In a new study, researchers have unveiled a major contributor to the decline in nighttime pollinator activity, with human activities being identified as a primary culprit.
The study reveals that nitrate radicals (NO3) in the atmosphere, resulting from the combustion of gas and coal, degrade the scent chemicals of common wildflowers, thus impairing the ability of nighttime pollinators to locate these flowers through smell.
The team was co-led by Jeff Riffell, a professor of Biology, and Joel Thornton, a professor of Atmospheric Sciences at the University of Washington.
The researchers focused on the pale evening primrose (Oenothera pallida), a wildflower native to the arid regions of the western United States.
This flower was chosen due to its white blossoms that emit a scent attracting a diverse group of pollinators, notably nocturnal moths, which are among its most crucial pollinators.
The experts embarked on a detailed analysis of the wildflower’s scent by collecting samples from field sites in eastern Washington. Utilizing advanced chemical analysis techniques, including mass spectrometry, they identified and examined the dozens of individual chemicals that constitute the flower’s scent.
“When you smell a rose, you’re smelling a diverse bouquet composed of different types of chemicals,” said Professor Riffell. “The same is true for almost any flower. Each has its own scent made up of a specific chemical recipe.”
The findings were startling. When exposed to NO3, certain key scent chemicals, particularly monoterpene compounds which moths find highly attractive, were nearly eradicated. This discovery was further explored through experiments with moths, which demonstrated a remarkable ability to detect scents, akin to dogs and far surpassing human olfactory capabilities.
The researchers used a wind tunnel and an odor-stimulus system to simulate the flower’s scent in the presence of NO3, observing a significant decrease in the moths’ ability to locate the scent source.
For instance, the accuracy of the tobacco hawkmoth (Manduca sexta) dropped by 50%, and the white-lined sphinx (Hyles lineata), a primary nocturnal pollinator, could not locate the source at all.
Further field experiments reinforced these findings, revealing a drastic reduction in moth visitation to flowers when their scent was altered by NO3.
Riffell emphasized the impact of NO3 on reducing the “reach” of a flower’s scent, severely limiting its ability to attract nighttime pollinators before the scent becomes undetectable.
The study also compared the effects of daytime versus nighttime pollution on floral scents, concluding that nighttime pollution poses a much greater threat due to the stability of NO3 in the absence of sunlight, which otherwise helps degrade NO3.
Utilizing a computer model to simulate global weather patterns and atmospheric chemistry, the researchers identified regions most at risk of experiencing disruptions in plant-pollinator communication due to NO3 pollution. These areas span across western North America, Europe, the Middle East, Central and South Asia, and southern Africa.
“Outside of human activity, some regions accumulate more NO3 because of natural sources, geography and atmospheric circulation,” said Professor Thornton, who added that natural sources of NO3 include wildfires and lightning.
“But human activity is producing more NO3 everywhere. We wanted to understand how those two sources – natural and human – combine and where levels could be so high that they could interfere with the ability of pollinators to find flowers.”
“Our approach could serve as a roadmap for others to investigate how pollutants impact plant-pollinator interactions, and to really get at the underlying mechanisms.”
“You need this kind of holistic approach, especially if you want to understand how widespread the breakdown in plant-pollinator interactions is and what the consequences will be.”
The study is published in the journal Science.
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