New species, new story: The impact of invasive non-host predators on host-trematode interactions

Predation is considered one of the key drivers shaping the relationships between animals. There is increasing evidence that non-host aquatic taxa use free-living parasite stages as prey, including motile cercariae of digenean trematodes. However, species-specific predator differences in cercarial consumption are poorly understood, especially when invasive consumers are involved. We used experimental approaches to investigate the possible predator-induced reduction in trematode-host encounters in freshwater benthic communities. Four gammarid species, two native European (Gammarus jazdzewskii and G. pulex) and two invasive Ponto-Caspian (Dikerogammarus villosus and Pontogammarus robustoides), were selected as predators of echinostomatid cercariae (Echinoparyphium aconiatum). We hypothesised that (i) all predators will prey on cercariae, but the amount of prey consumed will vary, the invaders will be more voracious than native species, and (ii) the effect of predator-induced dilution of cercariae will depend on the environmental condition, i.e., the presence of the natural target host (the planorbid snail Planorbarius corneus) and alternative food resource; the exposure of predators to these two basic environmental factors will reduce their consumption rates of cercariae. The extent of the dilution effect caused by gammarids was highly predator species-dependent and also varied depending on experimental conditions. Overall, all gammarids preyed on cercariae and reduced infection levels in the target snail host, but the invaders were more efficient than the native species. Furthermore, decreased consumption rates of cercarial prey were observed in the presence of both the target snail hosts and alternative food resources, except for invasive P. robustoides consuming more cercariae when a snail host was present. The observed differences among predators in the consumption of cercariae may be a result of complex interaction traits of parasites, predators and target hosts, such as different predatory behaviour of invasive and native amphipods, change in parasite behaviour in response to the presence/chemical signals of the target host and a predator-induced "ecology of fear" interfering with the dilution effect of parasites. We provided evidence that gammarids can cause a significant reduction in parasite-host encounters. The high consumption rates, especially in the presence of easily available alternative food resources, emphasise the importance of free-living cercariae as an energy resource in aquatic food webs. The observed predator differences in the consumption of cercariae are further evidence of the higher competitive ability of invasive species. Simultaneously, our results indicate that the appearance of new predators in the environment may interfere with established host-parasite interactions.