The damage that air pollution can cause is extensive and well-known: Chemicals produced by human activities can trap heat in the atmosphere, change the chemistry of the oceans, and harm human health in a myriad of ways.
Now, a new study shows that air pollution can also make flowers less attractive to pollinating insects. Compounds called nitrate radicals, which can be abundant in nighttime urban air, severely degrade the scent emitted by the pale evening primrose, reducing visits by pollinating hawkmoths, researchers reported in Science on Thursday.
This sensory pollution could have far-reaching effects, hindering plant reproduction and reducing the production of fruits that feed many species, including humans. It could also threaten pollinators, which rely on flower nectar for sustenance and are already experiencing global decline.
“We’re very concerned about human exposure to air pollution, but there’s a whole system of life out there that’s also exposed to the same pollutants,” said Joel Thornton, an atmospheric chemist at the University of Washington and an author of the new study. “We’re really just discovering how profound the effects of air pollution are.”
The project was led by Dr. Thornton. his colleague Jeff Riffell, a sensory neurobiologist and ecologist at the University of Washington. and their mutual PhD student, Jeremy Chan, who is now a researcher at the University of Naples.
The study focuses on the pale evening primrose, a plant with delicate flowers that open at night. Its primary pollinators include hawkmoths, which have highly sensitive odor-detecting antennae. “They are as good as a dog in terms of their chemical sensitivity,” said Dr. Riffell.
The scent of a flower is a complex olfactory bouquet containing many chemical compounds. To identify the components in the primrose’s characteristic scent, scientists attached plastic bags over the flowers, capturing samples of the scented air. When the team analyzed these samples in the lab, they identified 22 different chemical components.
The scientists then recorded the electrical activity of the moths’ antennae when they were exposed to these aromatic compounds. They found that the moths were particularly sensitive to a group of compounds called monoterpenes, which also help give conifers their fresh, evergreen smell.
Researchers used these attractive scents to create their own primrose fragrance. Then they added ozone and nitrate radicals, both of which can form when pollutants produced by burning fossil fuels enter the atmosphere. Ozone, formed in the presence of sunlight, is abundant during the day, while nitrate radicals, which are degraded by sunlight, are more predominant at night.
The scientists first added ozone to the primrose scent and observed some chemical degradation, with concentrations of two key monoterpenes dropping by about 30 percent. They then added nitrate radicals to the mix, which proved far more damaging, reducing these key moth attractants by up to 84 percent compared to their original levels. They were “almost completely gone,” said Dr. Thornton.
To assess the effects on two species of hawk moths, the scientists placed a fake flower, emitting the scent of primrose simulation, at one end of a wind tunnel. Moths released at the other end often found their way to the flower.
But when the fake flower gave off a scent that was degraded by the nitrate roots, the moths faltered. The rate of flower visits for tobacco hawk moths was reduced by 50 percent, while white-lined sphinx moths no longer visited the flower at all. Adding ozone alone had no effect on the moths’ behavior, the researchers found.
Scientists replicated these findings in the wild by placing artificial flowers on primrose plants. Flowers emitting a scent degraded by pollution received 70 percent fewer hawkmoth visits overnight of those that give off an intact aroma, the researchers found. This decline would reduce primrose pollination enough to significantly reduce fruit production, they estimated. “The chemical environment plays a really profound role in shaping these ecological communities,” said Dr. Riffell.
Researchers believe the problem extends far beyond the hawk moth and primrose. Many pollinators are sensitive to monoterpenes, which are common in flower scents. Using computer modeling, the researchers estimated that in many cities around the world, pollution has reduced scent detection distances by more than 75 percent since pre-industrial times.
