Many bumblebee species have vanished from places where they were once common. Now a new Canadian-led study finds that hotter temperatures during heat waves are to blame, and uses it to predict which bumblebees are most likely to face local extinction as the climate warms.

The researchers say the technique that could also be used to make predictions for other species at risk from climate change.

In recent years (between 2000 and 2014), your chance of seeing a bumblebee at a given location declined by nearly half (46 per cent) in North America compared to in the 20th century (between 1901 and 1974), reports the study by researchers at the University of Ottawa and University College London.

In Europe, the decline was 18 per cent, according to the study, which was published Thursday in Science.

“These are really severe declines,” said Peter Soroye, a PhD candidate at the University of Ottawa and the study’s lead author.

And that’s not just bad news for bumblebees — they’re crucial for pollinating agricultural crops from squash to berries to tomatoes, along with other flowers and plants that add colour to world, Soroye said.

“They’re probably among the best pollinators we have… they’re also really beautiful little animals,” he said. “We’d be missing a lot if they were to decline more than they have already.”

Not keeping up with climate change

Soroye’s supervisor, University of Ottawa biology Prof. Jeremy Kerr, led a previous study showing that bumblebee species have been squeezed out of warmer parts of their habitats by climate change since 1975.

On average, the 67 bumblebee species they studied were locally extinct in the southern 300 kilometres of their ranges by 2010.

“They’re not able to keep up with these changes in temperature,” said Soroye, whose followup study with Kerr echoes those results.

The fact that they were disappearing from the south pointed to climate change as a factor. But the researchers wanted to know exactly what aspect of climate change was to blame — changes in temperature, precipitation, or both?

In order to do that, they turned to a database of 550,000 bumblebee sightings from 66 species in North America and Europe between 1900 and 2015 from museum and research collections, along with sightings from citizen scientists. 

The new study found that bee declines were specifically linked to hotter maximum temperatures, Soroye said. 

“It’s kind of these extremes of climate throughout the year that climate change is causing.”

While that link was clear, he also noted that bees face a lot of other challenges, such as habitat loss and pesticide use. The researchers weren’t able to determine specifically how much of the bees’ decline was due to climate change.

Nor did they know exactly how extreme heat causes bee declines. It may either cause the bees’ death directly, or it could impact the plants and flowers they rely on for food.

By looking at the highest monthly maximum temperatures at different locations over time and comparing them to bees’ historical ranges, however, the researchers could identify the temperature limits that each species could withstand, and take things a step further.

“We found we were able to predict [local] extinctions in bumblebees really well,” Soroye said.

He suggested the technique could also be used to predict where climate change will put other species at risk, including birds, mammals and reptiles.

What you can do for bees

While most bumblebees appeared not able to move into cooler areas to adapt to climate change, and few seemed to thrive in warmer temperatures, there were some exceptions.

Soroye said he and his colleagues hope to figure out why and use that information to hopefully reduce or reverse declines in other species.

In the meantime, the findings also suggests ways anyone can help bees — by providing “little refuges” from extreme temperatures in the form of fallen logs, leaf litter and plants of different heights in their gardens, Soroye said.

Victoria MacPhail, a PhD candidate at York University, led a 2019 study that found that by 2016, one bumblebee species, the American bumblebee, had disappeared from 70 per cent of its range and lost 89 per cent of its population compared to the period between 1907 and 2006. That suggests it has become critically endangered.

MacPhail, who was not involved in Soroye’s study, said this latest research reinforces what she and other bumblebee researchers have also found.

“The majority of our bumblebee species will drastically decline in population size and range, and some may even become extinct,” she said.

The idea that those declines are linked specifically to extreme heat adds a new piece of the puzzle, she said.

“The question is how we can help.”

MacPhail thinks the results will help identify areas to prioritize for bumblebee conservation, such as places where multiple species live or the edges of species’ ranges, where they’re under the most stress.

She agrees that what the bees need most is somewhere they can find refuge to withstand hotter temperatures, and that people can help by restoring and managing bee habitat. That may include making sure they have flowering plants to feed on from spring through fall that places where they nest or spend the winter are protected.

But MacPhail thinks efforts need to go beyond bee conservation too.

“We need to take these findings seriously and increase our actions to keep global warming to 1.5 C above pre-industrial levels,” she said.

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