A severe drought in North Dakota in 2021 slashed bumble bee populations by 98%, and recovery has lagged for years, a new study shows. Researchers say the findings highlight that climate-driven drought can disrupt pollinators long after ecosystems appear to rebound. Among species observed in the study, Bombus griseocollis, a common species with a generalist diet, pictured here, recovered more successfully than some other species. (Ansel Oommen, Bugwood.org)

By Ed Ricciuti

Ed Ricciuti

Grasslands bumble bees decimated by drought face years of struggle before they are once again at home on the range, say researchers from North Dakota State University. They evaluated how populations of bumble bees (species in the genus Bombus) in North Dakota’s Prairie Pothole Region bottomed out during an extreme drought in 2021 and had not fully recovered even by 2024.

The investigation, reported in April in the journal Environmental Entomology, revealed a 98 percent drop in bumble bee abundance during the drought on the study site, a stretch of grazing land at a NDSU research facility. After it ended, bumble bee numbers and diversity slowly rebuilt but even three years later had not come back to pre-drought levels, reaching only 32 percent of prior abundance. A few species collected before the drought were not seen at all afterward.

The researchers were able to piece together some of the elements that promote recovery and, conversely, some that don’t.

“We were able to document bumble bee recovery following a major drought and tie the slow recovery trends we observed to specific factors,” says Bethany Roberton, a Ph.D. student and insect ecology research specialist at NDSU and lead author of the study. “Our concern has been that droughts may decrease bumble bee populations and have a negative impact on diversity in our area over time, and this study supports that idea as a possibility.”

The research focused on rangelands, the team writes, because “relatively little is known on how bumble bee communities respond to and recover from drought within rangeland ecosystems.”

Rangelands, with their rich communities of grasses and forbs, are a key but often overlooked foraging and nesting ground for bumble bees. They, in turn, return the favor by promoting abundance and diversity in rangeland floral communities, which feeds back into increasing both qualities in bumble bees.

A severe drought in North Dakota in 2021 slashed bumble bee populations by 98%, and recovery has lagged for years, a new study shows. Researchers say the findings highlight that climate-driven drought can disrupt pollinators long after ecosystems appear to rebound. The study was conducted on a stretch of grazing land (shown here) at research facility at North Dakota State University. (Photo courtesy of Bethany Roberton)

A severe drought in North Dakota in 2021 slashed bumble bee populations by 98%, and recovery has lagged for years, a new study shows. Researchers say the findings highlight that climate-driven drought can disrupt pollinators long after ecosystems appear to rebound. Here, research technician Jaeden Sears uses a sweepnet to survey for bees. (Photo courtesy of Bethany Roberton)

A severe drought in North Dakota in 2021 slashed bumble bee populations by 98%, and recovery has lagged for years, a new study shows. Researchers say the findings highlight that climate-driven drought can disrupt pollinators long after ecosystems appear to rebound. Here, Bethany Roberton, a Ph.D. student and insect ecology research specialist at North Dakota State University and lead author of the study, processes a bumble bee collected in a pasture. (Photo courtesy of Bethany Roberton)

Bumble bee species that recovered a measure of abundance did so starting two years after the drought, the researchers observed. The lag displayed by the bees occurred even though temperatures leveled off from a peak during the drought and the rangeland flower resources they use recovered, even boomed, after shrinking.

Resources used by bumble bees during one season contribute to the success of the next year’s population. The lack of these resources during the drought, the paper suggests, could account for the lag in recovery afterward.

Drought hammers not only bumble bees but the flowers that provide them with food, the researchers say, cutting down not only floral numbers but also variety and the amount of nectar produced. Assessing the relative importance of rangeland floral abundance and growing season weather conditions on bumble bee abundance was one goal of the study.

Flowers started a solid recovery soon after the drought ended, ahead of signs of bumble bee recovery. It may be, the researchers suggest, that despite floral flourishing, a change in species diversity among flowers resulting from drought may have had an impact on bumble bee survival.

“Therefore, future research should examine floral diversity and species composition across survey years in conjunction with abundance,” the researchers write.

Figuring out why some species of bumble bees living on rangelands regrouped better than others was another major goal. Perhaps reflecting the old saw about the rich getting richer, one of the findings was that bumble bee species that were most abundant before the drought recovered the quickest.

The researchers suggest that one reason is because bumble bees tend to have low populations in the first place, so those that are more numerous to begin with have a good leg up on others, which are more susceptible to decline.

A severe drought in North Dakota in 2021 slashed bumble bee populations by 98%, and recovery has lagged for years, a new study shows. Researchers say the findings highlight that climate-driven drought can disrupt pollinators long after ecosystems appear to rebound. Among species observed in the study, Bombus huntii, pictured here in a collection vial, recovered more successfully than some other species. (Photo courtesy of Bethany Roberton)

A severe drought in North Dakota in 2021 slashed bumble bee populations by 98%, and recovery has lagged for years, a new study shows. Researchers say the findings highlight that climate-driven drought can disrupt pollinators long after ecosystems appear to rebound. Among species observed in the study, Bombus griseocollis, a common species with a generalist diet, pictured here in a collection vial, recovered more successfully than some other species. (Photo courtesy of Bethany Roberton)

Species of bumble bees that got a head start on the season, the team hypothesize, “would be more resilient to drought because of their potential to create nesting sites and use early resources.” However, it turned out that the more resilient species typically had later active queens, in June rather than May.

Besides pre-drought abundance, three characteristics appeared linked to species that recovered more rapidly: medium queen body size, later peak queen activity, and a relatively short window (a month-and-a-half or less) between queen and worker peak activity. Large-bodied bees that are not particularly choosy about flowers also may have a better chance of surviving when drought diminishes floral diversity, the researchers say.

Out of the 10 bumble bee species detected in 2018 before the drought, three were found in only small numbers that year and were not found again post-drought, between 2021 and 2024: Bombus nevadensis, B. rufocinctus, and B. impatiens.

Five species, meanwhile, were most common at the site and potentially exhibited higher resiliency to drought conditions. There were B. griseocollis, a common species with a generalist diet; B. fervidus, a declining species; and the three other species that were abundant in 2018, B. borealis, B. ternarius, and B. huntii.

The period during which the study occurred was during a drought that parched not only North Dakota but much of the American West. The paper notes that, while the impact of climate change on bumble bees is still relatively unknown, there is no question that drought associated with warming temperatures decreases bumble bee abundance.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His most recent book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.