Are Frog Populations at Risk? Regional PVA of the Pelophylax esculentus complex in fragmented habitats of South Banat shows low extinction risk

Abstract

This study evaluates the spatial structure and viability of the Pelophylax esculentus complex metapopulation system in South Banat, Serbia. A habitat suitability model, based on 10 biotic and abiotic variables, was created to identify source and sink regions in the landscape, with a suitability threshold of HS=0.8. The dispersal function was modelled with an average distance of 0.7 km to reflect potential species dispersal within the region. The carrying capacity (K) of suitable regions was calculated using average habitat suitability and the number of suitable cells (K=AHS×NOC). A ceiling growth model was applied, assuming no density dependence below K. The population structure model identified 13 viable regions. We used RAMAS MetaPop to assess extinction risk and minimum viable population size (MVP). The demographic model was simulated for 25% stochasticity to reflect anthropogenic pressure on all demographic parameters, including dispersal. The initial population size (K/10) was distributed among age classes 2–6. Simulations were run over 100 years (25–30 generations) with 1,000 iterations. Results showed strong spatial structure and overall stability, with a slight decline (~5%) over time. The local extinction threshold of 10% indicated extinction in 6 of 13 demes in the defined time frame, while the remaining 7 demes exhibited lower extinction risk. Sensitivity analysis showed that increased egg and larval mortality, along with reduced fecundity in reproductive age classes, had the greatest impact on extinction risk. In contrast, a reduction in initial population size had minimal effect, while dispersal stabilised the dynamics. The estimated minimum size of the metapopulation was ~3,095 individuals, with an adult MVP of 250–300. Continued decline and demographic shifts—particularly in early life stages—could increase the extinction risk to 10%, meeting IUCN Red List criterion E for classification as Vulnerable in regional assessments.