Turning Night into Day : Does Skyglow affect Bat Activity and Timing of Emergence?

Abstract: Artificial brightness of the night sky caused by the backscatter of artificial light in the atmosphere is a consequence of ongoing urbanization. Skyglow covers 88 % of Europe’s surface and poses significant threats to biodiversity. Extensive research on responses of bats to direct light pollution already revealed significant impacts. However, evidence for the influence of skyglow is scarce. Therefore, this study investigates the effects of skyglow on bat activity patterns, namely Pipistrellus pygmaeus and Myotis species, in a rural area outside of Stockholm, Sweden. Additionally, the analysis includes the role of local habitat structures and landscape composition at multiple scales for both activity rate and timing of emergence.The two focal species showed diverging behaviour in their main habitat on the local scale. While Myotis spp. emergence was delayed by about half an hour above the water surface, P. pygmaeus appeared about 20 minutes earlier in forest edges under brighter night skies. Landscape-level skyglow significantly delays the emergence of Myotis spp. above water surfaces by 18 minutes, but P. pygmaeus did not respond to skyglow at the landscape-level. Both, Myotis species and P. pygmaeus, appeared earlier in forest edges than above water surfaces. Evidence on skyglow affecting bat activity rates was weak. Here, local habitat and landscape structure were more important for both species. The total length of all forest edges decreased bat activity at most scales, while activity was lower with the proportion of water and open land at different scales. In conclusion, bat conservation programmes need to consider the effect of skyglow as well as landscape characteristics.