Don’t stop it now: functional and structural habitat connectivity assessment suggests further expansion in Southern Europe for the Eurasian beaver

Abstract

Context After being close to extinction, the Eurasian beaver (Castor fiber) is now recovering throughout Europe. The Eurasian beaver is a keystone species, and an ecosystem engineer, capable of modifying ecosystems at multiple levels. In the last years, the species reappeared in Italy due to natural recolonization and illegal reintroductions, while in Spain, after being confined to the Ebro basin, the beaver has been recently reported at the border with Portugal. Objectives In this study, we evaluated both functional and structural connectivity for the Eurasian beaver in Italy and in the Iberian Peninsulas, to deepen the understanding of its expansion process, and to prioritize the barriers that occur in the areas where the species is reported, and that could limit this expansion. Methods We generated resistance layers for each study area, based on key variables for the target species. Circuit theory was applied to evaluate functional connectivity, summarizing it at the basin and protected areas (PAs) level. We applied different fluvial connectivity indices to characterize, at-reach-scale, structural connectivity, and identify barriers within each hydrographic network. Results In both study areas, there are several corridors mainly linked to rivers with high tree cover that occur in flat areas, like the valleys in the Apennines and the Cantabrian mountain systems. Despite the distances and the extant barriers, the occurrences in Central Italy and Spain are respectively well-connected. In general, in each hydrographic network, only a few barriers impede connectivity, with the Iberian Peninsula more impacted, especially in the Douro basin. In the Ebro basin, several barriers contribute to fragmentation, especially in the northern area considering the Population Connectivity Indices; however, high connectivity is found within the protected areas. Conclusions The recent return of the Eurasian beaver in Italy, and its recent expansion in the Iberian Peninsula, could continue in the near future, considering both the fair habitat connectivity and the adaptability of the species. Although both areas are impacted by man-made dams, only a specific set possibly limits the Eurasian beavers’ expansion. In general, the current PAs arrangement does not protect the areas important for the beaver connectivity, with some of them impacted by man-made dams.