Long-Term Increase in Hibernating Bats in Swedish Mines — Effect of Global Warming?

Abstract

We present the result of bat winter censuses in three old mines in southern Sweden from 1980 until present (2017). The Taberg and Kleva mines, each with about 1.5 km of accessible passages, have winter populations of 517 and 132 bats, respectively (maximum counts) belonging to six species, the highest numbers known in underground sites in Sweden. Adelfors is less extensive and has fewer individuals (maximum 22). The two former sites were protected and gated in the 1980′s while the third site still has no formal protection and is subject to disturbance. Generally Daubenton's bat Myotis daubentonii and the brown long-eared bat Plecotus auritus are common species and the numbers are stable. The whiskered and Brandt's bats M. mystacinus/brandtii and Natterer's bat M. nattereri have increased significantly, while the northern bat Eptesicus nilssonii, which is relatively rare in mines generally, has shown a slight but significant decline. At the species level the population trends conform well to those of the respective species in continental Europe and the British Isles. This suggests that there is a common factor behind the population changes across Europe. Although our data are very limited, the results question some previous explanations for the observed trends, but are in line with theoretical predictions based on global warming scenarios.