The decline in populations of bees and other flying pollinators in the US and Europe may be due to air pollution destroying the perfumes of flowering plants.

Bees, butterflies and other pollinators are having trouble picking up the fragrance of flowers because the plants' chemicals are blocked by air pollution from power plants and cars.

Investigators from the University of Virginia Department of Environmental Sciences say this could partially explain why wild populations of some pollinators, particularly bees – which need nectar for food – are declining in several areas of the world, including California and the Netherlands.

Dr Jose Fuentes, professor of Environmental Sciences at the University of Virginia and fellow researchers looked at the problem of air pollution destroying floral hydrocarbons.

Floral hydrocarbons provide essential signals to attract pollinators. As soon as they are emitted to the atmosphere, however, these hydrocarbons are destroyed by chemical reactions involving pollutants such as ozone.

Dr Fuentes said that the scent molecules produced by flowers in a less polluted environment, such as in the 1800s, could travel for roughly 1,000 to 1,200 metres, “but in today's polluted environment downwind of major cites, they may travel only 200 to 300 metres.

"This makes it increasingly difficult for pollinators to locate the flowers."

Various studies, as well as anecdotal evidence from farmers and fruit and vegetable producers have shown that populations of bees, particularly bumblebees, and butterflies have declined greatly in recent years in parts of the US and Europe.

Dr Fuentes said it is likely that a vicious cycle has developed where pollinators struggle to find enough food to sustain their populations, and populations of flowering plants, in turn, do not get pollinated sufficiently to proliferate and diversify. He believes air pollution may be a contributing factor.

How Far and How Fast Flower Scents Travel

To investigate the effect of air pollution, he and his team of U.Va. researchers, including Quinn McFrederick and James Kathilanka, created a mathematical model of how the scents of flowers travel with the wind.

They calculated scent levels and distances that scents can travel under different conditions, from relatively unpolluted pre-industrial revolution levels, to the conditions now existing in rural areas downwind from large cities.

Scent molecules produced by flowers are very volatile and they quickly bond with pollutants such as ozone, hydroxyl and nitrate radicals, which destroy the aromas they produce.

Instead of traveling intact for long distances with the wind, the scents are chemically altered and the flowers, in a sense, no longer smell like flowers. This forces pollinators to search farther and longer and possibly to rely more on sight and less on smell.

"It quickly became apparent that air pollution destroys the aroma of flowers, by as much as 90 percent from periods before automobiles and heavy industry," Fuentes said. "And the more air pollution there is in a region, the greater the destruction of the flower scents."

Pollinators Must Search by Sight

Because the distance the floral hydrocarbons emitted by flowering plants can travel before being destroyed by the pollutants in the atmosphere has been reduced to only a few hundred metres, the task of finding pollen is increasingly difficult for bees, bumblebees and butterflies.

“The increased destruction of floral signals in polluted air masses may have important implications for both pollinators and signalling plants, Dr Fuentes said.

“When patches of flowers are further apart than the visual range of pollinators, such as in fragmented landscapes, the loss of scent signals may mean that pollinators spend more time searching for patches and less time foraging.

“This decrease in pollinator foraging efficiency will simultaneously decrease the pollinator's reproductive output and the amount of pollen flow in flowering plants.”

The result could well be a vicious cycle where pollinators struggle to find enough food to sustain their populations, and populations of flowering plants, in turn, do not get pollinated sufficiently to proliferate and diversify.

The study, Air pollution modifies floral scent trails, appears online in the March 2008 issue of the journal Atmospheric Environment.