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Study of bumblebee decline points to climate ‘vise’

Southern habitats getting too hot, but bees not moving north

A study of the shrinking range of bumblebee habitat suggests farmers relying on those species to pollinate crops may soon have to look elsewhere, barring a reversal of climate change — or, perhaps, an “assisted migration.”

An international study of specimens from 31 species of bumblebees in North America and 36 such species in Europe, as curated between 1901 and 2010 in museums and labs, finds the bees being slowly forced out of their southernmost habitats by climate change — but unable or somehow unwilling to push further north.

The study, results of which were publicly released Thursday and published in the journal Science, looks into the database of about 423,000 geo-referenced bee observations and finds “rapid declines” in bumblebee species, in “often indistinguishable” patterns, on both continents.

“I’d suspected some (bumblebee species) may be declining, but not such a large proportion,” bee expert, study contributor and York University professor Laurence Packer said in a release. “The fact that at the northern edges of their ranges they are not moving north as the climate changes is actually really quite worrying.”

“For the North American species that I work on, we know that about a third of them are in decline and in some cases this has been quite dramatically, more than 90 per cent,” York University environmental studies professor Sheila Colla said in the same release.

For example, she said, where the rusty-patched bumblebee was the fourth most common species in southern Ontario in the 1970s and early 1980s, she has only seen two in 10 years despite “extensive” searching throughout its range in Canada and the U.S.

“One of the scariest parts of the work that I’ve done is just realizing how quickly the situation is changing,” she said. “The bumblebees that are in decline were doing fine 50 years ago. We’re talking about large changes in community composition of essential pollinators over just a few decades.”

While the study contributors don’t let agricultural pesticides off the hook as a danger to bee health, they emphasized during a conference call that they don’t believe their findings are a result of pesticide use, nor of changes in land use over time.

For example, comparing their data against U.S. data on neonicotinoid pesticide application, the researchers found losses in bumblebee range began before neonics came into wide use.

Rather, lead author Jeremy Kerr of the University of Ottawa said on the call, the study shows a “mechanism operating across two continents to crush bumblebee populations in kind of a ‘climate vise.’

“We’ve lost about 300 km from the ranges of bumblebees in southern Europe and North America,” he said. “The scale and pace of these losses are unprecedented.”

The researchers “don’t know for sure what is causing a stagnation at the northern end” of the habitat range, Paul Galpern of the University of Calgary said in a separate release.

“Bees should be able to start new colonies in places they did not historically occupy. But we don’t know why this is happening so slowly that it looks like the (northern) ranges are not moving at all.”

One theory considers the evolutionary difference between bumblebees and other insects. Many species, such as butterflies, originated and diversified from tropical climates, and thus are more likely to adapt. Bumblebees instead have “unusual evolutionary origins” in the cool-to-temperate Palearctic.

The study also shows its selected bumblebee species shifting to areas of habitat at higher elevation within their usual geographic range.

“Moving upslope doesn’t necessarily mean they’ve lost area there yet,” said study participant Leif Richardson of the University of Vermont, “but eventually, they may simply run out of hill.”

Governments and regulators will need to help manage threats to pollinators generally, and to co-operate internationally to stem the rate of climate change by reducing greenhouse gas emissions, the researchers said.

Also, Kerr said, an “assisted migration” — active intervention to encourage bumblebees to expand their northern ranges — may need to be considered.

The study authors grant the assisted migration idea is “controversial” in conservation biology circles, where it’s already been considered for over a decade, but is “gaining support as warming continues.”

Such a move, Kerr said, must not be attempted without first carefully considering the potential impacts on species already present in those northern reaches.

Galpern, an assistant professor of landscape ecology, said the study shows “an important service to ecosystems” in general under threat.

“Bumblebee species play critical roles as wild pollinators, not just for crops but of all sorts of plants,” he said in a separate release. “They help plants produce fruits, seeds and this in turn provides both food and habitat for other animals, and so on.”

The “very rapid” rates of losses shown on both continents in the study, Kerr said, are “not just something to worry about at some vague future time.”

It’s possible, he said, that to help bumblebees, people and governments “will need to intervene in a significant and expensive way.” — Network



Shifting ranges of bumblebees compared to other species as temperatures rise in both North American and Europe. (Graphic by Ann Sanderson and Sheila Colla)


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