As darkness fell over Manhattan on the ninth anniversary of the September 11 attack, two beams were shot into the sky at the site where the Twin Towers once stood. The commemorative lights had appeared annually since the towers fell, but in 2010 onlookers noticed something unusual: countless white sparkles glittering within the white beams.

The mysterious white objects turned out to be thousands of migrating birds. Although the public had just taken notice of this spectacle, conservationists had been aware of the phenomenon for several years. Shortly after the tribute first appeared, New York City Audubon, a conservation group, helped initiate a program to monitor the installation and temporarily shut off the lights whenever too many birds got caught in the beams. In a later analysis of the bird populations on memorial nights between 2008 and 2016, researchers found that, although the short-term shutdowns...

This annual demonstration of how artificial illumination can influence animal behavior is but one instance of a much bigger prob- lem. Around 80 percent of all humans—and more than 99 percent of people in the US and Europe—now live under light-polluted skies. In addition to direct lighting from urban infrastructure, light reflected from clouds and aerosols, known as skyglow, is brightening nights even in unlit habitats. As electric lights become more energy- and cost-efficient, the proportion of lit surfaces keeps rising. Meanwhile, the list of organisms that researchers document to be affected by Earth’s unnatural glow is growing right along with it.

In 2002, the University of Southern California geographer Travis Longcore, also science director of the Los Angeles–based nonprofit The Urban Wildlands Group, and colleagues organized the first North American conference on the ecological consequences of light pollution. This inspired a growing interest in the scientific community that eventually led to a handful of large-scale projects that launched in Europe around 2010, says Thomas Davies, a postdoctoral ecologist at Bangor University in the UK. “That’s when we started to see this exponential growth in the research output in this field.”

We have nothing in our genetic make-up that has been exposed to this type of challenge. It’s completely unprecedented in the history of the Earth.

—Therésa Jones, University of Melbourne

Over the last 16 years, researchers have uncovered the many nuanced ways that light can affect individual species and have started to build a bigger picture of the effects on ecosystems. “It’s become clear that light pollution is a major anthropogenic pressure on the environment,” says Kevin Gaston, an ecologist at the University of Exeter in the UK.

And it’s a uniquely disruptive pressure in that life on Earth evolved to the beat of the circadian cycle, and bright, constant light at night is a very recent phenomenon in evolutionary time, adds Therésa Jones, a behavioral ecologist at the University of Melbourne in Australia. “We have nothing in our genetic make-up that has been exposed to this type of challenge. It’s completely unprecedented in the history of the Earth.”

Fatal attraction

In the 1880s, Swedish-American ornithologist Ludwig Kumlien noted that a 200-foot-tall, illuminated observation tower in Milwaukee, Wisconsin, was attracting migrating birds in the evening—and that many perished after colliding with the lights or the surrounding electric wires.2 This was one of the earliest reports of what is now a well-known effect of artificial nighttime lighting: its ability to draw in wildlife.

Since then, researchers have identified several other animals that succumb to light’s fatal allure. Insects are perhaps the most obvious example—many of these critters are nocturnal, and a wide variety of species, including beetles, mayflies, and moths, will cluster around streetlamps, floodlights, and other sources of nighttime illumination. Although the factors underlying this so-called “flight-to-light” behavior remain unclear, the consequences are well documented: increased rates of injury, exhaustion, and predation.

Evening lighting can also fragment animals’ habitats, as strings of lamps limit the movement of organisms from one place to another. In one field experiment, Franz Hölker, a freshwater ecologist at the Berlin-based Leibniz-Institute of Freshwater Ecology and Inland Fisheries, and his colleagues discovered that street lights could draw in moths passing within a radius of approximately 23 meters.3 Given that lampposts—at least in Europe—are typically around 20 to 45 meters apart, Hölker explains, the area from which they draw in insects often overlaps, creating a magnet that traps the animals and reduces their ability to disperse through the environment.

