Degraded Darkness

By Ben Harder

Illustration ©Michael Gibbs

On a pair of sweltering days one recent July, Sharon Wise and her husband Bryant Buchanan rigged strands of white Christmas lights from tree to tree in the wilderness of Virginia. No nativity scene was in evidence, no plastic reindeer, and certainly no snow. Any neighbors the couple might have impressed with their early display of yuletide spirit were in distant Utica, New York, where both biologists live and teach.

But the redback salamanders (Plethodon cinereus) that hide all day beneath the leaf litter of Mountain Lake Biological Station certainly took note of the couple’s efforts. When the researchers and three of their students switched on the lights at sunset one evening, the nocturnal salamanders responded with the amphibian equivalent of pulling the covers over their heads. They waited an hour longer than usual to get up for breakfast.

That delay concerns Wise. Under normal conditions, she knows, the salamanders emerge soon after nightfall and forage for just a few hours. But artificial illumination from buildings, road lights, and distant urban glow increasingly bathes organisms that, like Mountain Lake’s redbacks, have adapted to live in the dark.

Does a later start for the salamanders mean fewer nightly meals and fewer calories? Does that cut into their fertility or increase their mortality? What does it mean for the insects that the salamanders eat and for the predators that, in turn, eat them? Wise doesn’t yet have answers. Nor do other researchers studying the incipient field of artificial light ecology have a full grasp on parallel questions about a broad range of organisms and ecosystems.

Nevertheless, as these scientists begin to assemble an understanding of the ecological consequences of artificial light, they are recognizing numerous impacts. A few examples—sea turtles, for instance, and migratory birds—may be familiar. Most are not.

Many of the effects of artificial light may resonate up and down food chains, dragging whole ecosystems into imbalance. And by modifying the playing field on which nocturnal organisms develop, interact, and reproduce, artificial light may sculpt not only their individual lives but also the biological evolution of their species. That, says Buchanan, “is the most overlooked of all of the potential ramifications of artificial light.”

As scientists grapple with artificial light’s effects, forward-thinking conservationists—and a handful of sympathetic developers and regulators—are working to integrate the infant discipline into their efforts to mitigate light’s unnatural ecological impacts.

Natural darkness, once a given, has become an ecological resource of shrinking abundance. Everyone who has squinted into a street lamp or stared at lights visible through a window has experienced glare—light that strikes the eye directly rather than hitting the object it’s intended to illuminate. Those who have also seen composite nighttime satellite photos of Earth recognize how effective humans have become at dispelling the dark. Thickly populated, industrialized regions of the globe appear so starkly illuminated in these images that it’s easy, for example, to discern the intricate outlines of the Great Lakes. Italian astronomer Pierantonio Cinzano has compiled the first world atlas of night sky brightness with satellite data and models of light propagation. It reveals that one-tenth of the world population, approximately 40 percent of the U.S. population, and one-sixth of the European Union population live where nighttime brightness is too intense for the human eye to use night vision.

Not all light that goes up comes down, but a considerable amount of it does. Reflected off moisture and dust in the atmosphere, it creates a nighttime twilight known as sky glow. “I’ve been amazed at how far light will travel,” says Chad Moore, a physical scientist with the National Park Service. Major cities can broadcast sky glow as far as 250 to 300 kilometers, perpetually illuminating a sector of the sky everywhere within that radius. “We’ve discovered a pollutant,” Moore says, “and we’ve saturated our environment with it.” When distant glow falls almost horizontally on hilly terrain, slopes that face the city can be bathed in light, whereas less-exposed hillsides retain nearly natural levels of darkness. That effectively fragments the habitat into areas that are more or less suitable for nocturnal species, depending solely on their orientation to the sky.

