As the moon rises on a warm evening in early summer, thousands of baby turtles emerge and begin their precarious journey to the ocean, while millions of moths and fireflies take to the skies to begin the complex process of mate-finding.
These nocturnal behaviors, and many other similar behaviors, evolved to take advantage of the darkness of the night. But today they are increasingly threatened by the presence of artificial light.
Essentially, artificial light at night (such as from street lighting) masks natural light cycles. Its presence blurs the transition from day to night and can dampen the moon’s natural cycle. We are increasingly realizing that this has dramatic physiological and behavioral consequences, including altering hormones associated with the day-night cycles of some species and their seasonal reproduction, and altering the timing of daily activities such as sleeping, foraging, or mating. .
The increasing intensity and diffusion of artificial light at night (estimates point to 2-6% per year) makes it one of the fastest growing global pollutants. Its presence has been associated with changes in the structure of animal communities and declines in biodiversity.
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How animals are affected by artificial light
Light at night can both attract and repel. Animals that live next to urban environments are often attracted to artificial light. Turtles can turn away from the safety of the oceans and head inland, where they can be run over by a vehicle or drown in a swimming pool. Thousands of moths and other invertebrates become trapped and disoriented around city lights until they fall to the ground or die without ever finding a mate. Female fireflies produce bioluminescent signals to attract a mate, but this light can’t compete with streetlights, so they too can’t reproduce.
It is estimated that millions of birds are harmed or killed each year because they are trapped in the beams of bright city lights. They are disoriented and bump into brightly lit structures, or are pulled away from their natural migratory routes into urban environments with limited resources and food, and more predators.
Other animals, such as bats and small mammals, shun light or avoid them altogether. This effectively reduces the habitats and resources available to them to live and reproduce. For these species, street lighting is a form of habitat destruction, where a light rather than a road (or perhaps both) cuts through the darkness necessary for their natural habitat. Unlike humans, who can return to their homes and block out the lights, wildlife may have no choice but to leave.
For some species, light at night does offer some advantages. Species that are usually active only during the day can extend their foraging time. Night spiders and geckos often come near lights because they can feast on the multitude of insects they attract. While these species can win on the surface, that doesn’t mean there aren’t hidden costs. Research with insects and spiders suggests that exposure to light at night can affect immune function and health, and alter their growth, development and number of offspring.
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How can we solve this?
There are some practical examples of effective mitigation strategies. In Florida, many urban beaches use amber lights (which are less attractive to turtles) and turn off street lights during the turtle nesting season. On Philip Island, Victoria, home to more than a million shearwaters, many new street lamps are also amber and switched off along known migration routes during the flight period to reduce deaths.
In New York, the Tribute in Light (which consists of 88 vertical searchlights that can be seen nearly 60 miles away) is turned off for 20 minutes to allow disoriented birds (and bats) to escape and reduce the attraction of the structure to migrating animals.
In all cases, these strategies have reduced the ecological impact of nighttime lighting and saved the lives of countless animals.
While these targeted measures are effective, they will not solve yet another global biodiversity crisis. Many countries have outdoor lighting standards and several independent guidelines have been written, but these are not always enforceable and are often open to interpretation.
As an individual, there are things you can do to help, such as:
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default on darkness: only light areas for a specific purpose
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embrace technology: use sensors and dimmers to manage light frequency and intensity
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location, location, location: keep lights close to the ground, screen at the back and direct light below the horizontal
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respect the spectrum: choose low-intensity lamps that limit the blue, violet and ultraviolet wavelengths. Wildlife is less sensitive to red, orange and amber light
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all that glitters: opt for non-reflective finishes for your home. This reduces the scattered light that contributes to the glow of the sky.
In a way, light pollution is relatively easy to fix – we just can’t turn on the lights and let moonlight illuminate the night naturally.
Logistically, this is usually not feasible, as lights are deployed for the benefit of people who are often reluctant to give them up. While artificial light allows humans to utilize the night for work, leisure and play, in doing so we are catastrophically changing the environment for many other species.
If we don’t turn off the lights, there are other management approaches we can take to mitigate its impact. We can limit their number; reduce their intensity and the time they are on; and possibly change their color. Animal species differ in their sensitivity to different colors of light, and research suggests that some colors (amber and red) may be less harmful than the blue-rich white lights that are becoming common around the world.