Is the exotic red swamp crayfish (Procambarus clarkii) a current threat for the Mediterranean tree frog (Hyla meridionalis) in the Camargue (Southern France)?

The non-indigenous red swamp crayfish (Procambarus clarkii) has been shown to be a threat for amphibian conservation. Many amphibian species breed in temporary ponds to diminish predation risk as such ecosystems are free of large predators. However P. clarkii, occurring as an invasive species in the Camargue delta, can readily disperse on the ground and thus colonize isolated ponds. We studied the current impact of the exotic crayfish on the reproductive success of the Mediterranean tree frog (Hyla meridionalis). In a mesocosm experiment, we tested the effect of two crayfish densities (1 and 3 crayfish/m²) on tadpole abundance. We also tested in a field experiment, within a temporary pond, the crayfish’s predation on the tree frog’s eggs. Finally, we developed site occupancy models using data from 20 ponds to assess the effect of crayfish abundance on tadpole abundance. Neither the experiments, nor the site occupancy models showed a negative impact of the current crayfish abundance on the tree frog populations breeding in ponds. We found that recorded crayfish densities were lower than in other areas where crayfish has impacted amphibian populations, but we hypothesize that current crayfish abundance in the area may increase in the future, thus impacting tree frog populations.