How artificial lights affect animals

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As the Moon rises on a warm early summer evening, thousands of baby turtles emerge and begin their precarious journey to the ocean, while millions of moths and fireflies take flight to begin the complex process of finding a partner.

These nocturnal behaviors, and many others like them, have evolved to take advantage of the darkness of the night. However, today they are increasingly threatened by the presence of artificial lighting.

Basically, artificial light at night (like that from street lamps) masks natural light cycles. Its presence blurs the transition from day to night and can dampen the Moon’s natural cycle. Increasingly, we are realizing that this has dramatic physiological and behavioral consequences, including altering the hormones associated with some species’ day-night cycles 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 suggest 2-6% per year) makes it one of the fastest growing global pollutants. Its presence has been linked to changes in the structure of animal communities and biodiversity decline.

How animals are affected

Light at night can both attract and repel. Animals living near urban environments are often attracted to artificial lights. Turtles may turn away from the safety of the oceans and head inland, where they may be run over by a vehicle or drown in a pool. Thousands of moths and other invertebrates are 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 cannot compete with street lighting, so they may also not reproduce.

Each year, we estimate million birds are injured or killed because they are trapped in the beams of city lights. They are disoriented and crash into brightly lit structures, or are pulled away from their natural migration routes in urban environments with limited resources and food, and more predators.

Other animals, such as bats and small mammals, fear lights or may avoid them altogether. This effectively reduces the habitats and resources they have to live and reproduce. For these species, street lighting is a form of habitat destruction, where a light rather than a road (or possibly both) cuts through the darkness necessary for their natural habitat. Unlike humans, who can return home and block out the lights, wild animals may have no choice but to leave.

For some species, night light offers certain advantages. Species that are typically only active during the day may extend their foraging time. Nocturnal spiders and geckos frequent the areas around the lights as they can feast on the multitude of insects they attract. However, while these species may gain on the surface, that doesn’t mean there aren’t hidden costs. Research on insects and spiders suggests that exposure to light at night can affect immune function and health and change their growth, development and number of offspring.

How can we fix this?

There are concrete examples of effective mitigation strategies. In Florida, many urban beaches use amber-colored lights (which are less attractive to turtles) and turn off the streetlights during turtle nesting season. On Philip Island, Victoria, home to over a million short-tailed shearwaters, many new streetlights are also orange and are deactivated along known migration routes during the fledging period to reduce fatalities.

In New York, the Tribute in Light (composed of 88 vertical projectors visible from nearly 100 km away) is off for periods of 20 minutes to allow disoriented birds (and bats) to escape and reduce the attraction of the structure to migrating animals. Either way, these strategies have reduced the ecological impact of nighttime lighting and saved the lives of countless animals.

However, while these targeted measures are effective, they do not solve what could be a new global biodiversity crisis. Many countries have outdoor lighting standards and several independent guidelines have been written, but these are not always applicable and often subject to interpretation.

As an individual, there are things you can do to help, such as only lighting areas for a specific purpose and using sensors and dimmers to manage the frequency and intensity of lighting. It is best to keep the lights low to the ground, the shield to the rear and the direct light below horizontal. Choosing low-intensity lights that limit blue, violet, and ultraviolet wavelengths and non-reflective finishes for your home can also help.

In a sense, light pollution is relatively easy to fix – we just can’t turn on the lights and allow the night to be naturally lit by moonlight.

Logistically, this is usually not feasible as the lights are deployed for the benefit of humans who are often reluctant to give them up. However, while artificial light allows humans to exploit the night for work, recreation and play, in doing so we are catastrophically altering the environment for many other species.

In the absence of turning off the lights, there are other management approaches we can take to lessen their impact. We can limit their number; reduce their intensity and duration of ignition; and, potentially change their color. Animal species differ in their sensitivity to different colors of light and research suggests that certain colors (ambers and reds) may be less harmful than the blue-rich white lights that are becoming commonplace around the world.

-The conversation

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