Light pollution forms 'eco-traps'

Mark Kinver, BBC News 16 Jan 09;

An international team of researchers has found another form of light pollution that could have an adverse effect on wildlife.

The scientists showed that as well as direct light sources, polarised light also triggered potentially dangerous changes in many species' behaviour.

They added that road surfaces and glass buildings were among the main sources of this form of light pollution.

The findings appear in Frontiers in Ecology and the Environment.

Co-author Bruce Robertson, an ecologist from Michigan State University, US, said polarised light from structures within the built environment overwhelmed natural cues that controlled animal behaviour.

"Environmental cues, such as the intensity of light, that animals use to make decisions occur at different levels of severity in the natural world," he said.

"When cues become unnaturally intense, animals can respond unnaturally strongly to them."

As a result, the false cues could create an "ecological trap" for species attracted by the light.

Double vision

Dr Robertson said that water was the primary source of horizontal polarised light in the natural world, and that many animals - including birds, insects and reptiles - had highly developed polarisation vision.

This particular form of light played a key role in the animals' lifecycle, such as finding breeding and feeding sites, he added.

A well documented example is the way that baby sea turtles rely on the direction of starlight and moonlight reflected off the water's surface in order to help them find the ocean when they emerged from their nests.

Yet, there are examples of turtles in urbanised areas heading towards the brighter buildings and street lamps.

Dr Robertson said that expanding urban areas meant that there were more structures and surfaces to confuse wildlife.

"Any kind of shiny, black object - oil, solar cells, asphalt - causes problems," he explained. "The closer they are to wetlands, the bigger the problem."

"Light from the sun is vibrating in all possible directions, but after bouncing off smooth flat surfaces, like water, it only vibrates in the horizontal direction; it has become polarised.

"This is why polarised sunglasses make it easier for us to see on a bright day - they remove only the horizontally polarised light that reflects off water and roads," he told BBC News.

Flying light

The team of US and Hungarian researchers said that more than 300 species of insects were known to use polarised light as the primary source for navigation.

"This is used to search for suitable water bodies to act as feeding/breeding habitats," they wrote.

"Because of their strong signature, artificial polarising surfaces - asphalt, gravestones, cars, plastic sheeting and glass windows - are commonly mistaken for bodies of water."

The team added that "polarised light pollution" (PLP) differed from traditional forms of light pollution, called ecological light pollution (ELP) because it occured at any time during the day.

"Because ELP results from the incidence of visible light at times and places where it does not occur naturally, ELP is predominantly a night-time phenomenon.

"In contrast, PLP can occur during both light and dark cycles in terrestrial environments."

The study also suggested that PLP could also disrupt entire food webs if predators followed their prey into the urban "ecological trap", or if a generation of prey species was wiped out without reproducing.

It added that PLP also had an impact on marine life: "When scattered light passed through the transparent body of small aquatic prey animals, its polarisation signature was altered, increasing the visual contrast.

"Plankton feeders are adept at detecting zooplankton in the water column that would otherwise be transparent."

But this hunting technique, it explained, caused problems for the feeders when they encountered marine litter.

"Underwater plastic garbage is another source of PLP, and may prompt aquatic organisms into consuming inappropriate and dangerous items.

"Turtles commonly ingest plastic, particularly transparent plastic bags, which have a polarisation signature similar to that of prey items they commonly target."

Despite the growing human impact on certain communities of animals, the researchers suggested that the worst examples of PLP could be reduced.

They recommended the use of alternative building materials or employing methods to mitigate the problem, such as adding white curtains to dark windows or adding white markings on road surfaces.

Dr Robertson said the team's findings could offer conservationists an alternative way to deal with problematic species, such as insects destroying trees.

"You may be able to create massive polarised light traps to crash bark beetle populations," he suggested.