The Dark Side of Light

While cities sleep, creatures of the night swoop through the skies overhead. You may not realize it, but many bat species live in urban areas. People usually don’t notice the animals because they’re nocturnal. By day, bats roost in trees in parks, beneath bridges, and under the awnings of buildings. But “at night, they are active—flying, feeding, drinking,” says Winifred Frick, a biologist at the University of California, Santa Cruz, and chief scientist at Bat Conservation International.

Living alongside people isn’t always easy for wildlife. One problem city-dwelling bats face is light pollution. The electric lighting that illuminates cities at night disorients these creatures, which are used to navigating in total darkness. A brighter nighttime environment can also disrupt their cycle of sleeping and waking, as well as interfere with feeding and reproduction. “Light pollution can change behavior in harmful ways for some bat species,” says Frick. Fortunately, researchers, engineers, and city planners are working together to remedy the problem and help bats reclaim the night.

As cities sleep, creatures of the night swoop through the skies above. You may not realize it, but many bat species live in cities. People usually don’t notice the animals because they’re nocturnal. Bats sleep during the day. They roost in trees in parks, beneath bridges, and under the awnings of buildings. But “at night, they are active—flying, feeding, drinking,” says Winifred Frick. She’s a biologist at the University of California, Santa Cruz, and chief scientist at Bat Conservation International.

Living near people isn’t always easy for wildlife. One problem for city bats is light pollution. Bats are used to finding their way in total darkness, and the nighttime electric lighting in cities confuses them. Too much light at night can also mix up their cycle of sleeping and waking. And it can interfere with feeding and reproduction. “Light pollution can change behavior in harmful ways for some bat species,” says Frick. Luckily, researchers, engineers, and city planners are working together to fix the problem. They want to help bats take back the night.

For bats, flying into an artificially lit area would be like a person having a flashlight shine right in his or her face: It’s blinding. That can affect urban bats’ behavior in several ways. It can alter their commuting routes, forcing them to detour from their usual flight path to avoid brightly lit areas. Researchers have even seen bats flap toward a brightly lit spot and, instead of flying into it or around it, give up and turn around. Imagine the endless line of glowing streetlamps along a lonely highway—good for drivers, bad for bats.

Light can also influence when bats leave their roosts to begin their nighttime activities and how they hunt for food. You’ve probably noticed that bugs swarm around lights at night. “Certain lights attract insects,” says Frick. “Some types of bats take advantage of this and fly into the illuminated area to feed.” Catching a midnight snack at one of these all-you-can-eat bug buffets might sound like a smart move. But actually, it’s super risky. In well-lit areas, bats are easy targets for predators such as peregrine falcons and owls. To stay safe, bats need the cover of darkness.

There’s another problem with the buggy swarm that gathers around electric lights. The glow that lures all those insects also alters their behavior. Brightness so captivates the creatures that they might neglect to mate, which means their populations dwindle over time. And that’s potentially disastrous for bat species that eat insects. Fewer bugs means less food for the bats to eat.

If a flashlight shines right in your face, it’s blinding. That’s what it’s like for bats to fly into an area with artificial lights. This can affect city bats’ behavior in several ways. It can change their travel routes. To avoid brightly lit areas, the bats may need to leave their usual flight path. Researchers have even seen bats flap toward a brightly lit spot but then not fly into it or around it. Instead, the creatures give up and turn around. Think of the long line of glowing streetlamps along a lonely highway. They’re good for drivers, bad for bats.

Light can also affect when bats leave their roosts at night. And it can change how they hunt for food. You’ve probably noticed that bugs flock around lights at night. “Certain lights attract insects,” says Frick. “Some types of bats take advantage of this and fly into the illuminated area to feed.” It’s like an all-you-can-eat bug buffet. It might sound smart to catch a midnight snack there. But it’s very risky. In well-lit areas, bats are easy targets. Predators such as peregrine falcons and owls can spot them. To stay safe, bats need the cover of darkness.

When bugs gather around electric lights, another problem develops. The glow attracts all those insects, but it also changes their behavior. Brightness charms the creatures so much that they might not mate. That means their numbers decrease over time. And that could spell disaster for bat species that eat insects. Fewer bugs means less food for bats to eat.

