We are banishing darkness from the night. Electric lights have been shining over cities and towns around the world for a century. But, increasingly, even rural areas glimmer through the night, with mixed – and largely unstudied – impacts on wildlife. Understanding these impacts is a crucial conservation challenge and bats, as almost exclusively nocturnal animals, are ideal subjects for exploring the effects of light pollution.
Previous studies have confirmed what many city dwellers have long noted: some bats enjoy a positive impact of illumination by learning to feed on insects attracted to streetlights. My research, however, demonstrates for the first time an important downside: artificial lighting can disrupt the commuting behavior of a threatened bat species. This project, using a novel experimental approach, was supported in part by BCI Student Research Scholarships.
Artificial lighting is a global phenomenon and the amount of light pollution is growing rapidly, with a 24 percent increase in England between 1993 and 2000. Since then, cultural restoration projects have brought lighting to old docks and riversides, placing important river corridors used by bats and other wildlife at risk of disturbance.
Studies of bats' foraging activity around streetlights find that these bats are usually fast-flying species that forage in open landscapes, typically species of Pipistrellus, Nyctalus, Vespertilio and Eptesicus. Such bats are better able than their slower cousins to evade hawks, owls and other birds of prey.
For our study, we chose the lesser horseshoe bat (Rhinolophus hipposideros), a shy, slow-?ying bat that typically travels no more than about 1.2 miles (2 kilometers) from its roost to forage each night, often flying no more than 16 feet (5 meters) from the ground. The species is adapted for feeding in cluttered, woodland environments. Its global populations are reported decreasing and the species is endangered in many countries of central Europe. The United Kingdom provides a European stronghold for the lesser horseshoe bat, with an estimated population of around 50,000.
These bats' slow flight leaves them especially vulnerable to birds of prey, so they leave their roosts only as the light fades and commute to foraging areas along linear features such as hedgerows. Hedgerows are densely wooded corridors of shrubs and small trees that typically separate fields from each other and from roadways. Such features are important commuting routes for many bat species, which use them for protection from predators and the elements. We suspected that lesser horseshoe bats would avoid illuminated areas, largely because of a heightened risk from raptors.
We conducted arti?cial-lighting experiments along hedgerows in eight sites around southern Britain. We first surveyed light levels at currently illuminated hedgerows, then duplicated those levels at our experimental hedgerow sites, all of them normally unlighted. We installed two temporary, generator-powered lights – about 100 feet (30 meters) apart – 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 acoustically, with mounted bat detectors, 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. We identified horseshoe bat calls to species and measured relative activity by counting the number of bat passes per species each night.
We found no significant difference in activity levels of lesser horseshoe bats between the control nights and either of the two noise nights, when the generators were running but the lights were off. The presence of the lighting units and the noise of the generators had no effect on bat activity.
The negative impacts came when we turned on the lights. We documented dramatic reductions in activity of lesser horseshoe bats during all of the illuminated nights. In our study, 42 percent of commuting bats continued flying through the lights; 30 percent 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. We also recorded some strange behavior on one night when two bats flew over the hedge in a dark area between two lights, then flew up and down repeatedly, as though trapped between the lights.
We examined the effects of light on the timing of bats' commuting activity. The bats began their commute, on average, 29.9 minutes after sunset on control nights, but 78.6 minutes after sunset when the lights were turned on. Light pollution significantly delayed the bats' commuting behavior. Interestingly, the activity began a few minutes earlier (23 minutes after sunset) on the first, but not the second, noise night. It is possible that some bats emerged early to investigate the generator noise.
We clearly demonstrated how artificial lighting disrupts the behavior of lesser horseshoe bats. We found no evidence of habituation: at least on our timescale, the bats did not become accustomed to the illumination and begin returning to normal activity or timing.
These results 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.
Light pollution is an increasing global problem with negative impacts on such important animal behaviors as foraging, reproduction and communication. Yet lighting is rarely considered in habitat-management plans and streetlights are specifically excluded from light-pollution legislation in England and Wales.
I plan to use these results as the basis for recommendations for changes in policy, conservation and management for bat habitat in areas that are subject to development. This knowledge is fundamental for understanding the factors that impact bat populations not only in the United Kingdom but around the world, and in developing effective bat-conservation actions. I hope these findings will also help guide further research.
Scientists need to determine what levels of lighting particular bat species can tolerate, so we can take appropriate measures to limit the impact. These might include reducing illumination at commuting times, directing light away from commuting routes and constructing alternative flight routes.
We sincerely hope this research and similar studies will cause both officials and the public to think more about the consequences of artificial lighting on bats and other wildlife.
EMMA STONE is a Ph.D. student at the University of Bristol and a researcher at the university's School of Biological Sciences. This project earned her the national Vincent Weir Scientific Award from the Bat Conservation Trust of the United Kingdom. Visit her project website for more information: www.batsandlighting.co.uk.
This research was originally published in the journal Current Biology, with co-authors Gareth Jones and Stephen Harris.