Interspecific variation in the defensive responses of brown frogs to alien predators

Alien species impact native amphibians through various direct effects, including predation, and pose a significant threat to naïve prey populations. In this study, we exposed tadpoles of three brown frog species (Rana dalmatina, Rana latastei, and Rana temporaria) to the olfactory cues of two alien predators, the pond slider (Trachemys scripta) and red swamp crayfish (Procambarus clarkii), and compared their responses to those induced by the aquatic larvae of a widespread native predator, the Southern hawker (Aeshna cyanea). We recorded two tadpole defensive behaviors, the proportion of time they were active and the number of freezing events. Both agile frog species, R. dalmatina and R. latastei, showed moderate responses to red swamp crayfish kairomones and strong responses to both odonate larvae and pond sliders. In contrast, the common frog (R. temporaria) displayed a less intense response to crayfish with respect to odonate larvae, and a negligible response to pond sliders. Long-lasting coexistence with either European pond turtles or pond sliders may explain the strength of agile frogs’ response toward the alien species; while, the historical range of the white-clawed crayfish (Austropotamobius pallipes) overlapped that of the common frog, enhancing the co-option of alien crayfish cues by this anuran species.