severely impacts naïve populations, ultimately conferring a competitive advantage invading hosts introduced them. Despite such host-switching events occurring in biological invasions, studies demonstrating the impact macroparasites on animal are surprisingly few. In Europe, red squirrels ( Sciurus vulgaris) replaced by North American grey S. carolinensis ) mainly through resource competition, and, only United Kingdom and Ireland, competition mediated viral disease. Italy disease absent, but an nematode Strongyloides robustus from known occur. Here, we used long-term (9 years) capture-mark-recapture parasitological data areas co-inhabited Northern investigate this helminth squirrels’ body mass, local survival, reproduction females. We found no negative effect parasite mass or reproductive success, intensity infection reduced survival both male female squirrels. Significantly, co-infected their nematode, Trypanoxyuris sciuri , was less affected suggesting protective against new infection. Hence, demonstrate adds detrimental effects stress induced squirrels, further reducing fitness presence invasive competitor.