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The City Dark

Premiere Date: July 5, 2012

'The City Dark' in Context

Effects of Light on Ecology

The City Dark explains that astronomers are not the only ones losing the night. Biologists along the Florida coast trace the death of thousands of hatching sea turtles to their disorientation by coastal lights. Volunteers at the Field Museum in Chicago collect from city sidewalks thousands of dead birds, about one-third of which are victims of light-induced disorientation and collisions into lit buildings.




As Chad Moore of the National Park Service explains, "When we add light to the environment it has the potential to disrupt habitat, just like running a bulldozer over the landscape can."

While our understanding of the full spectrum of ecological consequences of light pollution is limited, we do know that artificial light at night contributes to disruption of natural behaviors. Some less recognized implications include the interruption of reproduction, foraging and inter-species communication. These are all light-sensitive cycles that are disrupted during unexpected periods of attraction to or repulsion from artificial light sources. For example, sockeye salmon fry stop swimming downstream when exposed to any light above 0.1 lux (the standard of measurement for lighting) and often end up in low-velocity waters near shore, which brings them into close contact with predators. Researchers believe this phenomenon explains the recent decline in the sockeye salmon population in Cedar River, Washington, a location that is exposed to both direct light and sky glow.

Many species of slow-flying bats, such as the horseshoe bat, have increasing difficulty procuring food. Bats are genetically predisposed to avoid light, and now that the hours of darkness are limited, they don't have as much time to seek out food. Also, bats eat insects, which tend to swarm around lights, so fewer are left to be caught in the dark. Other species are affected, too. For example, when days were extended to 16 hours due to artificial light, white-tailed bucks began feeding two weeks earlier than usual and weighed 20 pounds more than previously at winter's end.

Territorial singing in birds is also changing. While northern mockingbirds have evolved to sing only shortly before sunrise during mating season, they have recently been observed singing at night in artificially lit areas. When exposed to high levels of artificial light, American robins will often initiate their morning songs earlier, sometimes up to 100 minutes before the onset of dawn. According to Wildlands CPR's quarterly journal, prolonged singing could result in higher energy demands, greater predation risk and earlier yearly feeding times. These changes may become serious for threatened and endangered species.

As awareness of the danger of artificial light to sea turtles grow, an increasing number of communities are restricting coastal illumination. Countries all over the world have passed ordinances that control the amount and type of light used in coastal environments. As the list grows, hatchling sea turtles are starting to be able to find the sea without the help of human volunteers to guide them. Learn more about local and regional action by visiting www.seaturtle.org. (International Dark Sky Association)

Photo caption: Millions of birds collide with buildings each year.   Credit: Wicked Delicate Films LLC.

Sources:
» Coastal Carolina University. "Does Light Intensity Influence Song Output by Northern Mockingbirds?"
» Ecological Consequences of Artificial Night Lighting, ed. by Catherine Rich and Travis Longcore. Washington, D.C.: Island Press, 2005.
» International Dark-Sky Association
» "Street Lights Pushing Slow Bats Towards Extinction." The Telegraph, June 19, 2009.
» University of British Columbia. "Light Pollution: Environmental Effects of Roadway Lighting."
» Wildlands CPR. "Effects of Artificial Lighting on Wildlife."



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