Jan. 9, 2009 -- The glinting surfaces of asphalt roads, shiny cars and dark skyscraper windows are treacherous lures for a variety of creatures that use polarized light to guide their behavior, according to new research.
These lures appear to draw insects and other animals away from similarly reflective rivers and lakes -- to which the animals have an evolved attraction -- trapping them on man-made surfaces where they can't survive or reproduce.
This type of polarized light pollution could be a significant conservation concern, according to the authors of a new study published in Frontiers in Ecology and the Environment.
The problem occurs in organisms that have evolved the ability to see polarized light and use it as a cue for behaviors such as finding a place to live or lay eggs. Water is a natural polarizing surface, while man-made polarizing surfaces include asphalt roads, dark shiny cars, dark windows and black plastic sheeting.
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An unpolarized light beam is made up of waves vibrating in every direction. When such light strikes a polarizing surface, such as a body of water or a road, the waves in the vertical direction are absorbed into the material, while the waves moving parallel to the surface are reflected. The beam of light leaving the surface is then polarized, meaning that the light waves are aligned in one direction.
Because the polarization is often more complete from man-made surfaces than from natural ones, the man-made surfaces are especially entrancing to the creatures.
"It's this giant, super-attractive habitat that happens to be the worst habitat around," said study author Bruce Robertson of Michigan State University's W.K. Kellogg Biological Station in Hickory Corners. "Even when you have a little asphalt road, it's reflecting polarized light better than water," he said.
The effect is best understood for insects such as dragonflies and mayflies that clearly use polarized light to find water.
"If they have a choice between a pond and a road, they'll lay their eggs on the road. They like it better," said Robertson. "We're creating giant bug zappers on our roads."
Water birds, fish, and many other animals also make use of polarized light, but it's not yet clear how their behavior is affected by artificial polarizing surfaces.
"The next stage of research is to quantify if this is going to be a big conservation problem for aquatic insects." Robertson said. The effects of disrupting these insect populations could move up the food chain, affecting fish or birds that feed on them.
"The fact that [artificial polarization] attracts insects and they end up dying is not good," agreed Justin Marshall of the University of Queensland in Australia. "What I'm not sure about is the quantity"-- whether the extent of insect loss is significant.
"Essentially I agree with what they're saying," he added. "We should ... think about ways to prevent this sort of attraction, in the same way that people try to prevent birds from flying into windows."
"There are several ways that you can change the way we build things," Robertson noted.
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