Ecological impacts of an invasive predator are mediated by the reproductive cycle

Predicting the ecological impacts of invasive species is impeded by context-dependencies that can mediate the strength of trophic interactions. Reproductive status is a pervasive context across trophic and taxonomic groups, yet has mostly been neglected in studies of invader impact. The present study examines the influence of the reproductive cycle on predatory impacts of a known damaging invasive freshwater crustacean, Gammarus pulex (Amphipoda), using a comparative functional response approach. Across females that were non-ovigerous, or had immature- or mature-stage embryonic broods, all G. pulex exhibited potentially prey destabilising Type II functional responses towards chironomid larvae. Attack rates were highest by immature embryonic brood stages, and significantly lower by mature embryonic brood stages. Conversely, handling times were consistently lower, and hence maximum feeding rates higher, where broods were present as compared to absent. These predatory patterns may reflect changing resource demands for progeny development across the reproductive cycle, the influence of brood bulk, female moult stage and/or motivational aspects of feeding, such as avoidance of filial cannibalism. Accordingly, many aspects of reproduction may alter the intensity of predatory impacts by invasive species. Future studies considering invader impacts should consider the influence of sex and reproductive status to more holistically quantify and predict population-level invader impacts.