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Rising temperatures threatening bumblebees, finds B.C. researcher

One of the most important pollinators in the world, bumblebees are facing increasing pressure from rising temperatures, finds a new study. Here are some ways you can help.
Bumble bee queen
A bumblebee queen from the Bombus kirbiellus species forages on a flower in British Columbia.

Bumblebees are among the most prolific insect pollinators in the world, helping to fertilize a range of crops and wildflowers in places honey bees often can’t survive. 

Their secret — they know how to shiver.

Like all insects, bumblebees are cold-blooded. But by controlling how they vibrate their bodies, they can raise their internal temperature up to 30 Celsius, the threshold they must hit to take flight. 

Add a layer of insulating hairs, and the relatively big and fluffy bumblebee can forage through rain, cold and wind, pollinating early summer crops like apple and cranberry. They have even been found bouncing from flower to flower at an altitude of 5,600 metres, high in the Himalayan alpine.

In the North American wild, the continent’s 46 species of bumblebee act as a catalyst for a web of life, in one instance pollinating huckleberry bushes, an important food for black and grizzly bears.

But the same traits that made bumblebees highly successful in a stable climate system have spelled disaster since the Industrial Revolution, when humanity began to release vast quantities of greenhouse gases into the atmosphere, largely through the burning of fossil fuels.

In May 2022, atmospheric carbon dioxide levels measured at the Mauna Loa Atmospheric Baseline Observatory came in at nearly 421 parts per million, 50 per cent higher than pre-Industrial levels. 

By trapping the sun’s energy and reflecting it back to Earth, greenhouse gases have led to a global average temperature rise of at least 1.1 degrees Celsius. In higher latitudes, that warming is amplified, meaning countries like Canada are warming twice as fast as the rest of the world.

That's a problem for honey bees, which in B.C. last year were found to have ejaculated to death during a record heat wave. 

It's also a problem for their wild cousin, the bumblebee, according to new research led by Simon Fraser University's Hanna Jackson, a master's student in the school's M’Gonigle Lab in Biological Sciences.

The study, published last week in the journal Biology Letters, found rising temperatures are the leading cause of bumblebee decline across most North American species. 

Over the past 120 years, she and her colleagues found temperature had an outsized negative impact on the bees, trumping the effects of precipitation and the availability of flowers.

To measure the shifting range of bumblebees, Jackson and her colleagues pored over a massive digital bumblebee database dating back to 1805 and drawn from museums, and a number of academic and private collections.

“Imagine, every record we have of a bumblebee, we have all of them for all of North America for the last 120 years,” said Jackson.

Using the database’s nearly 275,000 records, the researchers created two models, one looking at bumblebee populations through time and another looking at how environmental factors have shifted between 1900 and 2020. Together, the models helped Jackson and her team extrapolate how climate and shifting land-use patterns changed where bumblebees lived.

Six North American bumblebee species were found to have decreased their range since 1900. Another 22 species increased their range through time and 18 were found to have a population that remained geographically stable.

But when compared to an imaginary world where human-caused climate change never happened, the study found rising temperatures either shrunk the range or slowed its expansion in 37 of the continent’s 46 bumblebee species.

Rising temperatures tell a dangerous story

On the one hand, Jackson said their study suggests that the range of many North American bumblebee species might not be suffering as dramatically as some past studies have concluded. 

On the other, she says their results track with past research conducted across North America and Europe, which has found bumblebees are getting squeezed at their southern limits while failing to expand into northern ranges. The researchers also point to a study in Japan that found a correlation between rising temperatures and declines in five of six bumblebee populations. 

“It’s a bit of a tricky interpretation,” she said, stressing their study only looked at the range of bumblebees and not their abundance. “We don't want to say, ‘Look, we found that most of them are doing OK. Everything's fine. Don't worry about it…’ It could be that they're escaping from a scenario that they're not happy with anymore.” 

Imagine a bumblebee species in B.C.’s Lower Mainland, says Jackson. As temperatures climb, it might try to move further north or up mountainsides to find its ideal temperature. An incomplete observational data set could show that its range was expanding. But in reality, its population may well have been decimated and only a few individuals are seeking refuge elsewhere. 

“The mere fact that temperature had negative impacts on most species is enough to say that as temperature continues to change through time, we would expect the number of places that these specific bumblebees are found to decrease,” Jackson said.

While the study didn’t focus on how bumblebees are doing in B.C., Jackson said the province’s variety of temperate microbiomes create ideal habitats for several species. 

Two species found in B.C. — Bombus fervidus and Bombus terricola — appear to be losing range, found the study. 

“We also have many of the species that we showed to be negatively affected by temperature,” said Jackson. “We do know that many of these species that we predict to be negatively affected will likely have the same thing happen here.”

Jackson said that beyond a changing climate, a confluence of disease, competition with honey bees and an intensification of single-crop agriculture are threatening the long-term survival of bumblebees. 

How you can help bumblebees under a changing climate

Jackson's study paints a bleak picture for the future of bumblebees, But she also says there is a lot individuals can do to help. 

Outside of preventing runaway climate change, Jackson says backyard or community gardeners can help bumblebees by planting a variety of native wildflowers. 

“Something that people don't generally think about is having flowers throughout the growing season — so early spring, late spring, early summer, late summer, you sort of have a variety of plants that flower at different times,” she said, “That really helps cover the nutrients that they need throughout the whole season versus if you just have flowers that grow only in early summer.”

Another benefit, said Jackson, you get to enjoy flowers and the bees that come with them — year-round. 

You don’t need a garden to help bumblebees. When it comes to tracking a shift in species' ranges due to climate change, a growing body of data is coming from citizen scientists. 

Jackson encourages people to join groups like the Native Bee Society of British Columbia or download apps from the groups iNaturalist or Bumble Bee Watch that connect observers and their photos with researchers looking to track the health of the buzzing pollinators.

“Getting involved in that kind of stuff can be fun, and also, definitely very helpful,” said Jackson.

“Scientists only have so much money and time.”