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Full Story: Eos (10/28)
Air pollution comes with a host of unpleasant human health effects. New research suggests it’s not great for bees either.
Levels of fine particulate matter in the lower atmosphere affect the degree of linear polarization of sunlight, an important factor in insect navigation. Higher levels of atmospheric particulate matter could therefore make it harder for pollinators such as bees to get around, researchers report in Communications Earth and Environment.
Each light wave contains an electric field that oscillates perpendicularly to the direction in which the light wave is traveling. In linearly polarized light, those electric field oscillations align with each other in a single plane. In unpolarized light, the electric fields of each light wave are oriented in different directions.
Before it hits the atmosphere, sunlight is unpolarized. When light waves hit gas molecules in the atmosphere, however, they bounce off those molecules in such a way that the sunlight becomes partially polarized—meaning some, but not all, of the light waves’ electric fields line up.
“Light waves have to obey [a specific] geometry, and when they get scattered in the upper atmosphere, you get this very predictable pattern,” explained James Foster, a neurobiologist at Universität Konstanz in Germany who was not involved in the new research.
Polarization Navigation
Bees and other insects can use this polarization pattern to orient themselves in relation to the Sun, even when it is covered by a cloud.
The difference between polarized and unpolarized sunlight is undetectable to the human eye (though humans can see differences via tools such as polarized sunglasses or digital camera filters, which filter out specific angles of light). But bees and other insects can use this polarization pattern to orient themselves in relation to the Sun, even when it is covered by a cloud.
However, that polarization can be lost if the light bounces off other material in the atmosphere. Tiny particles of aerosols and pollutants—particularly PM2.5, or particles less than 2.5 micrometers in diameter (about 1/30th the width of a human hair)—scatter sunlight especially well.
Light on a clear, sunny day has a degree of polarization of around 60%–70%, said Yoori Cho, the study’s lead author, an ecologist at Seoul National University in Korea. Once the degree of polarization falls below 15%, insects experience “low-visibility days” in which they can’t rely on the polarized light as a guide.
In the new study, Cho and her colleagues examined the relationship between PM2.5 levels and light polarization at ground level near an air quality monitoring station in Seoul. To determine the degree of polarization of sunlight, they captured images with a digital camera equipped with a polarizer filter. The team found that the degree of polarization decreased as PM2.5 concentration increased, suggesting that rising PM2.5 levels could increase the number of days per year that bees struggle to navigate.
Particulate matter isn’t the only thing that can disrupt polarization patterns, noted Axel Kreuter, an atmospheric scientist at Medizinische Universität Innsbruck in Austria who was not involved with the study. Clouds or dust higher in the atmosphere could scatter sunlight well before it reaches particulate matter closer to the ground. That means that even on days with minimal pollution, the degree of polarization could still be low if other natural particles higher in the atmosphere are scattering sunlight.
Under a cloud-covered sky, “the effect of aerosols is completely negligible,” Kreuter said. The correlation that the researchers observed between PM2.5 levels and the degree of polarization was strongest on cloudless days when PM2.5 levels were very high, he noted.
Far-Reaching Effects
Cho and her colleagues previously observed that bees spent more time foraging for food on days with low air quality caused by dust storms or urban air pollution.
In the new study, the researchers suggest that a lower degree of polarization caused by particulate matter could be one reason for this. They predict more frequent low-visibility days for bees in places expected to experience poor air quality in the coming decades, such as northern India, parts of China, and areas where wildfires are becoming more frequent. Field studies suggest that beekeepers in India and China are already facing problems due to air pollution, Cho said.
“I would like to focus my work on revealing the effects of the reduced [polarization] on the micronutrient contents of fruits that are pollinated by bees.”
Though bees rely heavily on polarized light as a navigational guide, it isn’t the only tool in their orienteering arsenal. Insects have other means of navigating in the absence of polarized light, so it’s unclear how much a lack of polarized light alone would affect them in the wild, said Rickesh Patel, a navigation biologist at Lund University in Sweden who was not involved in the study. Bees can also rely on physical landmarks, the color of light across the sky, and the visual position of the Sun itself, he said. These intersecting factors mean scientists still have more to learn about exactly how much polarization changes may affect bee navigation.
“We really do have to know what exactly are the situations where bees are suffering the most from this loss of polarized light in the sky,” Foster said.
Cho said she plans to expand her research into how bees’ ability to navigate and pollinate food crops could affect food security and nutritional quality. Pollinators already struggle in areas with polluted air, and navigation difficulties could exacerbate the issue, she explained.
“I would like to focus my work on revealing the effects of the reduced [polarization] on the micronutrient contents of fruits that are pollinated by bees,” she said.
—Skyler Ware (@skylerdware), Science Writer
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