A new study shows that contrary to conventional wisdom, street lights do not benefit common bats. The research, conducted by a team of scientists from Bat Conservation Ireland and the University of Exeter, found that bats were generally less active in areas with street lights than in darker locations that had similar habitat. The study’s findings have significant implications for bat conservation, debunking the presumption that common bats benefit from street-lights, as they feed on flying insects that are attracted to- and congregate around the lights.
The study, which was recently published in the journal Philosophical Transactions of the Royal Society B, compared the activity of common species of bats in street-lit areas with the activities of bats in darker unlit areas. The authors found that activity of three species — notule, serotine and soprano pipistrelle bats — was either similar to- or lower than bat activity observed in darker unlit areas. They observed an increase in activity for the most common species — the common pipistrelle — but only at locations that offered a high degree of shelter in the form of hedgerows or trees. Leisler’s bat, which is rare in Britain but more common in Ireland, was the only species that appeared to benefit from artificial lighting provided by street-lights.
“People rarely see bats, and when they do it is usually because they are silhouetted by a light,” explains lead author, Dr Fiona Mathews, who is a research scientist at the University of Exeter. “Because clouds of insects accumulate around lights, there has been an assumption that the bats were getting an easy lunch. What our work shows is that they are actually usually just as active, if not more so, in adjacent dark areas. We already knew that lighting was bad news for rare species such as horseshoe bats. Now we have demonstrated that, for the common species of vital importance to our ecosystem, lighting is not helpful. Over recent decades, the number of streetlights, and the brightness of lighting, has grown enormously. We also use increasingly powerful lights to illuminate outdoor areas around our homes. We urgently need to reverse this trend.”
The researchers analysed large-scale bat surveys conducted in Great Britain and Ireland, consisting of over 265,000 bat calls recorded at over 600 sites. The study explored the link between bats and lighting at various spacial scales including short bicycle-surveys, longer surveys conducted by volunteers travelling by car across Ireland, and more in-depth monitoring over several nights at some sites.
Although bats are frequently portrayed as being blind, they have good eyesight that is specially adapted to foraging in low light conditions. However, this adaptation doesn’t fare well for them in brightly lit conditions.
“When we walk out of a lit house into the dark, it takes a while for our eyes to adapt to the darkness. The same is true in bats – they are dazzled by bright light and it takes time for their eyes to re-adjust,” explains Dr Mathews. “This could affect their ability to navigate. In addition, it seems that their ability to hunt insects is reduced in the light. So although a bat may be seen flying round and round a streetlamp, it may actually be struggling to catch anything.”
According to Dr Niamh Roche, a scientist with Bat Conservation Ireland and co-author of the paper: “Leisler’s bat is considered very special in Ireland since its population here is of international importance, so it is good to know that this species at least may not be so negatively impacted by street lighting. Nonetheless, we are extremely concerned that, with just one out of our nine Irish species showing a positive association with street lighting, much more needs to be done to lessen negative impacts of lighting.” He suggests that “this can be achieved by considering lighting scheme designs more thoroughly from the planning stage.”
Fiona Mathews, Niamh Roche, Tina Aughney, Nicholas Jones, Julie Day, James Baker, Steve Langton. Barriers and benefits: implications of artificial night-lighting for the distribution of common bats in Britain and Ireland. Philosophical Transactions of the Royal Society B, March 2015 DOI: 10.1098/rstb.2014.0124