Our warming world is changing the ranges of species everywhere. Some are being driven northward, some are expanding up in elevation, and some are increasingly overlapping with others. But it’s a different story for bumblebees: Their ranges are simply shrinking. According to a new Science paper, these important North American and European pollinators are failing to migrate north, and they’re losing hundreds of kilometers of range on both continents.
"Global warming has trapped bumblebee species in a kind of climate vise," University of Ottawa’s Jeremy Kerr says. They’re being crushed as climate change compresses their geographic ranges. "The scale and pace of these losses are unprecedented," he adds.
Kerr and a large international team arrived at this alarming conclusion after they developed a database of 423,000 geo-tagged observations of 67 bumblebee species from 1901 to 2010. Because the climate was cooler from 1901 to 1974, this period was considered a baseline. "The bumblebees that are in decline were doing fine 50 years ago," York University’s Sheila Colla explains. "We're talking about large changes in community composition of essential pollinators over just a few decades."
The team then compared baseline bumblebee activity to changes in each of the species’ northern and southern limits, warmest and coolest temperatures occupied, and mean elevation during these last few decades. Unlike many other terrestrial animals heading for the poles, bumblebees didn’t shift their ranges northward during these recent, warmer decades in either of the two continents.
At the same time, bumblebee populations have vanished from the southernmost parts of their ranges, enduring range losses of 300 kilometers (186 miles) in North America and Europe. "Climate change may be making things too hot for them in the south, but is not pulling them north as expected," University of Calgary’s Paul Galpern says. "We don't know for sure what is causing a stagnation at the northern end of things. Bees should be able to start new colonies in places they did not historically occupy." It might be because bumblebees evolved under cool conditions.
When the team examined the impacts of factors other than climate change – such as land use changes and pesticides – they found no major correlation with this range compression and the failure to pick up territory in the north. However, they did find that bumblebees are relocating upslope to higher elevation habitats. "But, eventually, they may simply run out of hill," Leif Richardson from University of Vermont adds.
Informational graphic related to shifting ranges of bumblebees in “Climate change impacts on bumblebees converge across continents,” by Kerr et al. The green represents other plants and animal species. Ann Sanderson, Sheila Colla