Fish Introductions Reshape Antipredator Sensitivity in Rana temporaria Tadpoles Across Alpine Lakes

Biological invasions are a major driver of biodiversity loss, particularly in isolated ecosystems such as alpine lakes. The introduction of non-native fish into historically fishless systems can profoundly alter predator–prey interactions, potentially modifying prey perception of predation risk. In this study, we investigated whether the presence of introduced fish affects background risk and antipredator sensitivity in larvae of the common frog (Rana temporaria). We conducted behavioral assays on tadpoles collected from six alpine lakes in the Western Italian Alps, representing a gradient of predation contexts: fishless lakes, fishless lakes located near waterbodies with fish, and lakes currently hosting introduced fish. Tadpoles were exposed to increasing concentrations of chemical cues released by a native predator (dragonfly larvae), and their freezing response was quantified as an indicator of perceived predation risk. Tadpole responses followed a clear concentration-response pattern, with increasing freezing behavior at higher cue concentrations. However, populations associated with fish presence, either locally or in the surrounding landscape, exhibited higher sensitivity to predator cues, responding significantly even at low concentrations compared with populations from isolated fishless lakes. These results suggest that the presence of introduced fish may modify sensitivity to predation risk in amphibian larvae. Moreover, the responses observed in populations from fishless lakes located in areas with fish indicate that these effects may extend beyond directly invaded habitats. Overall, our findings highlight that biological invasions can influence not only direct predation risk but also prey behavioral responses and non-consumptive effects, with potential consequences at both population and landscape scales.