Light pollution at the urban forest edge negatively impacts insectivorous bats

Connectivity and quality of vegetation in cities, including urban forests, can promote urban biodiversity. However the impact of anthropogenic pressures at the forest-matrix edge, particularly artificial light at night (ALAN), on connectivity has received little attention. We assessed the influence of artificial light at forest edges on insectivorous bats. We acoustically surveyed 31 forest edges across greater Sydney, Australia, half with mercury vapour streetlights and half in ambient darkness, and compared the bat assemblage and activity levels to urban forest interiors. We also sampled the flying insect community to establish whether changes in insect densities under lights drive changes in insectivorous bat activity. We recorded 9965 bat passes from 16 species or species groups throughout our acoustic survey. The activity of all bats, and bats hypothesised to be sensitive to artificial light, was consistently higher in forest interiors as opposed to edges. We found that slower flying bats adapted to cluttered vegetation or with a relatively high characteristic echolocation call frequency; Chalinolobus mono, Miniopterus australis, Vespadelus vulturnus, and Nyctophilus spp., were negatively affected by artificial light sources at the forest edge. The emergence time of Vespadelus vulturnus was also significantly delayed by the presence of streetlights at the forest edge. Conversely, generalist faster flying bats; Chalinolobus gouldii, Ozimops ridei, Austronomous australis, Saccolaimus fiaviventris, and Miniopterus orianae oceanensis, were unaffected by artificial light at the edge of urban forest, and used light and dark forest edges in a similar way. Insect surveys showed that larger lepidopterans seemed to be attracted to lit areas, but in low numbers. Artificial light sources on the edges of urban forest have diverse effects on bats and insects, and should be considered an anthropogenic edge effect that can reduce available habitat and decrease connectivity for light-sensitive species.