Shining New Light on Light Pollution

Web exclusive

• Laura Tangley
• Feb 01, 2009

SCIENTISTS HAVE LONG known that artificial lights can lead animals astray, causing them to crash into buildings or migrate in the wrong direction, for example. Perhaps the best known case is the threat that faces newly hatched sea turtles on densely populated beaches. Mistaking the bright lights of hotels, condos and streetlamps for moonlight shining on the water, the hatchlings travel inland rather than toward the sea.

Now a new study has shown that, beyond direct sources of artificial light, indirect light reflected off manmade structures also can throw animals off course. This is especially true of structures with dark, smooth surfaces, which cause reflected light to become highly polarized. "In nature, the primary source of horizontally polarized light is water," says Michigan State University ecologist Bruce Robertson. "Biologists discovered in the 1980s that such polarized light is an amazingly reliable cue for finding bodies of water."

Except when that light comes from objects such as dark-colored cars, black plastic sheeting, asphalt or the dark glass surface of buildings. In the recent study, published in the journal Frontiers in Ecology and the Environment, Robertson and his colleagues discovered that dragonflies and other insects, which tend to lay eggs in lakes, ponds and streams, are particularly affected by artificial polarized light. The consequences can be far-reaching. "Aquatic insects are the foundation of the food web," says Robertson, "and what's harmful to them is harmful to entire ecosystems and the services they provide."