A double edged sword: predation risk effects of a non-native species that acts as predator and prey

Non-native species are becoming increasingly pervasive worldwide. In an effort to help predict the impact of non-native species, they are often categorized into a single trophic level. However, many species can be both predator and prey to resident species, so generalizations made by assignments to either category alone may not accurately predict the effects of the non-native species. Here, we explored how a non-native species to New England rocky shores, the crab Hemigrapsus sanguineus that can be prey for the resident predator (Carcinus maenas) and also predator to the native prey (Nucella lapillus), impacts the risk responses of Nucella. We found that when Hemigrapsus was added to the food chain, Nucella spent similar time in refuge overall and consumed a similar amount of food but changed the specific refuge they used and grew less compared to exposure of Carcinus alone. When Nucella were exposed to Hemigrapsus alone, they altered the specific refuge they used but reduced their foraging and had similar growth compared to Carcinus alone. Exposure to each predator individually induced a fear response in Nucella, but these effects were non-additive when Nucella were exposed to both predators. We suggest this is due to a reduction in the predatory actions of both crabs on Nucella, given that Hemigrapsus could also have served as prey to Carcinus here. Our results demonstrate that only examining the dyadic interactions of non-native and resident species may not provide a good understanding of the actual impact of non-native species on resident food webs.