How Light Pollution Impacts Wildlife & How You Can Help

From cougars and bats to butterflies and sea turtles, wildlife are increasingly threatened by light pollution, but simple solutions can help

  • Jessica Snyder Sachs
  • Conservation
  • Jul 04, 2023

In the Santa Monica Mountains, a western screech owl (above) perches in a tree above Los Angeles, one of the country’s most light-polluted urban areas. The threatened western snowy plover (below) avoids roosting on beaches where artificial light is brighter than the light of a half-moon. (Photo above by Jason Klassi)

SEVERAL YEARS AGO, members of the Ventura Audubon Society approached the operators of a power plant about an hour north of Los Angeles. Its location—an otherwise undeveloped stretch of sandy beach—is a habitat that has become as rare as it is crucial for California shorebirds such as the threatened western snowy plover.

For more than a decade, the society had been counting 20 or more plover nests each year to the north and south of the generating station. But the birds never nested on the half-mile of sand directly in front of the plant, and the conservationists suspected the facility’s bright security lights might be the problem. Plovers need darkness to hide their open-sand nests from nighttime predators. The plant managers agreed to dim the lights, and the following year, in 2013, 10 snowy plover nests appeared on the previously barren beachfront.

An image of a Snowy Plover female incubating.

That small victory impressed Travis Longcore, an environmental biologist at the University of California, Los Angeles and a pioneer in the study of light pollution’s harmful effects on wildlife. He and colleagues Ariel Levi Simons and Karen Martin went on to measure just how much light pollution is too much for the western snowy plover as well as the California grunion, a silvery-blue fish that makes dramatic spawning runs on the same beaches. Plovers are far less likely to roost on beaches where artificial light exceeds that of a half-moon, they found, and grunion are far less likely to run on shores where it exceeds that of a full moon.

Before the mid-1800s, most of the world’s humans and other animals lived under night skies lit solely by the moon. Electric outdoor lighting became commonplace in the early 20th century and increased about 3 to 6 percent a year during the second half of the 20th century. The global extent of modern light pollution became clear in 2016, with publication of the first comprehensive global satellite measurement of nocturnal artificial light—“The New World Atlas of Artificial Night Sky Brightness”—in the journal Science Advances. Researchers found that more than 80 percent of the world population lives under light-polluted night skies—places where the glow from artificial lights is significant enough that stars begin to disappear from view. In the United States and Europe, 99 percent of residents live under light-polluted skies. A 2017 follow-up report found that global light pollution levels are rising at a rate of 2.2 percent a year.

For every human stargazer who bemoans the disappearance of the Milky Way, light pollution disorients countless animals, exposing many to dangers such as predators, exhaustion and starvation. In 2022, Longcore joined 20 colleagues from four countries to publish “A plea for a worldwide development of dark infrastructure for biodiversity” in the journal Landscape and Urban Planning. “Light pollution is an increasing worldwide pressure for biodiversity,” they wrote, “contributing to habitat loss and fragmentation.” The researchers urged conservation scientists and urban planners worldwide to include the crucial need for darkness in their plans for preserving and restoring wildlife habitat.

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An image of P-35 Mountain Lion at one of her deer kill cache sites.

Cougars that appear unfazed by buildings and other signs of human development will not cross brightly lit areas.

Barriers, disorientation and fatal attraction

Like roads and fences, artificial light can create barriers that fragment habitat. Many slow-flying bats, for example—including little brown, mouse-eared and long-eared bats—avoid feeding in or even passing through illuminated areas because it exposes them to predators such as owls and other birds of prey. Light shining on or close to the roosts of these light-sensitive animals also delays their emergence at dusk, when the insects they eat are most abundant. Sometimes, light will cause bats to abandon their roosts or become entombed in them, starving to death as they wait for darkness that never comes.

Even large predators such as cougars, which may roam freely through developed areas, often are stopped short by lights at night. This is a particular problem in the area surrounding Los Angeles, where cougars already are confined to small habitat fragments isolated by highways and other barriers that have caused inbreeding within their populations.

Light also can be a fatal lure to wildlife, as with the poster animals of this problem: sea turtle hatchlings. On beaches adjacent to roads and buildings, many emerging hatchlings head inland toward artificial lights instead of the ocean.

Researchers have documented similar attraction and disorientation among migratory songbirds. Many will circle brightly lit buildings throughout the night, leading to exhaustion and depletion of the energy stores they need for their journeys. Worse, birds often collide with lighted structures. Studies have shown that artificial nocturnal light also interferes with a migrating songbird’s ability to use natural polarized light from the sky to calibrate its internal compass.

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An image of fireflies on a forest path.

Even a brief flash of headlights can cause fireflies to cease or alter their mating flashes. In areas lit up by outdoor lamps—or where indoor light spills through windows—the insects’ bioluminescent signals may disappear completely.

Role in insect declines

Scientists say light pollution is among many factors driving a potentially catastrophic decline in some of the world’s insect populations. That threat is obvious to anyone who has witnessed moths and other nocturnal insects swarming around bright outdoor lights. The lights’ powerful lure may stem from the animals’ instinctual tendency to orient themselves by moonlight, explains Candace Fallon, an endangered species biologist with the Xerces Society for Invertebrate Conservation. “If you have insects fluttering around a street light all night, they’re depleting their energy reserves to the point of exhaustion, increasing exposure to predators and potentially missing courtship cues from mates,” she says.

