In a study published Thursday, a team of researchers has shown how air pollution is altering the sweet smell of flowers and altering the signals that nocturnal pollinators use to find nectar.
The research, published in the journal Science, is the first to show how pollution is causing a chemical chain reaction that degrades the odors that nocturnal insects depend on to find flowers. While light and noise pollution is well known, the work illustrates the little-known but detrimental effect that humans are having on wildlife that rely on smell to make sense of the world.
“There is a growing interest in sensory pollution,” said Jeff Riffell, a biologist at the University of Washington who co-wrote the study. Air pollution, he added, “could have widespread effects on a variety of different ecological processes.”
Smell brings order to much of the animal kingdom. Dogs greet each other with their noses. Salmon return to the streams where they were born to spawn by following their scent. Ants and other insects move around, leaving trails of pheromones on the ground.
Trapped in our own sensory bubble, people are largely unaware of all of this. “We are visual and auditory animals,” Riffell said.
A primrose for any other smell.
A species of insect called the hawk moth, that Riffell and his colleagues studied, uses antennae to sniff out the scent of flowers from more than a kilometer away. Once they reach a patch of wildflowers, the moths beat their wings so quickly to float in the air and suck nectar that they are often mistaken for hummingbirds.
One of her favorites is the pale evening primrose, a ghostly white flower that blooms at night. In the field in eastern Washington state, the research team placed the flowers in bags to test their scent. In the lab, they isolated the individual chemicals that made up the bouquet.
Up close, “it’s an intense smell,” said Joel Thornton, another University of Washington researcher who worked with Riffell on the research. But moths can detect it from afar.
“It’s totally fascinating that they can detect such low concentrations,” Thornton said. Atmospheric chemists like him build expensive equipment to detect small concentrations of aromatic compounds, but “biology has already done it,” he said.
Pollution, however, can alter that spectacular sense of smell.
The team discovered that certain aromatic chemicals that moths found attractive degrade in the presence of NO3, a potent pollutant formed from emissions from cars, coal plants and other industrial sources. NUMBER 3 it accumulates in the air at night because it is destroyed by sunlight.
In laboratory experiments, two moth species, the white-lined sphinx and the tobacco hawkmoth, struggled to fly toward the scent of primrose when subjected to levels of NO3 typically seen at night in cities. When the researchers returned to the field and treated the scent of artificial and real flowers with NO3, visits from wild moths were reduced by about 70 percent.
“The flowers only bloom once a night,” Riffell said. “Without their pollinators, there can be a big change in the plant community.”
Robbie Girling, a chemical ecologist at the University of Southern Queensland in Australia, who was not involved in the research, said the study is “the first in this area” of the effects of nighttime pollution on pollinators.
“It’s really exciting and also a little scary to see the results,” he said.
In its own experimentsGirling found that filling a field with diesel exhaust and ozone reduced the number of times bees and other daytime pollinators visited black mustard flowers.
Noting that the moth study involved only a handful of species, “to extrapolate that more broadly, you have to be very cautious,” Girling said. But she added that it is likely that air pollution “could have broader impacts” on ecosystems around the world.
“We’re at a very early stage of really understanding how important the problem is,” Girling said.