Is the Common Frog, Rana temporaria Linnaeus, 1758, becoming uncommon? Evidence from 23 years of monitoring brown frogs in the Pilis-Visegrád Hills, Hungary

Regular and annual monitoring of amphibians was initiated in Hungary in 2001, including within the Pilis-Visegrád Hills, one of the driest mountain regions in northern Hungary. The area is largely covered by forest and comprises ca. 140 ponds. These ponds are relatively small, many are temporary, and water levels can decrease significantly even in those that persist throughout the year. The 23-year-old monitoring program was initiated in one pond and after some conceptual changes it is currently being undertaken in 18 ponds. The distribution of Rana temporaria in Hungary is limited to regions with a higher and cooler microclimate, and it has long been observed in the Pilis-Visegrád Hills. In the first half of the monitoring period, we observed the species in most of the sampled ponds, although we consistently detected a lower number of eggs than those of Rana dalmatina, which utilises the same ponds for breeding in the same period as R. temporaria. Rana temporaria egg clutches have experienced a significant decline from the sampled ponds, with only one pond harbouring the species in 2023. The primary cause of this range reduction is quite likely the desiccation of the Pilis-Visegrád Hills, caused by the gradual but continuous increase in average temperature and decrease and more erratic precipitation events. A second factor may be the disturbance caused by wild game in the ponds, which decrease both water quantity and quality for breeding amphibians. We also outline other potential, and unlikely, factors which have been implicated in declines elsewhere.