Based at Bryce Canyon National Park in Utah, Moore has set out to inventory the night sky in as many U.S. national parks as possible and to establish baseline data that the government can use to monitor artificial light, just as it tracks other pollutants. Using a research-grade digital camera and a wide-angle lens, Moore and his colleagues take 360-degree snapshots of the night sky from vantage points within the nation’s natural treasures. So far, his team has surveyed about 20 national parks and monuments. At many of the sites, artificial illumination is comparable to at least the brightness of a crescent moon. That modest glow might not sound like much of a problem, but consider how little light it takes to produce biological effects. One lux, a unit of illumination, corresponds roughly to dim interior light or the halo of a street lamp. An unobscured full moon provides about 0.3 lux. In light measuring less than about 0.01 lux, says Utica College’s Buchanan, “you can see objects, but you would have trouble writing. You might not be able to tell the difference between an M&M and a deer pellet.” Squirrel tree frogs (Hyla squirella), by contrast, can see well enough to navigate and forage at 0.0001 lux or less, and they avoid activity when illumination exceeds 0.001 lux. Other frog species favor even darker conditions.

It’s tempting to assume that artificial light distresses only a few exquisitely sensitive species. But mounting evidence suggests that the disappearance of darkness can affect plants and animals in a variety of ecosystems.  Snake populations are declining in the vicinity of developing parts of California, for example. And intriguingly, it seems that not all the blame lies with familiar culprits like new roads and neighborhoods. Nocturnal snake species are thinning out more rapidly than diurnal snake species, even in areas where development isn’t cutting directly into snake habitat. “There are certain areas in southern California,” says biologist Robert Fisher of the U.S. Geological Survey, “that have what seems like suitable habitat for these nocturnal snakes. But they’re not there, even though their diurnal counterparts are.”

A few pioneering scientists have started down the long road toward understanding light pollution’s ecosystem-scale ramifications. Late on a summer’s night, you might spot Marianne Moore (no relation to Chad of the National Park Service) adrift in a rowboat on Boston’s Jamaica Pond. Hours after the sun’s last rays have faded, the Wellesley College limnologist and two of her colleagues put in and row out to the pond’s middle. While one researcher holds a light sensor aloft, another toils beneath a black shroud to keep the glow of a laptop screen from fouling their data collection.

“Somebody told us we look like we’re transporting illegal aliens,” chuckles Moore. She and her team have occasionally had to placate suspicious police officers or explain themselves to locals who jog on the lighted paths ringing the shore. In truth, the cruises are part of their study of how much nocturnal light penetrates the water’s surface at several lakes in New England.

Moore suspects that artificial illumination alters aquatic ecosystems from the smallest organisms on up. The implications are far reaching and could ultimately link light pollution to water quality. Minute zooplankton lurk well below the surface during the day to avoid predators, then rise to graze on algae at night. But artificial light discourages them from venturing toward the surface. “If their grazing is inhibited . . . effects will cascade up the food chain,” Moore says. Algae populations could explode in response to reduced predation, and those blooms would deplete dissolved oxygen critical to fish, crowd out other photosyn-thesizers, and cast unwanted daytime shade on submerged aquatic vegetation that provides habitat for juvenile fish.

When Moore and her colleagues experimentally blocked light from filtering into the depths, they found that some small taxa of zoo-plankton ascended two to three meters more than the organisms did under typical unnaturally bright conditions. Moore expects even stronger effects among larger kinds of zooplankton, some of which are known to avoid levels of illumination comparable to bright starlight. The intensity of artificial light shining on Jamaica Pond is considerably brighter than starlight, she has found, even on cloudless nights. When clouds roll in and reflect Boston’s glow downward onto the pond, light intensity can triple to almost two-thirds the luminosity of the full moon.


Moore has an idea for testing whether light pollution has steered evolution in Jamaica Pond during the century that it has been bathed in artificial light at night. Certain zooplankton lay eggs that settle to the bottom and can remain viable in a suspended state for decades, creating an “egg bank” that stores in the sediments the genetic traits of successive generations. Moore envisions digging up eggs, determining when they were laid based on the age of adjacent sediment, hatching them, and then exposing the organisms to light to see whether their responses vary according to their age.

“Invertebrates evolve very quickly, so it’s very reasonable to expect that they’ve responded to a history of artificial light,” Moore says. By contrast, longer-lived organisms such as fish probably need more time to adapt. That inequality could add to stress on fish populations, she says.