Certain organisms may adapt to light over time, potentially limiting the negative effects of the exposure. In 2016, a pair of Swiss researchers discovered that adult ermine moths (Yponomeuta cagnagella) from bright urban areas were less likely to be attracted to lights than their counterparts from dark, rural regions.4 This is likely a “genuine adaptation,” Gaston says. “The selection pressure to not fly to light is quite high if you are constantly exposed to it, and you’re suffering high mortality or energetic costs.”  

But so far, the ermine moths are the only documented example of such an adaptation to avoid artificial light. Whether the behavior of other animal populations will change in this way remains an open question, says Gaston. In addition, as light levels are one of many characteristics that differ between urban and rural areas, it is difficult for scientists to rule out the contributions of noise, air pollution, and other environmental stressors present in developed regions of the world.

Out of sync

Most organisms, from bacteria to people, have biological rhythms that help keep them aligned with the day-night cycles that occur as the planet rotates about its axis. These cadences are entrained by a variety of external signals, with light as the most important cue. As darkness disappears, that regulation can go awry.

Artificially extended days can modify the timing of tightly controlled daily activities, such as foraging and sleep.5 Some diurnal species, such as the great tit (Parus major), may continue searching for food later in the day, while nocturnal organisms—certain mice or bats, for example—spend less time out hunting or foraging.

Cosmic Timetraveler, unsplash

Light pollution can also distort seasonal and lunar rhythms, which are responsible for biological events such as reproduction and migration. Davide Dominoni, a postdoctoral ecologist at the Netherlands Institute of Ecology in Wageningen, and colleagues have found that constant, low levels of illumination at night (0.3 lux, 20 times lower than the intensity of the average street lamp in Munich, Germany, where the study took place) caused European blackbirds, Turdus merula, to develop their reproductive systems a month earlier than counterparts reared with dark nights.6 Other researchers have found that light pollution can delay birth in wallabies,7 advance egg laying in songbirds,8 and alter the migration patterns of salmon.9

Such perturbations may be mediated by changes in levels of melatonin, a hormone that is produced primarily at night and plays a key role in light’s effect on circadian cycles. The secretion of this chemical is known to be suppressed by blue light, which is present in high amounts in electronic devices and light-emitting diodes (LEDs), a type of lighting gaining popularity for use in street lamps thanks to its low cost and high energy efficiency. Reduced levels of melatonin have been measured in humans exposed to blue light.10 Last year, a group led by researchers at the University of Haifa in Israel found that people exposed to computer screens at night experienced modified circadian oscillations—specifically, lower nighttime melatonin production and a smaller nocturnal drop in body temperature—as well as altered sleep patterns.11

Experiments in the lab have shown that exposure to light at night can also dampen melatonin secretion in a variety of other animal species, including birds, fish, and insects. Jones and her team, for example, found that crickets reared under constant light had lower melatonin levels and impairments in immune function compared with those exposed to 12 hours of illumination per day.12

Despite the growing evidence that nighttime glow can alter daily and seasonal cycles, scientists currently have “very little evidence for strong effects, at least in vertebrates, on the fitness of animals,” Dominoni says. “I’m interested in trying to figure out whether the effects of light pollution on circadian and seasonal rhythms . . . have long-term consequences on the health of these animals.”

For some species, the potential harm of light pollution may be offset by benefits. In recent work published as a preprint earlier this year, Jones and colleagues found that Australian garden orb-weaving spiders (Eriophora biapicata) exposed to artificial light at night end up maturing faster and with fewer molts, being smaller as adults, and laying fewer eggs.13 Outside the lab, however, the researchers observed a compensating advantage: spiders living near streetlights ate more than individuals living in darker locales, due to the abundance of potential prey congregating around the lights (unpublished). “Physiologically they were being affected, but ultimately they were doing okay because they gained this benefit from the change in the predator-prey relationship,” Jones says.  