Paying attention to the well-being of bats is important. These night flyers are a vital part of many of our planet’s ecosystems—communities of living organisms interacting with their physical environments. Bats’ feeding habits help keep insect populations in balance. Of the 42 bat species found in the U.S. and Canada, all but three dine on insects. These bats are the main predators of night-flying bugs, including crop pests. Bats’ huge appetite for insects provides farmers with billions of dollars’ worth of completely natural pest control each year.

Other bats feed on the fruit of valuable commercial crops and disperse seeds in their droppings. Some bats also sip nectar from flowers. They’re important pollinators for more than 500 plant species. The bats pick up powdery pollen from flowers as they feed and carry the grains from plant to plant, helping them reproduce. “Bats provide great economic value to human society,” says Frick. “But their real value isn’t economic—it’s that they’re part of our planet’s biodiversity. We share Earth with many other living things.”

But, adds Frick, “bats are facing many urgent threats, and their numbers are rapidly declining.” Right now, the most devastating problem for North American bats is a fungal infection called white-nose syndrome. The disease has killed more than 6 million bats, leading several species to become threatened or endangered.

Paying attention to bats’ well-being is important. These night flyers are a vital part of many of Earth’s ecosystems. An ecosystem is a group of living organisms in their physical environment. Bats’ feeding habits help keep insect numbers in balance. Forty-two bat species live in the U.S. and Canada. All but three eat insects. These bats are the main predators of night-flying insects, including crop pests. With their huge appetite for bugs, bats help farmers. They provide billions of dollars’ worth of all-natural pest control each year.

Other bats eat the fruit of valuable commercial crops. Then they spread seeds in their droppings. Some bats also sip nectar from flowers. They’re important pollinators for more than 500 plant species. The bats pick up grains of pollen from flowers as they feed. They carry the grains from plant to plant, helping them reproduce. “Bats provide great economic value to human society,” says Frick. “But their real value isn’t economic—it’s that they’re part of our planet’s biodiversity [variety of life].” We share Earth with many other living things.

But, adds Frick, “bats are facing many urgent threats and their numbers are rapidly declining.” Right now, the biggest problem for North American bats is white-nose syndrome. This fungal infection has killed more than 6 million bats. Because of this disease, several species have become threatened or endangered.

Bats suffering from white-nose syndrome first have fuzzy white spots of fungus appear on their noses. The spots quickly spread to exposed skin on their wings and tail. The fungus strikes in winter, when bats enter a sleepy state called torpor, which includes a drop in heart rate and body temperature. Normally, a bat stays in torpor throughout the winter, waking up only a few times to briefly sip water. But the fungus disrupts this cycle by causing bats to become dehydrated and wake up more than they should. When awake, bats quickly burn the fat stored in their bodies and are not able to survive the winter.

In addition, as humans use more and more land, bats are pushed out of their natural habitats. Wind turbines—a fast-growing source of clean, green, renewable energy—are trouble for bats, too. As migratory bats travel between winter and summer roosts, they often collide with turbine blades and get hurt or killed.

“With all of these problems, other threats like light pollution are added stressors on bats trying hard to survive,” says Frick. Fortunately, people are taking action to make the spaces we share more bat-friendly—because what’s good for bats is good for us too.

Bats with white-nose syndrome develop fuzzy white spots. First, the spots appear on their noses. Quickly, the spots spread to bare skin on their wings and tail. The fungus strikes in the winter. That’s when bats enter a sleepy state called torpor. It includes a drop in heart rate and body temperature. Normally, a bat stays in torpor all winter. It wakes up only a few times for a quick sip of water. But the fungus upsets this cycle. It causes bats to become dehydrated and wake up more than they should. When awake, bats burn the fat stored in their bodies quickly. Then they can’t survive the winter.

Also, humans are using more and more land. That pushes bats out of their natural habitat. Wind turbines are a fast-growing source of clean, green, renewable energy. But they’re trouble for bats too. Some bats migrate. When they travel between winter and summer roosts, they often fly into turbine blades. Then they get hurt or killed.

“With all of these problems, other threats like light pollution are added stressors on bats trying hard to survive,” says Frick. It’s a good thing people are working to make our shared spaces more bat-friendly. After all, what’s good for bats is good for us too.