While the geographic ranges of many animals are expanding toward the poles in response to warming temperatures, the ranges of North American and European bumblebee species are shrinking, a new study published in the 10 July issue of the journal Science shows.
Not only are bumblebee populations completely disappearing from the southernmost and hottest parts of their ranges, they are also failing to migrate north as would be expected as a result of climate change.
"We stopped dead in our tracks when we observed this because the observation that species are expanding their ranges northward is so common now," said the study's lead author Jeremy Kerr, a professor in the department of biology at the University of Ottawa. "We hope this work will stimulate others to look for more than just positive evidence that species are doing what everyone expects."
Kerr's findings reveal the vulnerability of bumblebees, which play key roles in agriculture, to a warming world — hinting these species may experience more rapid climate-related decline than others.
"It is possible we will need to intervene in a significant way to help them adapt," Kerr said.
"If you imagine a car running out of coolant and blowing steam out of a hood, that's an analogy for what a bumblebee does when it gets too hot."
Jeremy Kerr
The researchers generated a database of geographically tagged observations of 67 European and North American bumblebee species from 1901 to 2010. They then compared changes in individual bee species' northward movements in recent decades against bumblebee activity from 1901 to 1974, when the climate was cooler.
"This study is the first to evaluate a group of bees in this way," Kerr said, referring to the study's long span of observations, including temperature data, at the species' northern and southern range limits. "And it was the first study to do so thoroughly with respect to an array of environmental changes that smaller-scale work has suggested could be smoking guns for pollinator declines globally."
To Kerr's team's surprise, bumblebees in recent, warmer decades didn't shift their ranges north. What's more, their populations disappeared from the southernmost and hottest parts of their ranges — reducing their southern territories by up to 300 kilometers on both continents.
Figuring out why bumblebees are not shifting their ranges northward like many other terrestrial animals will require further evaluation, Kerr said.
"Two of the critical factors that determine whether species can shift their northernmost territories further north in response to climate change are their dispersal capacities and their capacities to grow populations quickly once arrived in a new place," Kerr explained. "Bumblebees are pretty good dispersers, but we suspect they may not have very high population growth rates in these warming, northern areas. That might be the key limitation."
To understand possible causes of range losses in the southern regions — besides climate change — Kerr and colleagues evaluated the roles of factors like land use and pesticide application. They found no significant correlation between these factors and range loss. Having evolved in cooler climates, it seems bumblebees just aren't physiologically capable of adapting to these regions' warmer temperatures, Kerr explained.
"If you imagine a car running out of coolant and blowing steam out of a hood, that's an analogy for what a bumblebee does when it gets too hot," Kerr said at a 8 July AAAS press event at the University of Ottawa.
"Our results suggest we may need to contemplate assisted migration into northern areas across continents and for a substantial group of species," Kerr said. "If successful, it will help bumblebee species maintain themselves even if they continue losing ground from hot and southern areas. But we may also be able to manage bees better in those hotter places by considering whether we can find microclimates that provide refuge from increasingly frequent and extreme heat. Those places would have better shade, perhaps steeper slopes in some areas, and durable water supplies that could keep flowering plants producing nectar."
Kerr noted there are complicated ecological considerations around assisted migration. "We are not suggesting we should drop everything and start engaging in this," he said, "but we do need to have a thoughtful international conversation about whether these efforts are required."
[Credit for related teaser image: Jeremy T. Kerr]