As the climate changes, plants and animals are on the move. So far, many are redistributing in a similar pattern: As habitat that was once too cold warms up, species are expanding their ranges toward the poles, whereas boundaries closer to the equator have remained more static.
Bumble bees, however, appear to be a disturbing exception, according to a study in Science today. A comprehensive look at dozens of species, it finds that many North American and European bumble bees are failing to “track” warming by colonizing new habitats north of their historic range. Simultaneously, they are disappearing from the southern portions of their range.
“Climate change is crushing [bumble bee] species in a vice,” says ecologist Jeremy Kerr of the University of Ottawa in Canada, the study's lead author. The findings underscore the importance of conserving the habitat the insects currently persist in, says Rich Hatfield, a biologist with the Xerces Society for Insect Conservation in Portland, Oregon, who was not involved in the study. Where bumble bees vanish, the wild plants and crops they pollinate could also suffer.
To see how global climate change is affecting the bees, the researchers amassed a data set consisting of some 423,000 observations, dating back to 1901, of 67 bumble bee species in North America and Europe. Then they mapped large-scale changes in the species' territories and in their “thermal ranges”—the warmest and coolest places the bees live. They also built statistical models to test whether any range shifts were best explained by climate change, or whether two other factors—changes in land cover and the use of pesticides such as neonicotinoids, which have been implicated in smaller-scale bee declines—also played a key role.
Overall, they found that some bumble bees have retreated as many as 300 kilometers from the southern edge of their historic ranges since 1974. The rusty patched bumble bee (Bombus affinis), for instance, has disappeared from parts of the southeastern United States. Southern species are also retreating to higher elevations, shifting upward by an average of about 300 meters over the same time period. Meanwhile, few species have expanded their northern territories. And it turned out that climate change was the only factor that had a meaningful impact on the large-scale range shifts. (Data on pesticide use were available only in the United States, however, and the study did not examine whether populations were growing or shrinking.)
One clue to the importance of climate: Bumble bee ranges began shrinking “even before the neonicotinoid pesticides came into play in the 1980s,” says ecologist and coauthor Alana Pindar, a postdoctoral fellow at the University of Guelph in Canada. She says the retreat from southern territories is “a huge loss for bumble bee distributions” and happened surprisingly quickly. The researchers believe the retreat—and the move to higher elevations—may reflect the fact that bumble bees evolved in cooler climates than many other insects that haven't yet lost ground, and so are especially sensitive to warming temperatures.
More mysterious is their failure to push north. “What we can infer is that temperature in the northern latitudes is not what's limiting their spread,” says Ignasi Bartomeus, a researcher at Spain's Estación Biológica de Doñana in Seville, who was not involved in the study. Differences in daylight or food could hamper a march north, or bumble bee populations may simply be too slow-growing to quickly expand. Many bumble bees form small colonies, Kerr explains, limiting their ability to spread quickly. In contrast, species with high population growth rates are “more likely to be able to establish a new colony that represents a measurable difference in geographic range.” He notes that one outlier in the study, the buff-tailed bumble bee (Bombus terrestris), one of Europe's most common species, is known for its reproductive success and has moved north. The species “is kind of like the dandelion of the bumble bee world,” he says.
So far, says Bartomeus, the most common bumble bee species seem to be the most resilient. But “we have a lot of losers,” he cautions, including species that have specialized habitat requirements. And climate change could further strain species already struggling with dwindling habitat and other pressures, Kerr says. “We're hitting these animals with everything,” he says. “There's no way you can nail a bee with neonicotinoids, invasive pathogens, and climate change and come out with a happy bee.”
The loss of bee species could carry consequences for ecosystems and people. For instance, “plants that like their pollinators to be pretty loyal” could see declines in reproduction, says ecologist Laura Burkle of Montana State University, Bozeman. And given that wild bees help pollinate many crops, “we play with these things at our peril,” Kerr says. “The human enterprise is the top floor in a really big scaffold. What we're doing is reaching out and knocking out the supports.”