Bright Nights

As artificial light increases in volume and geographical coverage around the world, a variety of animal species are suffering ill effects. Some of these consequences are immediate and obvious—for example, a moth that flies into a streetlamp may die on impact. But there are also less visible, possibly more damaging effects, such as changes to predator-prey and plant-pollinator relationships that can reverberate through ecosystems.
See full infographic: WEB | PDF
© lucy conklin

Chain reactions

During the summers of 2014 and 2015, Eva Knop, a community ecologist at the University of Bern in Switzerland, and her team spent several nights wandering through meadows wearing night-vision goggles, in search of nocturnal pollinators. The researchers had positioned street lamps over seven cabbage thistle (Cirsium oleraceum) patches in remote meadows in the foothills of the Swiss Alps previously unexposed to nighttime illumination. They then compared insects on the plants in the lit areas to those on plants in nearby, unlit control regions.

This investigation revealed a 62 percent reduction in nocturnal visits to the cabbage thistles in light-polluted areas, which led to a corresponding 13 percent drop in fruit production among the illuminated plants.14 This decreased output could cause a decline in diurnal pollinator populations, which rely on the plants as a key source of food, the study authors suggest. “We need further experimental work to prove these indirect effects, but this shows that the negative effect at night could indirectly propagate into the day,” Knop says.  

A number of recent experiments in Europe have started to reveal how light pollution influences species interactions. At the University of Exeter, Gaston and his colleagues have set up grassland mesocosms, mini ecosystems within wooden cubes, each containing various plant, herbivore, and carnivore species. By exposing the enclosures to LEDs of varying intensities—low levels to mimic skyglow, medium levels corresponding to streetlamps, and high levels akin to stadium lighting—Gaston’s team has found that light pollution can have profound effects on predator-prey interactions.15

It’s become clear that light pollu­tion is a major anthropogenic pressure on the environment.

—Kevin Gaston, University of Exeter

In one experiment, for example, the team found that in 48 mesocosms that were exposed to low-intensity light for a few months, aphid populations shrank by approximately 50 percent due to increased predation by parasitoid wasps. Conversely, predators spent less time on the prey’s host plants lit by more intense lights, leading to fewer aphid deaths. “Because of the interactions [between species], you might actually see quite severe effects even at quite low light levels,” Gaston says. “It’s not just how you respond, it’s what your natural enemies and competitors are doing.”

Such ecosystem-level effects of light pollution can influence population dynamics and even community productivity. At the Westhavelland Nature Park, one of the darkest regions in Germany, Hölker and his colleagues compared microbial communities in freshwater sediments from two sites in a shallow agricultural drainage ditch—one site lit by artificial light at night and the other left dark. After five months, they found a significant increase in photosynthesizing microbes, suggesting that these organisms were using nighttime lighting as an energy source.16 Subsequent laboratory experiments revealed that exposing sediments to artificial light also perturbed the seasonal changes that typically occur in the microbial population. Without exposure to light pollution, there were clear winter and summer communities of bacteria and algae, Hölker says. “[But] after one year of illumination, this difference was no longer significant—the temporal structure was lost.”

This change in the composition of microbial communities corresponded with a shift in the ecosystem’s productivity. Under artificial lights, the microorganisms produced less carbon dioxide than those unexposed to evening illumination, likely a consequence of nighttime photosynthesis. In the long term, the researchers suggest, this could reduce the amount of carbon released from these freshwater systems into the biosphere over time.

“We naturally tend to think about the impacts [of light pollution] on individual species in isolation,” says Gaston. “But I think what’s becoming apparent now is that those networks of interactions are really vital to understanding the consequences of light at night.”

