Age structure and growth in populations of an endemic frog species Rana italica along a slight environmental/climate gradient

Even if amphibian extinction risks are high, their population dynamics are still poorly investigated. This is true for several Italian amphibian populations whose declines have been reported in a few studies focusing on a limited sample. This is the case of Rana italica, a species endemic to Italy whose ecology has been insufficiently explored so far, and the existing knowledge is based more on general observations than on specific studies. The main goal of this study was to contribute to its conservation status through the analysis of demographic traits in six populations along a slight environmental/climate gradient combining skeletochronology and capture-mark-recapture methods. Overall, 598 and 289 individuals were marked and aged, respectively. Population estimates (149–1523 individuals) suggest that most sites supported demes in good demographic conditions. According to Bergmann’s rule, a positive body size-elevation relationship was found, this being possibly related to differences in age and growth observed at different elevations and/or the action of local factors. Adult age ranged 2–6 years with individuals from high elevations being older and slower-growing confirming that colder climate, shorter breeding season, and lower predation risk result in a longer lifespan. Growth rates, moderate to high (0.365–0.515) and constant until four years of age, were affected by elevation, air temperature, and tree cover density. Amphibians, due to their limited thermoregulation abilities, confirm to be sensitive to such environmental conditions, influencing their physiological activities and, consequently, growth. In our study, the studied life-history traits varied even across a slight environmental/climatic gradient highlighting the need to monitor species on a landscape scale to obtain an exhaustive framework of their ecological needs and conservation status. The present study will be useful for future conservation studies at both local and regional scale of temperate amphibians, especially those experiencing declines and occurring in high-elevation, pristine habitats.