UC Davis Research: Climate Change Rapidly Restructuring North American Bumble Bee Communities

Climate change is rapidly restructuring North American bumble bee communities, according to newly published research from the laboratory of Professor Neal Williams, a pollination ecologist in the UC Davis Department of Entomology and Nematology.

The analytical paper, “Warming Summer Temperatures Are Rapidly Restructuring North American Bumble Bee Communities” --authored by ecologist Jeremy Hemberger, a former postdoctoral fellow in the Williams lab, and Professor Williams--appears in Ecology Letters.

“This is one of the first papers to show really substantial shifts in community composition in bumble bees due to climate, but also in insects more broadly,” said Hemberger, now a postdoctoral researcher at his alma mater, the University of Wisconsin, Madison. “We're also able to partition the effect we found to being driven by a loss of cold-adapted species, and a rapid rise in warm-adapted species across North America, but alarmingly we see that, above 50° parallel north, even warm-adapted species are declining.”

The 50th parallel northis a circle of latitude that is 50 degrees north of the Earth’s equatorial plane. It crosses Europe, Asia, the Pacific Ocean, North America and the Atlantic Ocean. At this latitude, the sun is visible for 16 hours and 22 minutes during the summer solstice, and 8 hours, 4 minutes during the winter solstice, according to Wikipedia.

The ecologists documented “an alarming trend suggesting that above 50° N, both cool- and warm-adapted species are declining in relative abundance, indicating that warming temperatures are outpacing the capacity of bumble bee species to respond or adapt.”

“Though additional confirmation is needed, our results suggest that northern bumble bee communities may be in crisis, with significant species turnover and declines in abundance that may threaten the persistence of populations in the coming decades,” they wrote.

“Overall, our work provides strong evidence of the pervasive impacts a warming planet has for insect biodiversity, particularly for historically cool-adapted species,” they related. “It also identifies regions of concern where anthropogenic climate warming is rapidly restructuring the communities of an ecologically important group of insects.”

They used along-term dataset of North American bumble bee species occurrences to determine whether the community temperature index (CTI), a measure of the balance of warm- and cool-adapted species in a community, has increased given warming temperatures. The database of 781,280 records from 1805 to 2020 was derived from a variety of sources, including natural history collections, research studies, and citizen science programs. To match the temporal range of available climate data, they used bumble bee records collected between 1960 and 2018.

“Over the last 29 years across the continent, bumble bee communities increasingly consist of fewer cool-adapted and more warm-adapted species with resultant increases in the community temperature index, a measure of the balance of warm- and cool-adapted species,” they wrote. “Changes are most pronounced at mid- to high latitudes and high elevations in the American Rockies, Intermountain West and central Mexico.”

Their project, launched in 2020 and completed in 2023, focused on 59 species. All California bumble bees were included in the analysis. The authors noted that the long-term, rising summer temperatures are “particularly alarming” to the Western bumble bee, Bombus occidentalis; the two-formed bumble bee, B. bifarius of Western North America; and the forest bumble bee, B. sylvicola, a high-altitude specialist native to North America and widely distributed in Canada.

B. occidentalis is one of the cool-adapted species that is declining. The yellow-faced bumble bee, B. vosnesenskii, native to the West Coast, is one of the "biggest winners” per the analysis.

The most severe responses to climate have tended to be at high latitudes, they said. “For example, regions north of 45° have experienced rapid increases in temperature leading to pronounced phenological shifts across taxa. Our results support this trend, revealing the largest rates of bumble bee CTI change at higher latitudes and high elevation. The bumble bee species in these locations tend to have narrower geographic ranges and be cold-adapted, traits identical to other insect taxa that have exhibited declines due to climate.”

“Alarmingly, our results found that even warm-adapted species are struggling to respond to the pace of warming temperatures at higher latitudes: both cool- and warm-adapted bumble bee species north of 50° N have exhibited significant declines in relative abundance,” they wrote. “This result supports previous work describing the limited capacity of bumble bees to track their northern range limits in accordance with warming temperatures. Though additional confirmation is needed, our results suggest that northern bumble bee communities may be in crisis, with significant species turnover and declines in abundance that may threaten the persistence of populations in the coming decades."

“As far as action items to be taken, obviously finding ways to halt carbon emissions is paramount,” Hemberger said, “but we also need to find ways to support species locally though climate refugia—that includes include heat- and drought-tolerant pollen and nectar sources, and structural heterogeneity to provide microclimate respites during heat waves. Ultimately, we don't know whether the balance of declining and increasing bumble bee species will impact pollination or ecosystem function, but we do know that the species being lost are often important pollinators in both natural and managed ecosystems.”

Hemberger’s grant from U.S. Department of Agriculture’s National Institute of Food and Agriculture (USDA-NIFA) funded the project. Hemberger was a USDA-NIFA postdoctoral fellow in the Williams lab from April 2020 to October 2022. He joined the University of Wisconsin, Madison, in September 2022.

Williams, who joined the UC Davis Department of Entomology and Nematology faculty in 2009, researches pollination biology, bee ecology evolution and behavior and agroecology. He was selected a Chancellor's Fellow in 2015, a five-year program supporting his research, teaching and public service. He was named a a Highly Cited Researcher by Clarivate Analytics in 2018, and a Fellow of the California Academy of Sciences in 2021.

A native of Madison, Wisc., Williams received his doctorate in ecology and evolution from Stony Brook University, New York in 1999.