On a hot summer day, urban areas of the Triangle can be up to five degrees warmer than surrounding rural locations, and the temperature gap grows after the sun sets, as acres of pavement, concrete and steel emit heat absorbed during the day.
The phenomenon is known as the “urban heat island” effect, and a recent N.C. State University study shows that many of North Carolina’s native bee species keep away from hot, urban areas. The study also offers a glimpse at how bees might be affected by rising temperatures due to climate change.
“We’re interested in how urban warming affects the ecology of insects and what implications that might have for understanding how global warming might impact insects outside of the city, “ said Elsa Youngsteadt, an entomology research associate at NCSU and co-lead author of the study.
North Carolina is home to 500 species of bees, both native and naturalized, which are directly linked to the success of its annual $84 billion agriculture industry.
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Domesticated European honeybees are often credited with pollinating crops. But in places like North Carolina, where fields tend to be smaller, native bees have been shown to do much of the pollination work. Bumblebees, mining bees, and mason or orchard bees are crucial for spring-flowering crops such as apples and blueberries, according to crop researchers.
Now scientists want to know how global warming will affect bees and whether it might result in fewer bees buzzing around where they are needed.
Two years ago, April Hamblin, an entomology graduate student at NCSU who carried out the field work and co-wrote the study, began looking at how some of North Carolina’s common native bee species tolerate increasing heat by placing individual bees in small test tubes. The tubes were then warmed slowly until the bee became incapacitated due to heat exhaustion.
She found that carpenter bees were the most heat tolerant, withstanding temperatures up to 124 degrees. Sweat bees and bumblebees, meanwhile, were the least heat tolerant, each becoming incapacitated at just below 113 degrees.
What about outside?
To see whether the lab test is an accurate predictor how bees respond to heat in the real world, Hamblin used urban heat islands to mimic climate change, following bee populations at 18 places around Wake County over two years. The abundance of each species was compared between warmer and cooler sites. It turned out that the bees with a low heat tolerance in the lab, including bumblebees and sweat bees, were less abundant in hotter areas.
“As a group, the bumblebees were consistently lower than average in their heat tolerance, and their populations tended to decline,” said Youngsteadt, adding, “Almost all of the species we looked at declined with warming.”
Sheila Colla, assistant professor of environmental studies at York University in Toronto, said recent studies she was involved with have reached similar conclusions: Bumblebees are especially susceptible to climate change.
Colla noted that The International Union for Conservation of Nature recently assessed North American bumblebees and found that one quarter to one third of the species are at risk of extinction due to climate change and other factors such as habitat loss and disease. Colla, who is active with bumblebee conservation groups, noted that unlike with animals that shift north or to higher elevations, the ranges for bumblebees are simply shrinking.
But bumblebees make up only a fraction of North Carolina’s bees, and the struggles faced by other native species are less clear. Most native bees are not closely monitored, but the available evidence suggests that their populations may be sensitive to warming, too. Adding to the problem, native bees, unlike honeybees, cannot be relocated to protect their populations.
Aside from climate change, the NCSU study brings awareness to all the challenges native bees face in urban areas. Youngsteadt recommends taking steps to reduce urban heat island effects in future development, such as adding more green spaces and green roofs to make urban areas more temperate for bees.
Planting flowering plants, especially native varieties, reducing pesticide use, and leaving natural areas intact are essential as well, she said.
“In addition to temperature, bees have to have floral resources – they need nectar and pollen to eat, they need places to nest,” Youngsteadt said. “And that includes undisturbed ground or cavities for nesting.”
Colla urges people to get involved with citizen science programs such as BumbleBeeWatch.org, which rely on people to snap and submit photos used to help scientists track how climate change is affecting native species throughout their ranges.
The N.C. Wildlife Resources Commission recently identified seven of the state’s struggling bumblebee species as being among the animals and plants in greatest need of conservation under its Wildlife Action Plan. NCSU’s work could help scientists hone those conservation efforts by determining which bee species are most at risk from climate change.
“Climate change is affecting temperatures on a much broader scale,” Youngsteadt said. “But in principle we have shown that by measuring one thing in the lab, you can tell something about how they’re going to respond in the field.”
Jeremy Frieling: 919-829-4826