Hibernating bats in the Schenkgroeve, an artificial limestone cave in south Limburg, the Netherlands
Between 1944 and 2010, sixty-one yearly winter bat surveys were carried out in the Schenkgroeve, a man-made, now abandoned, underground limestone excavation in Meerssen, in the south of the Province of Limburg, the Netherlands. This cave is divided into two separate parts, each with a different microclimate. The front part houses the majority of hibernating bats and is the focus of this paper. Over a period of 67 years, the numbers of hibernating bats of ten different species have changed considerably, although some general trends can be recognised. Because the census has had different objectives over the years, our evaluation of the census data is separated into three time periods: 1944-1959 (period A), 1960-1979 (period B) and 1980-2010 (period C). In period A bat banding was the main goal and this and other human disturbance led to a decline in numbers of lesser horseshoe bat (Rhinolophus hipposideros), Geoffroy’s bat (Myotis emarginatus) and greater mouse-eared bat (Myotis myotis), while the number of other species such as Daubenton’s bat (Myotis daubentonii), pond bat (Myotis dasycneme), whiskered/Brandt’s bat (Myotis mystacinus/brandtii), Natterer’s bat (Myotis nattereri) and brown/grey long-eared bat (Plecotus auritus/austriacus) remained more or less constant. In this period an average number of 70.3 bats per year were found, although between the years 1954 and 1959 the average was only 50.7 bats. In period B, when the main goal was to study the hibernation position of the bats in relation to climatic factors, the mean number of observed hibernating bats stayed at a low level of 62.8 bats per year. At the end of this period only Daubenton’s bat showed a slight increase. In the beginning of 1980 an iron gate was installed. The number of Natterer’s bats and Geoffroy’s bats and, to a lesser extent, Daubenton’s bats increased, while other species such as whiskered/Brandt’s bats, pond bats and long-eared bats showed no significant changes in number. The average annual number of hibernating bats during period C increased to 198.0 bats. Between 1980 and 2010, the index of the number of bats hibernating in the Schenkgroeve increased almost seven fold. We discuss possible factors that may have influenced changes in the numbers of different species over the years. The reasons for the decline in bat numbers in periods A and B include the excessive banding of bats in period A and the effects of mushroom culture. Both are generally thought to have negatively affected bat numbers in the Schenkgroeve and other caves in Limburg. We can only speculate about the causes of the spectacular positive trends in more recent years, especially for Geoffroy’s and Natterer’s bat in period C, a phenomenon also observed in other hibernacula in Limburg. We recommend continuation of annually monitoring the bats hibernating in the Schenkgroeve, which is one of the most important hibernacula in Limburg and the Netherlands.