Urban sprawl and light pollution disrupt commuting corridors of urban-roosting bats

Artificial light at night poses a serious threat to wildlife, particularly to nocturnal species. Bats that roost in urban areas during the day but forage in surrounding natural habitats at night face the dilemma of commuting through artificially lit environments on their way to dark foraging grounds. Such illumination may force bats to take energetically costly detours, threatening colonies by disconnecting them from areas of high prey density. Here, we studied commuting greater mouse-eared bats (Myotis myotis), a species that in central Europe forms maternity colonies almost exclusively in buildings. Using miniaturised GPS loggers, we tracked the movements of female bats at high spatial and temporal resolution, and compared the habitat around used versus available commuting routes to identify factors influencing route choice. In total, we analysed 38 paths from colonies in three cities of different sizes. At all study sites, bats flew farther from street lights than expected by chance. In the largest city, but not in the two smaller ones, bats avoided routes with a high density of street lights, especially when this density varied greatly along the flight path. At our study sites, several bats commuted near streams or rivers, as these constituted natural dark corridors. In the two larger towns, bats also avoided areas with high levels of impervious surfaces. The effects of grassland and vegetation cover varied across sites and scales. We conclude that maintaining dark flight corridors through the illuminated urban matrix is key to ensuring connectivity between roosts and dark foraging areas for greater mouse-eared bats and potentially many other light-sensitive species. Without such corridors, pervasive urban sprawl could threaten populations of urban species and further erode urban biodiversity. Hence, we emphasize the importance of conserving networks of dark corridors in urban areas.