Bumblebee Decline Linked With Extreme Heat Waves

Type “bumblebees” into Google and one of the first results is a University of Minnesota webpage describing how the furry, flying pollinators have “special adaptations for colder weather including their long, thick hair, and are more commonly found in colder climates.”

That’s a good clue that they will face challenges in a warming world, and new research by scientists at the University of Ottawa suggests that extreme heat waves have already driven some local North American and European bumblebee populations to the edge of extinction. 

Measurements of bumblebee species over time “provide evidence of rapid and widespread declines across Europe and North America,” the authors of the study wrote. More frequent extreme heat waves with temperatures higher than bees can tolerate help explain the “widespread bumblebee decline,” they added.

Bumblebee populations have been hardest hit in warming southern regions such as Spain and Mexico, where some species already live near the edge of their temperature range. Sometimes it just gets so hot that bees die and fall out of the sky, said University of Ottawa biologist Peter Soroye, lead author of the study, published Thursday in the journal Science.

“Bumblebees are disappearing from areas eight times as fast as they are recolonizing others,” he said. “They are the best pollinators in wild landscapes and really important for crops like tomatoes, squash and berries.” 

The results of the paper are surprisingly robust, given the lack of basic information in North America about bumblebee distribution and abundance, said University of Maryland biologist David Inouye, who has studied bumblebees at the Rocky Mountain Biological Laboratory in Gothic, Colorado. He was not involved in the new study, but some of his research is cited by the authors.

“There is still very little, if anything, known about how most of the 4,000 native bee species in the USA are doing,” Inouye said. A new federal monitoring effort could help improve the situation, he added.

The new study analyzed more than half a million bumblebee observations of 66 species in Europe and North America from two time periods; 1901 to 1974 and 2000 to 2014. The researchers mapped where the bees are now compared to where they used to be historically, and matched those records with changes in temperature and precipitation.

“They are disappearing from areas where it’s getting hotter fast,” Soroye said. “We could predict the changes for individual species and communities of bumblebees with surprising accuracy.” 

The average probability of a site being occupied dropped by 46 percent in North America and 17 percent in Europe from 2000 to 2014, compared to the earlier period. Future warm extremes will continue to put the important pollinators under pressure, the study’s authors wrote.

The new study also suggests that extreme heat poses risks for other species, including mammals, birds and reptiles. Bumblebees are an indicator species, he said. 

“We were trying to answer the question, ‘Is climate change causing extinctions’?  We decided to use bumblebees as a test example, and it turns out they work really well,” Soroye said. “In theory, these ideas are really universal for birds, reptiles and mammals.”

Crucial for Wild Plant Survival 

Bumblebees—46 species of them—are native to North America. 

Bumblebee populations are dwindling because of habitat loss and increasing pesticide use, and global warming makes for a triple whammy that is killing off some populations. Honey bees, which were imported to North America by European colonizers, are also struggling, because of exposure to pesticides and the spread of parasitic Varroa mites. 

Inouye said those threats to bumblebees and honey bees matter a great deal to humans and other species that depend on fruits, vegetables and nuts that require pollinators—animals that move pollen between flowers to fertilize seeds. 

“It makes sense to be concerned about reports of pollinator decline,” he said.

Bumblebees fly faster and farther than many other pollinating insects. They also start pollinating earlier in the spring and continue later into the fall. That makes them critical pollinators for some early and late blooming plants. And their tolerance for cool conditions and long-distance flights means they are very important pollinators in extreme habitats like high mountains and deserts, where individual plants needing pollination may be miles apart.

Jeanette Whitton, a botanist and biodiversity researcher at the University of British Columbia who was not involved in the study, said it was a powerful way to determine if increased temperature and precipitation extremes are driving the changes in bumblebee populations.

“We now have a stronger case linking changes in climate extremes to changes in abundance and distribution of bees,” Whitton said. “We know that many plants rely on bees for their reproduction, so it’s not a big leap to say that this matters to plants. If bees decline, this won’t be good for the plants that rely on the bees.”

Dave Goulson, a University of Sussex biologist and bumblebee expert, said the results of the study were not surprising.

“Bumblebees thrive in cool, temperate climates. They are scarce in warmer regions where they tend to overheat in hot weather,” he said. “ It seems likely that a rapidly warming future climate may be the final straw for many of them.”

Biodiversity Loss Goes Hand in Hand with Global Warming

In a related “Perspective” article in the same journal, two scientists, Jon Bridle, an ecologist and evolutionary genetics researcher at the University of Bristol, and Alexandra van Rensburg, an evolutionary biologist at the University of Zurich, wrote that the new study shows how extreme climate events will increasingly drive biodiversity loss.

The research “adds to a growing body of evidence for alarming, widespread losses of biodiversity and for rates of global change that now exceed the critical limits of ecosystem resilience,” they wrote.

The study is part of a relatively new area of research linking the global biodiversity crisis with climate disruption, much of it done by younger researchers. Soroye, for example, is in an age group for whom global warming is part of nearly every biological equation. They are not asking “if” global warming drives biodiversity decline. They want to know how that happens and what can be done about it.

The current biodiversity crisis is “entirely driven by human activities,” Soroye said, and stopping it requires knowing where and why the extinctions happen. For bumblebees, that means understanding direct heat mortality, as well as threats to ground-nesting colonies from increased precipitation linked to global warming. And global warming also disrupts seasonal plant-pollinator cycles. Warm weather earlier in late winter or early spring, for example, can make some plants bloom too early, before bumblebee colonies are active, and by the time they emerge, their source of food may be dwindling or gone.

What Can We Do?

Besides ending greenhouse gas emissions, there are a few short-term conservation strategies that could help bumblebees persist until global warming is curbed and the climate is stabilized.

The first, most important step would be to halt the introduction of non-native bees to new areas to minimize the spread of disease, said London Museum of Natural History entomologist Paul Williams.

Additional measures to support bumblebees include reducing the use of pesticides and re-establishing tall, flower-rich grasslands in areas where the biggest population declines happened as agriculture expanded and destroyed habitat in recent decades. Williams said citizen science, in the form of bumblebee observations, is crucial to fill in data gaps. The U.S. Phenology Network also tracks bumblebees.

Homeowners can make their property more bee friendly by using certain types of plants, and they can become advocates for bumblebee conservation by participating in community planning and decision-making to ensure that there are bee-friendly areas like parks and highway medians. The Xerces Society, a nonprofit insect conservation advocacy group, has tools for community organizers interested in working on this, and citizen science is also an important part of that group’s efforts.

Whitton said political action is also important.

“If I was to say what people can do, I would say vote for people who recognize the importance of science as a basis for sound environmental, social and other policies,” she said.