City dwellers often see bats flying around streetlights, happily chasing insects that are attracted to the glow. But such lights, especially in rural areas, can also have a dark side for bats. Research by graduate student Emma Stone in the United Kingdom found that artificial lighting can interrupt some bats’ nightly journey from roost to foraging habitats.
Stone’s project, which used a novel experimental approach, was supported in part by Bat Conservation International Student Research Scholarships.
Light pollution is fast becoming a global phenomenon, but the impact on wildlife, including bats, is rarely studied. The bats reported foraging at streetlights are usually fast-flying species accustomed to open landscapes. Such bats are better than their slower cousins at evading hawks, owls and other birds of prey.
The lesser horseshoe bat that Stone studied is, by contrast, a shy, slow-ﬂying bat that’s adapted to cluttered woodlands. It typically travels no more than about 1.2 miles (2 kilometers) from its roost to forage each night. These bats frequently fly along hedgerows – densely wooded of shrubs and small trees – for protection from predators and the elements.
Stone conducted her artiﬁcial-lighting experiments along hedgerows in eight sites around southern Britain. She first surveyed light levels at currently illuminated hedgerows, then duplicated those levels at experimental sites. Her team installed two temporary, generator-powered lights that mimic the intensity and light spectra of streetlights. Each site was near a maternity colony and along confirmed commuting routes of lesser horseshoe bats.
Bat activity at each site was monitored with bat detectors, which allowed identification of horseshow bats during four specific treatments: control (with no lights); noise (generator on and lights installed but switched off); lit (full illumination all night for four consecutive nights); and another night of noise only.
No significant difference in activity levels was detected between the control nights and noise nights. The impacts came when the lights were switched on. “We documented dramatic reductions in activity of lesser horseshoe bats during all illuminated nights,” she reports. Thirty percent of horseshoe bats reversed direction and left before reaching the lights; 17 percent flew over the hedgerows; 9 percent flew through the thick hedgerow vegetation; and 2 percent circled high or wide to avoid the lights.
These results, Stone said, suggest that light pollution may fragment the network of commuting routes used by lesser horseshoe bats, causing them to seek alternate, and probably longer, paths between roosting and foraging habitats. For some bats, this increased flight time can increase energy costs and stress, with potential impacts on reproductive success. It is critical, therefore, that light pollution be considered in conservation efforts.