In the 1950s, Dutch physiologist Frans Verheijen began to study how lights attract animals and interfere with their behaviour. And during the 1970s, more biological observations of the impacts of light started popping up in the literature. But it took two lateral-thinking biogeographers — Catherine Rich, president of the Urban Wildlands Group in Los Angeles, California, and Travis Longcore, now at the University of Southern California in Los Angeles — to see the links between them and organize a conference in 2002, followed by a book,Ecological Consequences of Artificial Night Lighting (Island, 2006), pointing out how far the tendrils of the illuminated night extend.
For the vast majority of organisms — whether human, cockroach or wisp of plankton — the cycle of light and dark is an influential regulator of behaviour. It mediates courtship, reproduction, migration and more. “Since life evolved, Earth has changed dramatically, but there have always been light days and dark nights,” says Christopher Kyba, a physicist at the German Research Centre for Geosciences in Potsdam. “When you change it, you have the worry that it could screw up a lot of things”.
The pace of that change is increasing. Striking images from space over the past two decades reveal the extent to which the night is disappearing. Estimates suggest that more than one-tenth of the planet’s land area experiences artificial light at night1 — and that rises to 23% if skyglow is included2. The extent of artificially lit outdoor areas spread3by 2% every year from 2012 to 2016. An unexpected driver of the trend is the widespread installation of light emitting diodes (LEDs), which are growing in popularity because they are more energy efficient than other bulbs. They tend to emit a broad-spectrum white light that includes most of the frequencies important to the natural world.
The trend has had profound impacts on some species; lights are well known to disorient migrating birds and sea turtles, for example. Scientists have also found that disappearing darkness disturbs the behaviour of crickets, moths and bats, and even increases disease transmission in birds.
The most lethal effects are perhaps on insects — vital food sources and pollinators in many ecosystems. An estimate of the effects of street lamps in Germany suggested that the light could wipe out more than 60 billion insects over a single summer4. Some insects fly straight into lamps and sizzle; some collapse after circling them for hours.
Fewer studies have examined plants, but those that have suggest that light is disrupting them, too. In a study in the United Kingdom5, scientists took a 13-year record of the timing of bud opening in trees, and matched it up with satellite imagery of night-time lighting. After controlling for urban heat, they found that artificial lighting was linked with trees bursting their buds more than a week earlier — a magnitude similar to that predicted for 2 °C of global warming. A study of soya-bean farms in Illinois6 found that the light from adjacent roads and passing cars could be delaying the maturation of crops by up to seven weeks, as well as reducing yield.
Ecosystem effects
Now, the results of some ambitious experiments are coming in. One of the largest is a field experiment in the Netherlands, where eight locations in nature reserves and dark places host several rows of street lamps. The rows are different colours — green, red, white and a control row turned off — and run from a grassland or heath field into a forest7. For six years now, scientists and volunteers have used camera traps to monitor the activity of small mammals; automatic bat detectors to record echolocation calls; mist nets for trapping birds; and nest boxes to assess the timing and success of breeding. Botanists are studying the vegetation underneath the lamps.
Artificial light can also have impacts on ecosystem services — the benefits that ecosystems provide to humans. A study published in Naturelast year found that illuminating a set of Swiss meadows stopped nocturnal insects pollinating plants13. A team led by Eva Knop of the Institute of Ecology and Evolution at the University of Berne, found that insect visits to the plants dropped by nearly two-thirds under artificial light and that daytime pollination couldn’t compensate: the plants produced 13% less fruit. Knop’s team forecast that these changes had the potential to cascade to the daytime pollinator community by reducing the amount of food available. “This is a very important study, which clearly demonstrates that artificial light at night is a threat to pollination,” says Hölker
Light skies
Much of Earth remains free of direct artificial light, but skyglow — light that is scattered back to Earth by aerosols and clouds — is more widespread. It can be so faint that humans can’t see it, but researchers say it could still threaten the 30% of vertebrates and 60% of invertebrates that are nocturnal and exquisitely sensitive to light.
Skyglow “almost certainly” has an impact on biodiversity, Gaston says, because the level is well above the thresholds for triggering many biological responses. And yet, he says, “it’s actually quite hard to do the definitive study”
Bright future
It’s slow, meticulous work, but the field is coalescing as evidence accumulates, says Gaston. “The last two or three years has seen a dramatic improvement in the level of our understanding,” he says.
Nonetheless, there are improvements to make. Even measuring exposure is hard. In the field, the light an organism receives can be difficult to measure; a bird could retreat to the shadow of a nearby tree to avoid illumination, for example. So some scientists have tried strapping light meters to birds to get a better idea of dosage.
As the results seep out, one thing that both frustrates and inspires ecologists is that the remedy is at hand.
Longcore is now gathering published data on how different species, such as shearwaters and sea turtles, respond to different parts of the spectrum, and matching the results to the spectra emitted by different types of lighting. He wants to inform decisions about lighting — for example, which type of lamp to use on a bridge and which at a seaside resort.
Engineers and ecologists know that well-considered lighting can perform its task without “spraying light into the sky”, as Kyba puts it. LEDs can be tweaked to shine in certain parts of the spectrum, to dim and to switch off remotely. “My vision,” says Kyba, “is that in 30 years’ time, the streets will be nicely lit — better than today — but we’ll use one-tenth of the light.”
That would be great news for ecological systems, says Hölker, because darkness is one of the most profound forces to shape nature. “Half of the globe is always dark,” he says. “The night is half the story.”
Nature 553, 268-270 (2018)
doi: 10.1038/d41586-018-00665-7
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