Abstract
It is widely assumed that spillover of alien parasites to native host species severely impacts naïve populations, ultimately conferring a competitive advantage to invading hosts that introduced them. Despite such host-switching events occurring in biological invasions, studies demonstrating the impact of alien macroparasites on native animal hosts are surprisingly few. In Europe, native red squirrels (Sciurus vulgaris) are replaced by introduced North American grey squirrels (S. carolinensis) mainly through resource competition, and, only in the United Kingdom and Ireland, by competition mediated by a viral disease. In Italy such disease is absent, but spillover of an introduced North American nematode (Strongyloides robustus) from grey to red squirrels is known to occur. Here, we used long-term (9 years) capture-mark-recapture and parasitological data of red squirrels in areas co-inhabited by grey squirrels in Northern Italy to investigate the impact of this alien helminth on naïve native squirrels’ body mass, local survival, and reproduction of females. We found no negative effect of the alien parasite on body mass or reproductive success, but intensity of infection by S. robustus reduced survival of both male and female squirrels. Significantly, survival of squirrels co-infected by their native nematode, Trypanoxyuris sciuri, was less affected by S. robustus, suggesting a protective effect of the native helminth against the new infection. Hence, we demonstrate that alien S. robustus spillover adds to the detrimental effects of resource competition and stress induced by grey squirrels, further reducing the fitness of the native species in the presence of the invasive competitor.
Highlights
Biological invasions are considered one of the major threats to wildlife conservation (Clavero and GarcıaBerthou 2005; Vilaet al. 2010; Simberloff 2011)
We investigated the effects of infection by alien S. robustus and native T. sciuri nematodes on body mass, local survival and reproduction of native red squirrels
It is widely assumed that spillover of an alien parasite from an invading to a closely-related native species will be severely detrimental to the new host, because the naıve species will represent a physiologically similar host, but lack any coevolved tolerance or resistance to the parasite (Prenter et al 2004; Strauss et al 2012; McIntire and Juliano 2021)
Summary
Biological invasions are considered one of the major threats to wildlife conservation (Clavero and GarcıaBerthou 2005; Vilaet al. 2010; Simberloff 2011). Once Invasive alien species (IAS) are introduced, they can threaten native species through several ecological processes, including competition, predation and hybridization (Largiader 2008; Doherty et al 2016; Mazzamuto et al 2017a). Invading hosts will likely introduce at least some parasite species from their native range and these alien parasites may spill-over to a susceptible native species (Power and Mitchell 2004; Telfer and Bown 2012). It is widely assumed that such shared parasites may mediate the competition between the two species: if the alien parasite has a greater impact on the naıve species than on the invader, the latter will gain a competitive advantage and the probability of a successful invasion will increase (disease-mediated invasions) (Strauss et al 2012; McIntire and Juliano 2021). Alien species may acquire local parasites, amplifying (spillback: Kelly et al 2009; Sherrard-Smith et al 2015) or diluting their circulation (dilution effect: Thieltges et al 2009; Telfer and Brown 2012)
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