Researchers show how light pollution can harm imperiled bats
By Kristen Pope
A floodlight on a porch might be an effective way of lighting up a yard, but it could also be detrimental for nearby bats. Around half of the land in the U.S. is impacted by Artificial Light at Night (ALAN), which is a global pollutant that comes from a variety of sources, including residential, industrial development, and even transportation infrastructure. Studies have found outdoor light emissions have increased globally by about 10% each year since 2013.
Nocturnal bats are specially adapted to darkness, including sensory and behavioral adaptations, and ALAN can disrupt their circadian rhythms, physiological processes, and behaviors, which in turn can make them less fit and negatively impact their survival.
Dr. Amanda Adams, BCI’s Director of Research Coordination, and coauthors from Great Hollow Nature Preserve & Ecological Research Center in Connecticut set out to study how ALAN impacts bats in a recent paper. The article, “Far-reaching displacement effects of artificial light at night in a North American bat community” was published in Global Ecology and Conservation in 2023.
They set up an experiment at Great Hollow Nature Preserve, mounting three white LED floodlights—the same type your neighbors might use to light up their backyard—along the edge of a wetland that is a popular foraging habitat for bats. Some nights, they flipped the lights on, exposing the foraging habitat to ALAN, and other nights, they kept them off to record bat activity under natural conditions. Bat detectors from Wildlife Acoustics were set up at intervals, with one right by the lights and others 25, 50, and 75 meters away.
During the study, they recorded five different species of bats: imperiled little brown (Myotis lucifugus), big brown (Eptesicus fuscus), eastern red (Lasiurus borealis), and hoary bats (Lasiurus cinereus), as well as silver-haired bats (Lasionycteris noctivagans), which were only recorded sporadically.
Little brown bats avoided the light the most, with no bats of the species recorded right near the lights when they were on, and fewer of the species recorded at even the most distant site. This shows this imperiled species was disturbed at least 75 meters (246 feet) away from the light. At this distance, humans cannot perceive the light, but it makes a big difference to the bats. Myotis species are slow fliers, and the authors noted their low light tolerance may be related to the potentially greater exposure to predators they could face in brightly lit areas.
Big brown bats also decreased their activity when the lights were on, with 48–75% as many recordings of this species at the stations when the lights were on as opposed to off.
However, not every species seemed to mind the bright lights. Eastern red and hoary bats did not appear to change their behavior due to the lights. This was not surprising; eastern red bats are a species you may frequently see feeding on insects around streetlights in the summer
While some bats avoid brightly lit areas, others may prefer well-lit areas that lure in moths and other insects to eat. ALAN can change the bat composition at a site, keeping some away while attracting others.
When bats avoid an area due to light, this limits the habitat they can use for foraging, roosting, and other purposes. These findings are concerning to scientists; ALAN may reduce the already-limited habitat available to light-sensitive species, especially species like the little brown bat that are already imperiled by white-nose syndrome (WNS), which has killed over 90% of the species in less than 10 years.
“What our results tell us is that even small-scale, residential lighting can have far-reaching displacement effects on light-averse North American bats and drive shifts in overall community composition that extend well beyond the primary area of illumination,” says Dr. Chad Seewagen, who is the study’s lead author and executive director of Great Hollow Nature Preserve. “Habitat degradation for the imperiled little brown bat and the big brown bat caused by the spread of light pollution into dark landscapes could be substantial.”
Limiting ALAN can help reduce the stress humans are causing species that are already fighting for their survival. The study’s authors hope natural resources regulators and land-use planners can use this information to more effectively buffer important bat habitats from ALAN and better evaluate potential impacts to bats from proposed land-use changes that would generate light pollution. You can help by thinking twice before leaving a light on at night, or at the very least, using a motion control sensor or timer to reduce the amount of time the light is on.
This study was funded by the Connecticut Department of Energy & Environmental Protection.
Amanda Adams, Ph.D. – Director of Research Coordination
Amanda joined BCI in 2019 and has worked with bats for 20 years. As the Director of Research Coordination, she maintains communication and collaboration among internal and external partners for conservation research across BCI. She runs BCI’s Student Research Scholarship Program and is passionate about developing capacity for bat conservation. Amanda specializes in bioacoustics and has broad research experience, particularly in behavioral and sensory ecology, and is adjunct faculty in the Department of Biology at Texas A&M University. She received her Ph.D. from Western University in Canada and a B.Sc. and M.Sc. from the University of California, San Diego. She completed postdoctoral research at Ben-Gurion University of the Negev and Texas A&M University.