Bumblebees, among the most important pollinators, are in trouble. Fuzzy and buzzy, they excel at spreading pollen and fertilizing many types of wild flora, as well as crucial agricultural crops like tomatoes, blueberries, and squash.
But their numbers are dropping. New research using a massive dataset found that the insects are far less common than they used to be; in North America, you are nearly 50 percent less likely to see a bumblebee in any given area than you were prior to 1974.
Moreover, several once-common species have disappeared from many areas they were once found, becoming locally extinct in those places. For example, the rusty patched bumblebee, which used to flourish in Ontario, is no longer found in all of Canada—in the U.S., it’s endangered.
In a new paper published this week in the journal Science, researchers used a complex modeling process to suggest that their decline is driven in large part by climate change.
Specifically, the scientists found that in areas that have become hotter in the last generation, or have experienced more extreme temperature swings, bumblebees are less abundant. In Europe, they are 17 percent less plentiful than they were in the early 20th century. The scientists examined the abundance of 66 species across the two continents.
The approach suggests “climate chaos” is a primary driver of the drop in bumblebees, says study leader Peter Soroye, a doctoral student at the University of Ottawa.
“These declines are linked to species being pushed beyond temperatures they haven’t previously had to tolerate,” Soroye says. Their disappearance from a region means that they’ve either moved elsewhere or died.
It has long been known that bumblebees are more suited to cold weather, with their fuzzy bodies and ability to generate heat while flying, which often allows them to be the first bees out in the spring. Exactly how vulnerable they are to heat waves and weather fluctuations still isn’t clear for most species, though this study suggests there’s a limit to their adaptability.
And it is indeed warming up. The last five years were the hottest ever recorded in the 139 years that the U.S. National Oceanic and Atmospheric Administration has tracked global heat.
There are several mechanisms at play, says study co-author Jeremy Kerr. The insects can simply overheat, as lab experiments have shown, but there may also be indirect impacts on vegetation and flowers that could lead to the bees starving, he adds.
Bumblebees only live one year at most, and queens often spend the winter in leaf litter or in the ground. Here, they are still vulnerable to shifts in temperature, and things like unusually early ice melts and re-freezes, Kerr says.
The decline is dangerous for the environment since bumblebees’ pollination services are necessary for many flowering plants to reproduce, says Matthew Austin, a Ph.D. student and researcher at the University of Missouri in St. Louis who wasn't involved in the paper.
“As these plants are then used by myriad other organisms, the decline of bumblebees can have cascading ecological [effects] that may collectively cause biodiversity loss.”
There could also be economic costs. By one measure, bees contribute more than $15 billion to the U.S. economy by pollinating crops.
Other drivers of decline
Climate change is not the only factor behind the insects’ decline. They are also threatened by pesticides like neonicotinoids—which are extremely toxic to all bees—destruction of habitat by development and conversion of wildlands into agriculture, the spread of pathogens, and the release of non-native bees for commercial pollination.
“This study will be impactful in drawing scientists’ attention more to the role climate may play in the declines of these bees,” say Heather Hines, a researcher at Penn State University who wasn’t involved in the research. “That said, their data shows that while climate can explain declines to a large degree, it is not the only factor involved in explaining the overall decline in species richness observed over time.”
The authors agree, and note that the paper shows habitat loss was also a driver of local extinction. Kerr stresses that “we’re not arguing against the role of habitat loss and pesticide misuse as [drivers] of decline... we think the case for those things is strong, but just different.”
“What we’re pointing out is that there’s a strong climate change signal,” he adds. “If you ignore the climate change signal, you can’t understand extinction risk clearly.” (Read more: Why insect populations are plummeting—and why it matters.)
“While bees might be able to cope with one stressor alone, the combination of several stressors may bring a population over the tipping point,” says Matthias Becher, an ecologist with Exeter University in the U.K.
Some researchers went further. Jamie Strange, chair of the entomology department at Ohio State University, says that focusing on climate change could be problematic, because it ignores the many other causes of decline.
“My concern is not that their science is wrong, but that this work will draw the focus from some of the issues that desperately need to be addressed to save bee populations,” Strange says, which “are all equally or more pressing than climate change to impacting bee populations worldwide.”
How to help
There is good news, however, Soroye adds.
Since the paper suggests that extreme temperatures can impact the bumblebees, creating more parks or planting trees and shrubs in urban environments—which are often cooler than surrounding built spaces—could give them places to shelter from the heat, he says.
There are also other things people can do to help the bees. (Read how you can support bumblebees in your own backyard.)
Among the easiest are bee-friendly yard practices like planting native flowers that bumblebees can feed on, and avoiding the use of pesticides like neonicotinoids. Creating flower beds that are continuously in bloom can also help, Austin says, as well as waiting until spring to remove leaf litter, a prime denning spot for the insects.