For fireflies, even a momentary flash of headlights can cause the animals to cease or alter their mating flashes, and their bioluminescent signals decrease or disappear altogether in areas lit by outdoor lamps or where indoor light spills into yards through windows. Studies suggest that exposure to artificial light at night can harm day-active insects as well. When exposed to nocturnal light, migrating monarch butterflies will flit and flutter when they should be resting, and the next day, they appear to be disoriented from their migration route.

U.S. Fish and Wildlife Service (FWS) biologist Roger Tabor found a similar disruption of nighttime rest and migration among newly hatched Chinook and sockeye salmon in Washington state’s Lake Washington. Juvenile salmon are drawn to patches of water lit by bridge, street or wharf lights where they can become easy pickings for predators such as great blue herons and sculpins. “On a couple of occasions, great blue herons swooped down into our lighted experimental unit, even though our lighted shoreline sections were temporary and only covered a small area,” he says.

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An image of dead birds found at the World Trade Center.

Disoriented by light, many warblers, thrushes and other songbirds (above) collide with buildings in New York City. One deadly site is the Tribute in Light (below), which now can shut down when large numbers of birds (seen as white streaks) approach.

Bringing back the dark

“The good news is that addressing light pollution has a clear and immediate impact,” says Ashley Wilson, former director of conservation for the International Dark-Sky Association, a global leader in combating the problem. “As soon as you turn that light off, the pollutant disappears from the environment, and species can begin to return.”

Wilson cites several proven ways to reduce the disruptive effects of nocturnal lighting on animals. First and foremost, “remember that there is no such thing as wildlife-friendly lighting,” she says. “The best solution is no light beyond natural conditions.” She recommends carefully evaluating if an outdoor light is actually needed. Wilson, Longcore and other experts say that in many cases, the answer is no, citing a 2015 study in the Journal of Epidemiology and Community Health that found no increase in car accidents or crime when British authorities reduced nighttime street lighting to save money and cut carbon emissions.

An image of birds flying through the twin beams at the Tribute in Light.

If light truly is needed, reduce the amount that spills into wildlife habitat through actions such as dimming, focusing, shielding or lowering the height of lamps. Newer LED microarray lamps, for example, have the advantage of precisely focused beams that have little of the sideways and upward light scatter of conventional lighting. Light spill also can be decreased with shields or shades on outdoor lights, and drapes or blinds to block indoor light from spilling out.

FWS biologist Jason Davis recalls how shielding a single streetlamp in Delaware coincided with the return of a small population of the endangered Bethany Beach firefly. In 2019, he failed to find the insect in a spot where he’d previously seen its distinctive flashing and suspected the reason was a bright light cast by a streetlamp that hadn’t been there before. “The local homeowners and power company agreed to install a new light and shroud it to reduce how much light pollution spilled into the beach’s dunes and swales,” Davis says. When he returned the following summer, Davis once again saw the endangered firefly’s characteristic flashing.

In California, creating a haven from light pollution was one of the chief challenges in designing what will become the world’s largest wildlife crossing. The Wallis Annenberg Wildlife Crossing over the 101 Freeway—a project the National Wildlife Federation has supported by raising funds and public awareness of a need for the crossing to save the lives of cougars and other wildlife—is scheduled for completion in 2025. “To be successful, the corridor has to feel safe to approaching animals that travel under the cover of darkness and avoid brightly lit areas,” says Longcore, who serves as a lighting advisor to the project. “This is a particular challenge given the area’s high level of light pollution.”

To meet that challenge, the crossing’s architects are employing several strategies that include adding a tannish-brown color to the concrete used to build the bridge. “In essence, this will reduce the amount of glow from the concrete that comes from artificial light bouncing off its surface,” says Robert Rock, principal architect of Living Habitats, the landscape architectural firm doing the design. His team also worked with the California Department of Transportation to lower the height of road lights on either side of the crossing and install lamp shields that focus illumination on the highway. Doing so will significantly reduce the amount of light reaching the wildlife crossing and its adjacent habitat.

Researchers are also exploring which light wavelengths are the most or least disruptive at night. There is no one spectrum that avoids affecting all wildlife species, Wilson says. A 2013 study in the Journal of Neuroscience suggests that light on the blue, or shorter, wavelength end of the spectrum has a greater effect on circadian rhythms—particularly nocturnal behaviors such as sleep—than does light on the red end. Fluorescent lights tend to emit more blue light than incandescent bulbs, while multicolor LED lights can be adjusted for color tone as well as brightness, Wilson notes.

Beyond dimming, focusing and adjusting the wavelength of light, timers and motion detectors can limit how long light disrupts the darkness. Adopting that approach on a large scale, seasonal restrictions mandate lowering or extinguishing lights during crucial migratory or mating seasons. In 2022, for example, New York City became the largest U.S. urban area to adopt a migratory-bird-friendly Lights Out policy, requiring all city-owned and leased buildings to turn off outdoor lights between 11 p.m. and 6 a.m. during spring and fall migration. More than two dozen U.S. cities have similar programs.

But you needn’t wait for governments to act, Wilson emphasizes. “When it comes to light pollution, every person has the ability to make a difference,” she says. “Whether turning off a light, talking to neighbors or raising the issue at a town council meeting, this is an opportunity to enhance your community while benefiting wildlife and reducing energy consumption. It’s a win-win-win situation.”

Jessica Snyder Sachs is a science writer based in New Jersey.

Light’s Deadly Allure

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