Scentless Spring? Flower Smells Blocked by Pollution

Brian Handwerk
for National Geographic News
April 11, 2008
Soon it may be harder to stop and smell the roses.

Growing levels of air pollution from power plants and automobiles have reduced the potency of flower fragrances by up to 90 percent as compared with pre-industrial levels in the United States, a new study has found.

The trend is unpleasant for human noses, but may be life-threatening for pollinators such as bees and butterflies.

"Many insects find flowers by detecting the scent produced by those flowers," said study lead author Jose D. Fuentes, an environmental scientist at the University of Virginia in Charlottesville.

"This [pollution] makes it increasingly difficult for pollinators to locate the flowers [and feed on their nectar]."

Flowers also stand to suffer when this symbiotic relationship falters.

If insects can't find enough flower-based food to survive, their movements won't pollinate plant species.

(Read how flowering plants changed the world.)

Overwhelming Ozone

Flowers produce volatile scent molecules that bond with pollutants such as ground-level ozone, in the process breaking down the plants' sweet smell.

With more pollution in the air, the aromatic molecules don't remain potent as long and travel shorter distances on the wind.

The new study's model suggests that in the mid-19th century, when pollution levels were first recorded, scent molecules would have been able to travel some 3,300 to 3,900 feet (1,000 to 1,200 meters).

Today, in the polluted air found downwind of large metropolises, scents may only make it some 650 to 980 feet (200 to 300 meters).

The impact is especially pronounced during high-pollution "code red" days in summer.

"Lots of vehicles are releasing nitrogen oxides," Fuentes said. "When [the gases] are in the presence of sunlight they are converted into these molecules that we call ozone—one of the main pollutants that we find in the eastern U.S. in the summer months.

"Fragrances are overwhelmed by it."

Fuentes and colleagues published their findings recently in the journal Atmospheric Environment.

(Related news: "Japan's Cherry Trees Bloom in Fall; Warming to Blame?" [November 27, 2007].)

Bad News For Bees?

With bee populations dropping dramatically in many parts of the world, could these missing scents be a factor?

Scientists trying to pinpoint the cause of bee declines have variously blamed viruses, mites, bacteria, pesticides, and even cell-phone radiation.

Jay Evans, an entomologist at the U.S. Department of Agriculture's bee research laboratory in Beltsville, Maryland, was intrigued by the new study but hasn't seen bee behavior that suggests trouble with scents.

"Over the last couple of summers I don't think the bees in this area were bringing in much less food," he said.

"It might be that they had to work harder, but it seems like as long as there were bees to collect food they were finding flowers somewhere."

Evans also noted that beekeepers didn't report big drops in their honey yields, which would have occurred had food been harder to find.

But lead author Fuentes fears that the fading smell of flowers may stress insects already faced with an array of other threats.

"The [effects shown in] these studies will simply exacerbate whatever the bees are going through right now," he said.

"It's something that is really worthwhile paying attention to."

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