Bumblebee territory shrinking under climate change

As climate change marches on, bumblebees are losing ground.

Rising temperatures are quickly stamping out bumblebees from their southernmost territories in North America and Europe. Yet surprisingly, many Bombus species are holding steady on their northern borders instead of surging into cooler climes, researchers report in the July 10 Science. 

It’s the first time that scientists have firmly fingered climate change as a widespread cause of shrinking bumblebee habitat. The study raises concerns that populations of these valuable plant and crop pollinators, already facing decline from pesticides and pathogens, will continue to die out and may even disappear in some areas.

“This is something that is not a problem for the future; it’s happening right now and has actually now been going on for decades,” says study coauthor Jeremy Kerr, an ecologist at the University of Ottawa in Canada.

Mining collections from museums and other places, Kerr and colleagues assembled data on more than 423,000 bumblebees caught from 1901 to 2010 in North America and Europe. The bumblebees represented 67 species, around half of the species that live in those regions.

The records also included locations where the bumblebees were caught, allowing the researchers to assess how the insects’ ranges changed in that time.

From 1974 to 2010, some of the bumblebees’ southern borders receded by around 300 kilometers. That’s huge, says Kerr. The rusty-patched bumblebee, Bombus affinis, for instance, once a common sight in gardens in the southern United States, has largely vanished from that area. Some other insect species, such as butterflies, aren’t losing any ground.  

The authors note that during the same time frame, the bumblebees’ native ranges in North America and Europe saw a 2.5 degree Celsius jump in average maximum temperatures. Bumblebees, which are particularly sensitive to heat, seem to be retreating in lockstep with warming, Kerr says. The ranges of mountain-dwelling bumblebee species have also shifted, moving about 300 meters upward in elevation.

Yet, along the northern limits, most of the bumblebees are holding the line, which is “really weird,” says Kerr. Many other species are expanding their ranges northward in response to climate change, he says.

“The results are both intriguing and concerning,” says biologist Scott Hayward of the University of Birmingham in England. To understand why many bumblebees aren’t budging, researchers will need to focus on their biology, which in some ways is “very much a black box,” he says