Bioacoustics survey revealed the occurrence of alien green frogs across an urban gradient

The introduction and spread of alien amphibians pose significant threats to native biodiversity, particularly within vulnerable ecosystems. In Southern Europe, alien green frogs Pelophylax ridibundus represent a major conservation concern due to their ecological plasticity, competitive behaviour, and potential for genetic pollution. In this study, we employed bioacoustic surveys to assess the distribution and habitat associations of P. ridibundus across an urban gradient in the metropolitan area of Florence, Central Italy, where the species has been previously genetically identified. A total of 47 wetland sites were surveyed monthly during the breeding seasons of 2021–2022. We recorded spontaneous frog calls during nighttime surveys to model species occupancy using single-season occupancy models. Our results revealed that P. ridibundus occupied approximately 47% of surveyed wetlands. Occupancy was significantly influenced by the combination of aquatic vegetation cover and proximity to human-modified environments, indicating that the species may benefit from both habitat complexity and anthropogenic disturbance. Detection probability was relatively high (p = 0.66), and we also estimated that four nocturnal survey visits were sufficient to confidently confirm the species presence at a given site. These findings showed the high adaptability of P. ridibundus to urban environments, and suggest that suburban and urban wetlands with dense aquatic vegetation are particularly susceptible to invasion. Given the potential impacts of P. ridibundus on native amphibian communities, including competition and hybridization, early detection and regular monitoring are essential. Bioacoustic monitoring emerges as a cost-effective and reliable tool to monitor invasive anurans in human-dominated landscapes, supporting well-addressed conservation efforts to preserve urban biodiversity.