The Ecological Impact of the Invasive Freshwater Killer Shrimp Dikerogammarus villosus on Native UK Amphibians

Invasive alien species are one of several drivers implicated in the ever-increasing rate of global amphibian declines, contributing through intense predation of early life stages, but also through the spread of pathogenic agents. In this thesis I investigate the potential for the invasive freshwater amphipod crustacean Dikerogammarus villosus to adversely affect UK amphibians. First, I examine the possibility for D. villosus to impact upon amphibians directly as a predator of early life stages. Using functional response (FR) analyses, I quantify differences in per capita predatory behaviours between invasive D. villosus and the native amphipod Gammarus pulex towards the embryos and larvae of several UK anurans. Using the relative impact potential metric (RIP), I supplement FR measurements with native and invasive amphipod abundance data, recorded during the surveillance of uninvaded and invaded UK field sites, and from previously published literature. Compared to native G. pulex, invasive D. villosus demonstrated an exceptionally stronger predicted predatory impact, as a result of differential body size and population abundance. Second, I examine how interactions between conspecific invaders may alter the predicted ecological impact of D. villosus towards larval anurans. Per capita predation decreased as a function of increasing predator abundance, caused by strong conspecific aggregation and the emergence of mutual interference between amphipod predators. Non-aggressive interference between interacting conspecifics is predicted to cause a reduction in predatory pressures by D. villosus towards larval amphibians. Third, I explore how predatory pressures imposed by D. villosus towards larval UK anurans may change when alternative prey species are present, either in equal quantities or at varying provisioning ratios. Dikerogammarus villosus exhibited significantly greater selectivity towards alternative prey rather than anuran larvae, regardless of relative abundance. In the absence of frequency-dependent prey-switching behaviours, larval amphibians are expected to experience a reduced risk of predation by D. villosus when more preferred prey types are available. Fourth, I address the possibility for D. villosus to indirectly affect native amphibian populations, as a potential carrier of generalist amphibian pathogens. I verify that D. villosus can acquire infections, localised to the exterior surface of the cuticle only. Infections did not cause any discernible pathologies in amphipod hosts, but instead appeared to be transient, with some amphipod hosts clearing infections by shedding the infected cuticle. I finish by highlighting that, although these findings suggest the potential for invasive D. villosus to directly and/or indirectly affect amphibian populations, evidence of previous field-based interactions between larval amphibians and invasive amphipods is limited, and as such is an area that warrants further investigation.