Evidence for distance and illuminance thresholds in the effects of artificial lighting on bat activity

Light pollution is a major threat to biodiversity worldwide. There is a crucial need to elaborate artificial lighting recommendations to mitigate its impact on wildlife. In the present study, we investigated how streetlight spatial position and light trespass impacted the use of ecological corridors by transiting bats in anthropogenic landscapes. Through a paired, in situ experiment, we estimated how streetlight distance of impact and vertical and horizontal illuminance influenced the transiting activity of 6 species and 2 genera of bats. We selected 27 pairs composed of 1 lit site and 1 control unlit site in areas practicing either part-night or full-night lighting. We recorded bat activity at 0, 10, 25, 50 and 100 m; and measured vertical and horizontal light illuminance at the 5 distance steps (range = 0.1-30.2 lx). While streetlight attraction effect was mostly limited to a 10 m radius for Pipistrellus sp. and Nyctalus sp., streetlight avoidance was detected at up to 25 and 50 m for Myotis sp. and Eptesicus serotinus, respectively. Streetlight effects on Myotis sp. and Nyctalus sp. remained after lamps were turned-off. Illuminance had a negative effect on Myotis sp. below I lx, a mixed effect on E. serotinus, and a positive effect on the other species, although a peak of activity was observed between 1 and 5 lx for P. pipiswellus and N. leisleri. We recommend separating streetlights from ecological corridors by at least 50 m and avoiding vertical light trespass beyond 0.1 lx to ensure their use by light-sensitive bats.