What do sky scrapers have in common with ponds? The way they polarise light acts as a magnet to some animals, say researchers.
This unwanted side effect from some industrial materials – including road surfaces and automobiles – is called polarised light pollution, and Bruce Robertson of Michigan State University says the phenomenon is widespread enough that it is disrupting ecosystems.
When light bounces off smooth, dark surfaces it becomes polarised – meaning the light wave is aligned in one plane.
In natural environments, this most commonly happens around water, but humans excel at making smooth surfaces. “Cars, asphalt, oil pools, and windows polarise light more strongly than water,” says Robertson.
To animals tuned to distinguish polarised light and use it as an environmental cue, “these objects look more like water than water,” he says. “Even when given the choice between water and human-made surfaces, some insects prefer to lay their eggs on – and settle near – the latter.”
To begin to assess the effect of this newly identified form of pollution, Robertson and colleagues have gathered examples of how human-made surfaces are, or could be, disrupting the life cycles of some species.
They say roads and other surfaces are “ecological traps” for insects that are more attracted to these artificial surfaces than they are to water. Such traps are forcing some species to evolve to life in the new, artificial conditions.
The team say at least 300 aquatic insects are likely to be affected by polarised light. Male dragonflies, for example, often perch on car antennas, attracted to the reflective car paint. Female aquatic insects have been found to lay their eggs on car bonnets, cement floors, and roads – where they have no chance of hatching.
The effects are likely to cascade up the food chain, says Robertson. According to the researchers, the attraction of roads and black plastic sheets to insects causes these objects to function, effectively, as bird feeders. Previous work has shown that some bird species hunt insects around such surfaces.
“We can speculate on the more indirect effects of polarised light pollution – like reducing food availability for fish and amphibians that prey on aquatic insects – but the evidence is still lacking on community level effects,” says Robertson.
He and his colleagues say that although the problem has been largely ignored so far, it is a relatively easy one to fix.
Roads would polarise light less if the asphalt had more gravel, making them rougher, and buildings can be built of less reflective materials. If the ecologists have their way, dark glass-covered sky-scrapers could soon go out of fashion.
Journal reference: Frontiers in Ecology and the Environment (DOI: 10.1890/080129)