The effects of global warming are shrinking the geographic home range of North American and European bumblebees, and the insects appear unable to adapt to the changing conditions — a troubling discovery for an important group of pollinators critical to the world’s food supply.
Unlike other animal species, the bees are not migrating northward where it is cooler, and their failure to do so is prompting dramatic losses of bumblebee species from the hottest areas across two continents, according to a study published Thursday in Science, the journal of the American Association for the Advancement of Science.
“Global warming has trapped bumblebee species in a kind of climate vise,” said Jeremy Kerr, professor of biology at the University of Ottawa and one of the study authors. “For species that evolved under cool conditions, like bumblebees, global warming might be the kind of threat that causes many of them to disappear for good.”
For species that evolved under cool conditions, like bumblebees, global warming might be the kind of threat that causes many of them to disappear for good
This could prove disastrous for the planet’s ecosystems and for the human agriculture enterprise, as bees — including bumblebees and honeybees — are vital pollinators of crops and wild plants. Their decline is especially worrisome, given that bees are responsible for pollinating an estimated one-third of the food that humans eat.
“Anyone who has eaten a tomato should thank a bumblebee,” Kerr said. “Bumblebees are essential pollinators for tomatoes and contribute to pollination for many other species, like clover — great for livestock, for instance — strawberries, blueberries, cherries, different kinds of nuts, sunflowers, and the list goes on and on.”
Kerr and his colleagues found that bumblebees have not shifted their geographical ranges toward the polar region or higher elevations in response to increasing temperatures, resulting in range losses of up to 300 kilometers (about 186 miles) in both North America and Europe. It’s a finding that raises disturbing questions about the bees’ ability to survive if the planet continues to warm.
“Climate plays a strong role in affecting where species are found,” Kerr said. “That’s why there aren’t palm trees in the Arctic, for example. When climate changes, the limitations on species’ ranges drop, and they can move to new areas that might have been too cold in the past.”
There are at least two key processes — population growth and the ability to disperse — that determine how quickly species do this, Kerr said.
“Species do better when they can grow their populations very quickly,” he said. “Also, if they can move a long way, they have a better chance of keeping up with shifting climatic conditions. Bumblebees are pretty good dispersers, but we suspect that they may have a very hard time growing their populations quickly enough in the uncertain, northern climates to get properly established. The result is that over the last 35 years or so, the general trend is that they have been stuck along their northern frontiers.”
To investigate bumblebees’ responses to climate change, Kerr and his colleagues first generated a database of geotagged observations of 67 European and North American bumblebee species from 1901 to 2010. They compared changes in individual bee species’ northward movements in recent decades, against baseline bumblebee activity from 1901 to 1974, when the climate was cooler.
To their surprise, bumblebees in recent, warmer decades didn’t shift their ranges north. Simultaneously, bumblebee populations disappeared from the southernmost and hottest parts of their ranges, with bumblebees in those locations moving to higher, cooler elevations, where possible.
Increasingly frequent weather extremes, like heat waves, can hit bumblebee species hard
The researchers also evaluated the roles of factors besides climate change — like land use and pesticide application — in causing bumblebee range losses and found no significant correlation.
“Bumblebee disappearances from warm, southern areas are just as likely when there is no pesticide use and little agriculture,” Kerr said. “But we know that increasingly frequent weather extremes, like heat waves, can hit bumblebee species hard, and climate change poses threats that are already being felt.”
David Inouye, professor emeritus of biology at the University of Maryland, studies the impact of climate change on the environment but was not involved in the study. He said he believes the research provides important new insights into the challenges climate change poses for pollinator species.
“If additional work supports their conclusion, the outlook is not good for this important group of pollinators,” he said. “We know from long-term census data in the Colorado Rocky Mountains that some bumblebee species are moving up in altitude in response to the change climate, but perhaps the shorter distances involved for queens to fly facilitates this kind or response, in contrast to the long-distance movements required to change latitudinal distributions.”
Berry Brosi, assistant professor of environmental sciences at Emory University, who also was not involved in the study, agreed.
“While changing climate has moved bumblebees’ southern range boundaries northward, they have not been able to compensate for this by a northward expansion of their northern range limits,” he said. “There have been some well-documented range contractions in bumblebee species, but this study is the first to my knowledge to show that these range contractions are geographically biased, and have not included concomitant expansions into areas that would seemingly provide good habitat. This finding is troubling, as it implies a lack of resilience of bumblebee communities to climate change.”
Pollinators and plants already appear stressed by climate change. As science is increasingly showing that having a diverse group of pollinators is important for both crops and wild plants, Brosi said that knowing about how climate change will affect bumblebees and other pollinators is particularly important.
“Plants and the animals that pollinate them rely on different environmental cues to tell them when to bloom for plants or emerge from overwintering from pollinators,” he said. “These mis-matches in timing are already throwing some plants and pollinators off. If bumblebees and other important pollinators cannot fluidly change their geographic ranges to keep pace with climate change, that could have further negative implications for the pollination of crops and wild plants.”
[Bumblebees] matter intrinsically; they are beautiful and enrich the world we live in, and they matter practically in terms of food security and the economics of agriculture
Society must try to find ways to mitigate the damage by helping these species establish new colonies, Kerr said.
“In northern and cooler areas, we need to have a thoughtful, international discussion about whether we should be helping bumblebee species establish colonies in areas further north, called ‘assisted migration,’” he said. “This would help bumblebee species maintain their geographical ranges and might lower the risk of their extinction. In southern, warm areas, we need to identify and protect places which have cooler microclimates and better water availability.”
Wild bumblebees should not be confused with managed honeybees, although both are important players in pollination. In recent years, honeybees have been suffering major losses, partially due to the still mysterious Colony Collapse Disorder (CCD) but also from stressors like varroa mites. Bumblebees, on the other hand, seem unaffected by CCD, Kerr said.
“Bumblebees have a different kind of colony that is way smaller and that usually doesn’t last more than one year,” he said. “They are always being renewed by new queens at the end of each year. So, the underlying biology with bumbles is a bit different.”
James P. Strange, a research entomologist with the Department of Agriculture’s agricultural research service, believes it is unlikely that climate threats to bumblebees will have a damaging impact on the food supply at this point, given that some species are still doing well.
“That doesn’t mean that it is not an ecological tragedy, but we will still be eating blueberries in the future,” he said.
Still, the threat that climate change presents to bumblebees is alarming, Kerr said.
“Bumblebees matter in many ways,” he said. “They matter intrinsically; they are beautiful and enrich the world we live in, and they matter practically in terms of food security and the economics of agriculture. We need the bees, and not just honeybees. We need wild bees at least as much and maybe much more, because wild bees keep wild plants producing fruits and seeds.”
Furthermore, “pollination is an ecosystem service we need to keep,” Kerr added. “Losing bumblebee species from some areas means we are losing options to secure food production. We should not be cavalier about the life support systems we rely on. There is no backup plan if ecosystem services are impaired by our own disregard for our well-being.”
Marlene Cimons, a former Los Angeles Times Washington reporter, is a freelance writer who specializes in science, health, and the environment.