Predatory behaviour of an invasive amphipod in response to varying conspecific densities under higher-order predation risk

Behavioural responses of invasive animals to biotic interactions can inform predictions of their consumptive impacts; however, such biotic contexts are often overlooked. Here, we assessed the interacting effect of conspecific and higher-order predation risk on the per capita consumption and behaviours of the invasive freshwater amphipod Gammarus pulex, using field microcosm and video-recorded lab experiments in Northern Ireland. Gammarus pulex exhibited higher per capita consumption in the presence of conspecifics, owing to reduced handling time of prey, regardless of fish presence and despite reduced swimming time and increased time spent physically interacting with each other. Consumption was lower in the presence of fish in the field, and handling time decreased with greater amphipod densities in the presence of fish cue in the lab. Our results show that impacts of G. pulex are independently influenced by conspecifics and predation risk, whereas handling time revealed an interacting effect of conspecific density and predation risk. Further assessments of the responses of invasive animals to biotic interactions could help explain variability in their impacts at local spatial scales.