If the science relating to light pollution’s ecological impacts remains in its infancy, regulation to minimize its effects in sensitive areas has barely been conceived, says Sara Wan of the California Coastal Commission.

Nevertheless, perceived ecological threats from light pollution have helped stop a smattering of incautious development initiatives. In 1999 the commission rejected an application to add dazzling ornamental floodlighting on the Vincent Thomas Bridge in Los Angeles after hearing ecologists’ objections. One of the gravest concerns was that powerful beams of light would disorient and fatally attract migratory birds, says Catherine Rich, a cofounder of The Urban Wildlands Group who presented to the commission scientific data against the project. The data did not make an absolutely airtight case, but they raised substantial cause for concern, says Wan, who holds advanced degrees in both biology and electrical engineering. Developers subsequently modified the proposed lighting design to reduce the light emissions. The final plan is ecologically so much more sound than the original proposal, Rich says, that her conservation think tank endorses it.

In Bangor, Maine, environmental activists successfully challenged a proposal to build a Wal-Mart Supercenter. In March 2003, the state’s Board of Environmental Protection ruled that development of the site, including lights shining over the parking lot, would pose unacceptable risks to neighboring wetlands. In Seattle, critics of a proposal to construct lighted sports fields at Sand Point Magnuson Park have made the impacts on wildlife and the night sky central to their opposition. Light pollution from the fields would shine onto adjacent wetlands and lakeshore.


Most regulators, however, rarely hear conservationists object to development proposals on the basis of light pollution, Wan believes. Scientists and conservationists may not feel tempted to appear at public hearings without rock-solid cases, she says, but their frequent silence means some regulators remain wholly unaware of the issue.

On another front, Michael Mesure of the Toronto-based nonprofit, Fatal Light Awareness Program, has been leading a campaign against light pollution in Toronto since 1996. The first time Mesure witnessed the aftermath of the phenomenon known as “tower kill,” he could hardly believe his eyes. Even before he climbed out of his car, he could see in downtown Toronto’s pre-dawn gloom a bird carcass on the sidewalk. Others lay nearby. The migrating animals had been lured off course by lights on high-rise office buildings and had fatally collided with the structures.

On some mornings since that day in 1988, Mesure and an army of volunteers have identified more than 1,000 birds that had perished in this way during the previous night. The Fatal Light Awareness Program targets tenants in downtown Toronto’s high-rises, and it advocates the use of window shades or blinds and directed task lighting at workstations, as well as switching off lights in unused areas at night. Over the past five years, a 17-percent reduction in the amount of light escaping from those buildings has noticeably reduced the volume of nocturnal tower kill, Mesure says.

Even where steps have been taken to mitigate light pollution, organisms face the threat of ever-bigger, ever-brighter civilization. At the loggerhead turtle nesting grounds in Florida, says turtle researcher Michael Salmon of Florida Atlantic University, “the problem is fast becoming not the amount of light at the beach but rather sky glow from inland.” Salmon argues that the growing threat to darkness must be attacked at its source—population centers. “Nothing covers that,” he says, “except having a national policy that governs how lighting is used everywhere.”

That idea might sound more farfetched than it is. The Czech Republic’s parliament set an international precedent in 2002 when it passed the first national law to address light pollution, which requires shielding of public lights among other measures. At least 11 U.S. states and numerous towns and counties have also enacted laws or ordinances that regulate at least some aspects of light pollution.

That sort of sky change would be welcome elsewhere. In upstate New York, Bryant Buchanan is gathering data on nocturnal illumination in one of his favorite scientific stomping grounds, Utica Marsh. Near a brightly lit roadway that cuts through the wetland, he pauses and holds up a sensor to measure light from each of the cardinal directions. Then, in the columns and rows of his field notebook, he records the data. Even without a flashlight, he has no trouble writing.

For More Information

Miscellaneous papers on the ecological consequences of artificial night lighting can be downloaded from

Ben Harder covers the environment for Science News and has written in print or online for National Geographic, Science, and U.S. News & World Report.