The damage that air pollution can cause is vast and well-known: chemicals produced by human activities can trap heat in the atmosphere, change ocean chemistry, and harm human health in myriad ways.
Now a new study suggests that air pollution could also make flowers less attractive to pollinating insects. Compounds called nitrate radicals, which can be abundant in urban nighttime air, severely degrade the odor emitted by the pale evening primrose, thereby reducing visits by pollinating hawk moths, the researchers reported in science on Thursday.
This sensory pollution could have profound effects, interfering with plant reproduction and decreasing the production of fruits that feed many species, including humans. It could also threaten pollinators, which rely on flower nectar for their livelihoods and are already experiencing global decline.
“We worry a lot about human exposure to air pollution, but there is an entire system of life that is also exposed to the same pollutants,” said Joel Thornton, an atmospheric chemist at the University of Washington and author of the new study. “We are discovering just how profound the impacts 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 joint doctoral student, Jeremy Chan, now a researcher at the University of Naples.
The study focuses on pale evening primrose, a plant with delicate flowers that open at night. Its main pollinators include moths, which have extremely sensitive odor-detecting antennae. “They are as good as a dog in terms of chemical sensitivity,” Dr. Riffell said.
The scent of a flower is a complex olfactory bouquet that contains many chemical compounds. To identify the ingredients of primrose’s characteristic scent, scientists tied plastic bags over the flowers, capturing samples of the fragrant air. When the team analyzed these samples in the laboratory, they identified 22 distinct chemical components.
The scientists then recorded the electrical activity of the butterflies’ antennae when they were exposed to these odorous compounds. They found that butterflies were particularly sensitive to a group of compounds called monoterpenes, which also help give evergreens their fresh, lingering scent.
Researchers used these appealing aromas to concoct their own simulated evening primrose scent. Then they added ozone and nitrate radicals, both of which can form when pollutants produced by burning fossil fuels enter the atmosphere. Ozone, which forms in the presence of sunlight, is abundant during the day, while nitrate radicals, which are broken down by sunlight, are more dominant at night.
The scientists first added ozone to the evening primrose scent and observed some chemical degradation, with concentrations of two key monoterpenes decreasing by about 30 percent. They then added nitrate radicals to the mix, which proved much more damaging, reducing these key moth attractants by up to 84 percent from their original levels. They had “almost completely disappeared,” Dr. Thornton said.
To assess the effects on two species of hawkmoths, scientists placed a fake flower, emitting the simulated scent of primrose, at one end of a wind tunnel. The butterflies released at the other end often flew towards the flower.
But when the fake flower gave off a scent degraded by nitrate radicals, the butterflies hesitated. The rate of flower visitation by tobacco hornworms dropped by 50 percent, while white-lined hawkmoths no longer visited the flower at all. Adding ozone alone had no effect on the butterflies’ behavior, the researchers found.
Scientists have replicated these findings in nature by placing artificial flowers in primrose plants. Flowers emitting scent degraded by pollution received 70% fewer hawkmoth visits over the course of a night than those giving off an intact odor, the researchers found. This decline would reduce primrose pollination enough to significantly reduce fruit production, they calculated. “The chemical environment plays a very important role in shaping these ecological communities,” Dr Riffell said.
Researchers believe the problem extends well beyond the hawkmoth and primrose. Many pollinators are sensitive to monoterpenes, common in floral scents. Using computer modeling, researchers calculated that in many cities around the world, pollution has reduced odor detection distances by more than 75 percent since pre-industrial times.