The Impacts of Light Pollution on Bats
We exist in a world where artificial light is ubiquitous, but it hasn’t always been.
Humans once had a much different relationship with nighttime. Once upon a time, we navigated our evenings with lanterns, stars, and moonlit skies. It was commonplace to be able to see the Milky Way; this is no longer the case. We started illuminating our roads and cities at night in the mid-1800s to help ourselves and our neighbors feel safer at night; there wasn’t ill-intent to that.
And yet, it is only now, nearing two centuries later, that we are starting to understand the consequences of light expansion - and pollution. According to a 2016 study in Science Advances, 80% of the world, and a staggering 99% of US residents live in light-pollution; 80% of people living in North America cannot see the Milky Way, and about half of the U.S. experiences light polluted nights. And it is not necessarily just urban cities that suffer from light pollution - we can see light from hundreds of kilometers away.
Outdoor electric lighting increased 3-6% per year in the second half of the 20th century. This light pollution stems from streetlights, house lights, and buildings in general, and is produced by anything that generates light, not necessarily limited to lightbulbs. Bright white lights produce more light pollution than amber and red light, disrupting our Circadian rhythm – part of the reason we’re encouraged to put our phones away a couple hours before bed. Additionally, light may not help us as much as we think it does; one 2015 study noted that evidence illustrating a causative relationship between streelighting reductions and increases in crime and traffic collisions was limited at best. And on top of our own relationship with it, artificial lighting impacts the environment, and other creatures around us. And while it affects us, it also influences hibernation, migration, and even plant phenology. Artificial light can fragment habitat, and alter animal behavior. Illuminated buildings confuse birds and lure hatchling sea turtles. Insects flit around streetlights and other sources of bright light at night, ultimately exhausting themselves. They become more vulnerable to predators, but also may not have energy to mate, or meet their own survival needs. And bats do not necessarily benefit from light influencing insect behavior. Some insectivorous bats avoid bright light. Others may be drawn to it, and in such, expose themselves to other predators. Additionally, bright light on or near bat roosts can alter their behaviour; they may not be able to tell when dusk is falling, and miss one of their more important meal times of the day.
There are a few ways that we can help. On a smaller scale, switching off lights at night is a good place to start. Additionally, if you’re in a situation where having outdoor lighting at night provides a certain level of security, switching to motion-sensing lights can make a large difference - and if that is not possible, swapping out bulbs to use a more amber-toned or red light can also be beneficial. Recent research points to red lighting as a potential way to provide light to those who need it, as well as mitigate potential impacts to bats and other nocturnal and light-influenced species.
On a broader scale, some states are working to help cut their light pollution. New York State recently announced the ‘Lights Out’ initiative, where state-owned and managed buildings turn off their lights from spring through May 31st, and during peak autumn migration times - August 15th-November 15th.
Researchers are currently exploring the utility of switching off streetlights at certain times, staggering streetlights so that fewer are illuminated, and switching to warmer, amber and red shades of light. No conclusions have been reached at this time about how to best alter street lighting to benefit bats, but reducing light intensity did benefit arthropods. Further research is needed to determine bat responses to light mitigation strategies.
Until then, think like a bat. Don’t point lights at their roosts, or over waterways, which tend to attract insects - and thus bats. Look at your landscape - planting shrubs and trees to block out light from your yard can help bats. And turn off lights at night wherever and whenever possible - the best solution is for us to begin reducing the amount of light we each put out, and bringing back some darkness so that the bats can eat, the birds can migrate, and we can begin to see the stars again.
Want to learn more about light pollution? Here's our reference list -
“Lights Out” Initiative: https://www.dec.ny.gov/press/125415.html
Cole, H and Barber, J (2020). “Rewilding the night sky: Mitigating the costs of light pollution for bats and insects.”
Haddock et. al (2019). “Light pollution at the forest edge negatively impacts insectivorous bats.” https://doi.org/10.1016/j.biocon.2019.05.016
Laforge et. al (2019). “Reducing light pollution improves connectivity for bats in urban landscapes.” https://link.springer.com/article/10.1007/s10980-019-00803-0
Mariton et. al (2022). “Even low level light pollution levels affect the spatial distribution and
timing of activity of ‘light tolerant’ bat species.” https://doi.org/10.1016/j.envpol.2022.119267