Wild boars scrounge for a meal in Barcelona. (Laurent Geslin)

On warm nights beginning in July, baby loggerhead turtles hatch from nests buried on Southeastern U.S. beaches. Tiny fins churn the sand with such vigor that biologists call the scene a “boil.” Moonlight beckons. The new turtles, a hundred per clutch, follow the light sparkling off the water into the ocean.

That’s how their first hours on Earth are supposed to go. Except the night isn’t what it once was.

Because of light and noise pollution, strange odors, urban development, agriculture and other disturbances, nocturnal habitats have been transformed. People have pushed some species to retreat into the dark while dazzling others with artificial lights. And scientists are starting to quantify just how profound a change we’ve caused to these ecosystems.

Light pollution is an old problem — records from the late 1800s describe birds flying into lighthouses. But modern research into lights and wildlife began relatively recently, with high-profile studies in the past five to seven years, according to Travis Longcore, who studies urban ecology and conservation at the University of Southern California.

For young turtles, light pollution can be their first and final encounter with human influence. At Hilton Head Island in South Carolina, beach-facing lamps and second-story lights must go dark at 10 p.m. to avoid drawing the turtles away from the ocean. Flashlights must be capped with red filter stickers. Most vacationers abide by the rules, said Amber Kuehn, a marine biologist with the Hilton Head Island Sea Turtle Protection Project, as long as they are aware that endangered turtles benefit.

Artificial light still slips through. During a recent hatching season, a man set down a path of rose petals and luminaries to propose to his fiancee. Kuehn discovered the luminaries, and the whorl of turtle tracks orbiting the candles, early the next morning. There are never any infant turtles to rescue, even at the start of her 5 a.m. patrol, she said: Predators pick off disoriented hatchlings too quickly.

Now artificial lights are changing — and presenting even more problems for wildlife. “It used to be there were few light sources that were economically viable to use for outdoor lighting,” Longcore said. Orange, from high-pressure sodium lamps, was the most prevalent street lamp color until Congress passed a stimulus package in 2009 during the Great Recession.

A switch from sodium lights to more efficient LEDs was an easy, “shovel-ready project,” Longcore said. Fueled by Recovery Act funds, blue and white LEDs replaced orange lights above U.S. streets.

There was little regard, though, for the ecological impact of this color change, Longcore said. He and his colleagues, in research published this month in the Journal of Experimental Zoology, wove together previous studies to see what that might be. “We needed to get something out there that looked at lamps holistically,” he said.

They charted the relationship between light color and its impact on wildlife by combing through previous reports on insects, sea turtles, salmon and a type of endangered bird called the Newell's shearwater. The study authors included data involving dozens of lamp types, including kerosene lamps, LEDs, halides, fluorescent and sodium lights.

Longcore and his colleagues discovered a peak response, measured by animals' behavioral changes or their visual sensitivity, broadly centered around cold blue lights. The most intense of the blues and whites, with names like Yard Blaster, or one made by a manufacturer that added violet to its LEDs to make them “vivid,” were closest to the sun's brightness. Longcore is not anti-LED, but you “can make them pretty hideous if you want.”

Wildlife sensitivity plotted against correlated color temperature. D65 is the standard for midday light. ( Travis Longcore )

“When you add the potential for sky glow, the scattering of the light, and the human response of circadian disruption,” said Longcore, it becomes “very clear that lamps that have less blue in them are going to have on average less impact.” There’s variance among taxonomic groups; blue light more clearly affects insects, for example, than shearwater birds.

Longcore worries that we have too zealously embraced LEDs. A study published in November in Science Advances found that the artificially lit areas of Earth grew by 2.2 percent each year from 2012 to 2016, as LEDs became much more common.

“When something becomes more efficient and cheaper, we use more of it,” Longcore said, describing a phenomenon called the rebound effect. “And so LEDs are so much more efficient that people actually now overuse them.”

The authors predict that filtered yellow‐green or amber LEDs should have the least effects on wildlife. The goal should be enough light to safely get by without turning night to day, Longcore said.

For many mammal species, day has already shifted to night. A recent meta-analysis, published in Science, reveals that normally day-dwelling animals are becoming more active at night.

The study's authors examined 141 case studies, representing 62 mammal species on all continents except Antarctica. In each case study, GPS, camera traps or other trackers recorded animals' behavior. Some lived in areas of high human disturbance, others in protected areas or remote habitats.

In areas disturbed by humans, animals were 1.36 times more nocturnal on average. Put another way: “An animal that naturally split its activity evenly between the day and night, for example, would increase its nighttime activity to 68 percent around people,” said Kaitlyn Gaynor, a doctoral student at the University of California at Berkeley and an author of the Science paper.

Hunting, urban development and agriculture were all risk factors for increasing nighttime activity. But more mundane behaviors, such as nature hikes, chased animals into the night, too. “Daytime walking, particularly with people with dogs, even dogs on leashes, had these powerful effects in displacing wildlife species,” said Justin Brashares, a wildlife ecologist at Berkeley and an author of the study.

Larger animals were more likely to spend increasing amounts of time active at night. In undisturbed parts of Indonesia, sun bears were active after sundown about 19 percent of the time. Near humans, though, the sun bears' habits flipped: They performed 90 percent of their activity at night. Lions in Tanzania had a similarly pronounced trend. So did wild boars in Europe and brown bears in Alaska.

“We're creating novel sets of interactions,” Brashares said. “Animals that haven't traditionally seen each other, or have been avoiding each other temporally rather than spatially, are now forced to make it work together.” All told, 83 percent of the species studied showed an increase in nocturnal activity.

“There's an encouraging aspect to knowing that these animals do have the plasticity, the flexibility, to live among us,” Brashares said. But his concerns outweighed this encouragement.

For wildlife, the effects of moving to the night aren’t well understood. (Shift work in one species of mammal — humans — has small but increased risks for diseases like diabetes. And the International Agency for Research on Cancer concluded in 2007 that shifts that disturb natural clock rhythms are “probably carcinogenic.") It's possible these night-shifted species are more exposed to predators or illness and less likely to find food.

These night habitats could be what ecologists call “sinks,” which provide just enough resources for survival — but not enough for reproduction or a way out. “We don't really know,” Brashares said.

Mammals found refuge in the night once before, to escape another outsize threat: dinosaurs. Like us, Gaynor points out, dinosaurs were ubiquitous, terrifying and active during the day. “Only after the extinction of the dinosaurs did mammals explore the daylight,” she said.

But dinosaurs were just the thunder lizards. Humans are plenty thunderous — and we have lighting, too.

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