The study looked at where 67 bumblebee species were found in North America and Europe from 1901 to 2010. Since 1975, bee species have been squeezed north by about nine kilometres a year, and are now locally extinct in the southern 300 kilometres of their ranges, said Jeremy Kerr, lead author of a new paper describing the findings.
"The rates of loss are very rapid and are nearly the same across continents," he said at a news conference organized by Science where the paper is being published today.
The researchers from Canada, the U.S., the U.K., and Belgium analysed a database of 423,000 observations of bees from museum and university collections around the world, along with more recent monitoring programs.
Over the century the bees were tracked, temperatures warmed about 2.5 C in the northern parts of the areas where they lived. While the bees moved to cooler, higher elevations in some areas, they haven't expanded north at all to compensate for their disappearance in the south. That came as a surprise to the researchers, as many other species such as butterflies, birds, fish and other species are moving north rather quickly as the climate changes.
Crushed in 'climate vise'
Bumblebees' lack of aptitude to cope with climate change could put them at risk of extinction if humans don't take action, the researchers suggest. Some species, such as North America's rusty-patched bumblebee, have already gone nearly extinct.
"We already know that extreme heat in southern Europe, for example, has wiped out local populations of some bumblebee species," said Kerr, a biology professor and research chair in macroecology at the University of Ottawa. "In this paper, we show that this mechanism may be operating across two continents to crush bumblebee species in a kind of climate vise."
Kerr and his co-authors are particularly concerned that bumblebees are being squeezed into smaller and smaller geographic areas while their populations are also taking a hit from threats such as habitat loss and pesticides – factors that may kill off individual bees and hives, but don't seem to affect the overall geographic range of the whole population.
"We are essentially hitting them with everything we have in our global change arsenal."
Better pollinators than other bees
The loss of bumblebees is a problem because we rely heavily on bees to pollinate our crops, said Alana Pindar, a postdoctoral researcher at the University of Guelph in Ontario. She co-authored the paper and took part in the news conference, which also included co-authors from the University of Calgary and York University in Toronto.
Wild bees are actually better pollinators than honeybees, and bumblebees are better pollinators than other wild bees because they're active for most of the year, from spring until fall, and can therefore pollinate a huge range of plants. Many other bee species that typically only pollinate a few species of plants for a few weeks of the year.
Bees don't deal well with warmer temperatures, Pindar said, because they evolved in cooler places, away from the tropics.
The researchers aren't sure why the bees aren't moving north, but they believe it's because they're unable to establish new populations fast enough. One bumblebee species that has the rare ability to expand northward is Europe's buff-tailed bumblebee, which builds unusually large colonies, says Kerr.
So what can be done to save the bees? One possibility that the researchers suggest is for humans to physically move some bees north.
David Goulson, professor of biology at the University of Sussex, who was not involved in the research, said he is not convinced that would work.
"Bumblebees are pretty mobile creatures," he said in a statement. "I think it is likely that they could move northwards on their own if suitable habitat were available to move in to."
Nathalie Pettorelli, a research fellow with the Zoological Society of London, and who was not involved in the research, said in order to save the bees, scientists will need to figure out why they can't move north.
"Without this knowledge, efficient mitigation strategies are difficult to identify," she said in a statement.
For now, the researchers behind the study suggest focusing on conservation in areas hit by climate change, but making sure bees have the habitats they need to survive. This could involve:- Preserving natural areas in higher-altitude or cooler "microclimates."
- Planting gardens with bee-friendly plants such as raspberries.
- Ensuring there are places left for wildflowers and bee nesting areas at the edge of farmers' fields.
"Above all," Kerr said, "we must reduce greenhouse gas pollution and come to grips convincingly with the climate change threat."