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Climate change is putting a deadly squeeze on bumblebee populations worldwide

Tomatoes, sweet peppers, and strawberries are among the plants that benefit from bumblebee pollination

Jeremy T. Kerr

Bumblebees are losing the worldwide battle against climate change. Across North America and Europe, species that live in the South are dying off, whereas bumblebees that reside in the North are failing to migrate to cooler areas, according to a study published today in Science. This means that bumblebee populations are now highly compressed — leaving a void that other wild bee species probably won't be able to fill. Bumblebees are among the only bees that can pollinate flowers throughout most of the year.

"These species are at serious and immediate risk."

"These species are at serious and immediate risk from rapid, human-induced climate change," says Jeremy Kerr, an ecologist at the University of Ottawa in Canada and one of the co-authors of the study. "They just aren’t colonizing new areas and establishing new populations fast enough."

Credit: Melissa Tonge

Bumblebees didn't evolve in the tropics; they actually originated about 5 to 10 million years ago in cool-to-temperate regions of the world, which means that they tend to like a cooler environment. And that's great for humans because, unlike other wild bees, they can pollinate flowers from late April through October. Tomatoes, strawberries and sweet peppers are among the plants that benefit from their particular form of pollination. Unfortunately, their evolutionary history is probably the source of their inability to adapt to warmer temperatures — an entirely different problem from "colony collapse disorder," which only affects honey bees.

Bumblebees originate from cool regions of the world

The researchers looked at bumblebee data that date all the way back to 1901. "We wanted to know if species' northern and southern range limits had shifted through time," says Paul Galpern, a landscape ecologist at the University of Calgary and one of the co-authors of the study. To do that, the researchers used observations of 420,000 bee specimens across North America and Europe — in addition to gathering data about temperature, habitat loss, and pesticide use.

The researchers found that the disappearance of bumblebee populations from hotter and southern regions around the world can be linked to rising temperatures. "Bumblebees have retreated by nearly 300 kilometers, or about 190 miles," in the southernmost areas, Kerr says. "This is a huge loss and it happened very quickly, nearly 5 kilometers per year in southern areas."

Credit: York University

These departures are probably the result of a mass die-off, rather than northern migrations, the researchers suggest. Scientists have observed in other research that rapid warming — particularly the impact of extreme weather events — causes bumblebees to go locally extinct, Kerr says. And it's those local extinctions that are likely leading to a large-scale decline. "Bumblebee species are generally failing to take advantage of warming conditions to colonize new areas," Kerr says. And "we're simply losing the Southern populations."

Bumblebees in the North aren't doing much better. Despite rising temperatures that should have allowed them to move northward, they haven't budged. Instead, some species are moving uphill — a coping mechanism that won't do them much good when they run out of flowery hillsides. Thus, climate change has shrunk the amount of land that bumblebees use, and humans are paying the price.

Bumblebees are "failing to take advantage of warming conditions."

If bumblebees can't pollinate crops, food prices will rise and some crops will become harder to grow successfully, or plentifully. And these impacts are large and immediate, Kerr says. This isn't something that we should start to worry about "at some vague, future time."

"The results aren't entirely surprising — we know that bumblebees thrive in cool climates so we'd expect them to be impacted by climate change," says Dave Goulson, a biologist at the University of Sussex who didn't participate in the study. "But nonetheless this is the first clear evidence that this is happening across many species and across huge geographic scales."

"First clear evidence that this is happening across many species."

The researchers weren't able to "detectably link" other factors, like human land use and pesticide use, to global bee decline, Kerr explains. "We know that such factors can seriously harm pollinator species — there is no doubt about that — but those factors appear to be unrelated to changes along the margins of species ranges in Europe or in North America."

The scientists don't know why bumblebees have been unable to migrate north. It's possible that this has something to do bumblebees' capacity to grow their populations quickly when they arrive in a new location. Bumblebees are generally pretty good at getting around, but for some reason they might not be able to sustain significant growth in new, cooler areas.

Of course, it's also possible that the global temperature increase that the planet has experienced so far isn't large enough to make them move, says Sydney Cameron, an entomologist at the University of Illinois who didn't take part in the study. "It's way too early to say they won't move in the future just because they haven't yet moved."

But if that's not the case, then humans may have to act. Already, researchers are thinking of ways to help bumblebee populations move northward. One possible solution is "assisted migration," a practice that would involve moving various species northward and helping them get established.

Bumblebees "may need human intervention to help them establish new populations."

"Just as human activities cause climate change today, bumblebee species may need human intervention to help them establish new populations in cooler and more northerly environments," Kerr says. That said, scientists shouldn't start moving bumblebees north without thinking about how this might impact species that are already in those areas, he says. Humans could cause a lot of damage if bumblebees start competing with other, equally useful animals. In addition, some bumblebee species — like Europe's buff-tailed bumblebee — are doing pretty well, so moving them wouldn't do much good. "Big global patterns are a step," Cameron says, but scientists need to understand the details, too.

For now, gardeners can help bumblebees by adding wild flowers to their yards, the researchers say. But planting wild flowers might not be enough in the long term. Bumblebees are already battling habitat loss and pesticide use — factors that may place an eventual comeback out of reach.

"It's likely that the combined stresses from all of these pressures will have devastating impacts on bumblebees in the not-too-distant future," Goulson says. "Climate change is beginning to take a toll": a thought that Goulson describes as "depressing."