Polluted flowers smell less sweet to pollinators, research shows
The damage that air pollution can cause is widespread and well known: the chemicals produced by human activities can trap heat in the atmosphere, change the chemistry of the oceans, and harm human health in countless ways.
Now a new study suggests that air pollution may also make flowers less attractive to pollinating insects. Compounds called nitrate radicals, which can be abundant in nighttime city air, seriously affect the scent of the pale evening primrose, reducing the number of visits by pollinating hawk moths. researchers reported in Science on Thursday.
This sensory pollution can have far-reaching consequences, disrupting plant reproduction and reducing the production of fruits that nourish many species, including humans. It could also threaten pollinators, which depend on flower nectar for their livelihoods and are already facing declines worldwide.
“We worry a lot about people's exposure to air pollution, but there is an entire life system 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're really just discovering how deep the effects of air pollution go.”
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, who is now a researcher at the University of Naples.
The research focuses on the pale evening primrose, a plant with delicate flowers that open at night. Among the most important pollinators are hawk moths, which have extremely sensitive scent-detecting antennae. “They're as good as a dog in terms of their chemical sensitivity,” said Dr. Riffell.
The scent of a flower is a complex olfactory bouquet that contains many chemical compounds. To identify the ingredients in the distinctive primrose scent, the scientists attached plastic bags over the flowers, capturing samples of the fragrant air. When the team analyzed these samples in the laboratory, they identified 22 different chemical components.
The scientists then recorded the electrical activity of the moths' antennae when they were exposed to these odor compounds. They found that the moths were particularly sensitive to a group of compounds called monoterpenes, which also help give conifers their fresh, evergreen scent.
The researchers used these attractive aromas to concoct their own simulated primrose scent. They then added ozone and nitrate radicals, both of which can be created when pollutants produced by the combustion of fossil fuels are released into the atmosphere. Ozone, which is formed 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 primrose scent and observed some chemical breakdown, with concentrations of two key monoterpenes dropping by about 30 percent. They then added nitrate radicals to the mixture, which proved much more harmful, reducing these important moth attractants by as much as 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 simulated primrose scent, at one end of a wind tunnel. Moths released on the other side often found their way into the flower.
But when the fake flower gave off a odor degraded by nitrate radicals, the moths hesitated. The flower visit rate for tobacco hawk moths dropped by 50 percent, while white-lined sphinx moths stopped visiting the flower altogether. Adding ozone alone had no effect on the moths' behavior, the researchers found.
The scientists replicated these findings in the wild by placing artificial flowers in primrose plants. Flowers that gave off a pollution-tainted odor received 70 percent fewer hawk moth visits over the course of a night than those that emit an intact odor, the researchers found. That decline would reduce primrose pollination enough to significantly reduce fruit production, they calculated. “The chemical environment plays a very profound role in shaping these ecological communities,” said Dr. Riffell.
The researchers believe the problem extends far beyond the hawk moth and cowslip. Many pollinators are sensitive to monoterpenes, which are common in floral scents. Using computational models, the researchers calculated that in many cities around the world, pollution has reduced odor detection distances by more than 75 percent since pre-industrial times.