Street lighting disturbs commuting bats
Press release issued: 18 June 2009
Light pollution could have serious consequences for slow-flying bats on their nightly commute from the roost, according to new research from the University of Bristol, published today in Current Biology.
Artificial street lights were placed along hedgerows used as flight routes by lesser horseshoe bats (Rhinolophus hipposideros) when they leave their colonies at night. These experimental lights mimicked the spectral content and intensity of light from orange (high pressure sodium) streetlights that are prevalent around the world.
Previous studies have suggested that some bat species might benefit from street lighting as the lights attract insects and so fast-flying bats that catch insects on the wing often feed around them. However, bats with wing shapes adapted for flying in woodland (such as the lesser horseshoe bat) are rarely seen around street lights. It is these species that are often at greater risk of extinction.
The Bristol study found that lesser horseshoe bats did avoid flying along the lit hedgerows and were probably forced instead to use less ideal commuting routes (ones that led less directly to feeding areas). In some bat species, this has a detrimental effect as increased distances travelled to feed reduces the growth rates of infants.
The bats avoided the illuminated hedgerows because they are vulnerable to attack from birds of prey if they fly in lit conditions. Bats are not well adapted for detecting predators as the high frequencies they use in echolocation are directional and limited in range. While fast-flying bat species (that feed on insects around street lights) are better able to avoid predators, slow-flying species, like the lesser horseshoe bat, are particularly vulnerable to predators, and emerge in very dark conditions. Such bats seem to be hard-wired to avoid light.
Emma Stone, a PhD student in the University’s School of Biological Sciences and lead author of the research paper, said: “Man-made light pollution is an increasing global problem which has a negative impact on such important animal behaviours as foraging, reproduction and communication yet such pollution is rarely considered in habitat management plans and street lighting is excluded from English and Welsh light pollution legislation.
“We really need to know what levels of lighting particular bat species can tolerate, and take appropriate mitigation measures such as reducing illumination at commuting times, directing light away from commuting routes and constructing alternative flight routes. Win-win compromises are possible and we hope the public and planners will think more about the detrimental consequences of artificial lighting when considering whether to install it in the future.”
‘Street lighting disturbs commuting bats’ by Emma Louise Stone, Gareth Jones and Stephen Harris Current Biology 19, 1-5, July 14 2009