Shrinking numbers

Last year, researchers reported that flying-insect populations in Germany had dropped by more than 75 percent over the past three decades.17 This dramatic loss in invertebrate life made headlines, and a coauthor of the study warned that such declines have set the Earth on course for an “ecological Armageddon.” Of course, the question on everyone’s mind was: What’s the cause? “When this study came out, they were thinking about land-use change, climate change, and pesticides,” Hölker says. But these factors alone could not explain the population plunge. Light pollution might be the missing piece of the puzzle, adds Hölker, whose team recently discovered that the decimated regions also had high levels of evening illumination.18


Light pollution could dramatically alter populations of vertebrates as well. Field experiments by Kamiel Spoelstra, a biologist at the Netherlands Institute of Ecology, and colleagues have revealed that fast-flying Pipistrellus bats accumulate under certain colors of light that slow-flying Myotis and Plecotus bats avoid.19 The slow bats may be light-shy because exposing themselves under light could make them more vulnerable to predators, whereas agile bats may be able to enjoy the feast of insects that accumulate under the lights, Spoelstra explains.

Over time, it is possible that “if you have many lights outside, these light-shy bats simply lose habitats,” Spoelstra says. “It may be that the more lighting we have, the more common the common species and the rarer the less-common species become.”

But examining the effects of artificial light on animal populations is difficult, and strong evidence of light pollution’s long-term repercussions remains scarce. To appreciate the true scale of light pollution’s effects, the best place to look would be in regions that have only recently been exposed to nighttime lights. A recent analysis found that between 2012 and 2016, the artificially lit outdoor surface area of the earth increased at an estimated rate of 2.2 percent per year, with much of the growth occurring in South America, Africa, and Asia.20 This expansion has largely been facilitated by energy-efficient LEDs, Davies says. “So you get remote lights that are being put up in parts of the world that have previously been dark for the whole of evolutionary time.”

As scientists uncover more and more evidence of the harms of nighttime light, they are beginning to work with designers, architects, and government officials to protect the planet’s wildlife. Spoelstra, for example, has worked with Dutch policy makers to illuminate some areas with red light instead of white to prevent disruption to bat populations. However, this problem “can’t be solved by changing the spectrum alone,” Spoelstra says. Other modifications, such as limiting the times when roads are illuminated, putting motion sensors on lights, and shielding lamps so light does not spill into the sky or adjacent forests, are also necessary, he adds.

By themselves, these solutions are not enough to fight the effects of the increasingly luminous nights across the globe. Animals will not be able to evolve fast enough to adapt to the changes humans make on the planet, Longcore says. “We need to make either individual or collective decisions to not make the world even more light polluted than it already is.”  

Evening Hues

Artificial lights come in a range of colors. Low-pressure sodium lamps, which are typically used to brighten streets at night, have a distinct yellow hue. Light-emitting diodes (LEDs), on the other hand, offer illumination that is more energy-efficient, but the commonly used white LEDs typically produce large amounts of blue light.    
Scientists have long suspected that blue-rich lighting is the most harmful to wildlife. Decades of research have revealed that the cool hue is attractive to many animals—particularly insects—and can suppress the production of melatonin, a crucial hormone for regulating circadian rhythms.

In a recent analysis, University of Southern California geographer Travis Longcore and colleagues compiled previously published data on organisms’ responses to light across the spectrum to calculate the predicted effects of different lighting types. This work demonstrated that blue-rich lights indeed pose the greatest risk for the well-being of a wide variety of species, including insects, birds, and fish (J Exp Zool, doi:10.1002/jez.2184, 2018).

Now, conservationists are looking to capitalize on this information to protect wildlife. In Florida, for instance, the Fish and Wildlife Conservation Commission now recommends the use of red or amber LEDs to avoid attracting hatchling sea turtles. Similarly, some areas in the Netherlands have installed red lights to make their evening skies safer for bats.  

But even these colors can have adverse effects. For example, red lights tend to attract migrating birds—a problem recently recognized by the US Federal Aviation Administration, which announced in 2015 that it would require communication tower operators to replace steady red lights with flashing ones to reduce their allure.



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