Moonlight rather than moon phase influences activity and habitat use in an invasive amphibian predator and its native amphibian prey

Predation affects population dynamics via both direct mortality and behavioural responses to the threat of death. Environmental factors are known to influence the activity of both predator and prey in amphibians by interacting with their physiological constraints. For instance, lunar cues (moonlight and moon cycle) influence amphibian predation risk and foraging efficiency. However, little information is available on how the moonlight affects activity and habitat use. Here, we investigated whether lunar light flux, environmental variables, and lunar phases influence abundance and habitat use in a predator-prey system consisting of an invasive predator (i.e., the American bullfrog Lithobates catesbeianus) and a native prey (i.e., the green frogs Pelophylax spp.). In accordance with both the “visual acuity” and “predation risk” hypotheses, the activity of the invasive predator was enhanced in moonlit nights, especially in open habitats, whereas the activity of the native prey species seemed to be not affected by the intensity of moonlight. Contrary to the “habitat-mediated predation risk hypothesis”, no suppressive effect of moonlight in open habitats was found for L. catesbeianus, this possibly evidencing the lack of perceived predation risk by this alien species. Moon cycle had no effect on American bullfrog abundance and weakly affected the activity of green frogs especially in open habitats between the first quarter and the full moon. Our study shows how moonlight, lunar phases and environmental parameters influence the behaviour and ecology of the American bullfrog and one of its native prey. Since indirect effects have the potential to moderate or amplify the impacts of invasive species on native populations, our findings could help to understand the spreading of American bullfrogs and to possibly mitigate their impacts on native fauna through habitat restoration.