Pollination perturbation: Is climate change putting bumblebees in a vise?

One long-term study suggests that rising temperatures are prompting bumblebee populations to retreat from the southernmost borders of their habitats.

Scientists say climate change has some insects heading for hills. But bumblebees are lagging – and that’s bad news for our most important pollinators.

A new study, which tracked the upper and lower limits of North American and European bumblebee populations, found that most species are losing ground at their southern boundaries. Surprisingly, the displaced bees are also failing to extend their northern range. Lead authors Jeremy Kerr and Leif Richardson propose that this “range compression” is a result of global climate change. Their findings were published Thursday in the journal Science.

By digitizing over 400,000 existing specimen records, researchers were able to map 110 years of bumblebee distribution. They found, both theoretically and observationally, that bumblebees were receding from their southernmost reaches. On both continents, that trend appeared to coincide with shifting climatic conditions.

“The net effect is that climate change is kind of like a vise, where [bumblebee populations] are being crushed from the southern edges of their range inward,” says Dr. Kerr, a professor of biology at the University of Ottawa.

Other pollinating insects, like butterflies, have also lost habitat at their southern limits. But as once-too-cold regions have warmed, most have managed to extend their northern boundaries. But bumblebees, generally speaking, have not.

“That’s a sign that they’re adapting to some aspect of climate change,” Kerr says. “In general, bumblebee species are not doing this. It’s like they’ve been locked down in the North.”

It’s the latest in an ongoing trend of hardship for bumblebees, who have been increasingly facing disease and decline in recent years. But what exactly is keeping them from buzzing poleward?

“The short answer is, we don’t know,” says Dr. Richardson, a researcher at the University of Vermont’s Taylor Ricketts Lab. “But there are other factors involved that could be interacting with climate warming and bee ecology to prevent them for expanding northward into novel geographic areas. These could include things like interactions with host plants, interactions with parasites, and other ecological factors. This is an area that probably warrants additional research.”

But this distribution shift wasn’t limited to just north and south. Richardson and Kerr found that bee populations were also seeking new elevations.

“We find that bumblebees in the more southern areas of range are moving upslope, thereby tracking their thermal niche,” Richardson says. “But in northern areas, we actually see an overall decline in mean elevation occupied by the bees.”

Richardson suspects this trend may have something to do with changes in vegetation. Many bumblebee species live in shrub-dominated alpine (and subalpine) habitats. These habitats may disappear as the tree line moves up in elevation, potentially forcing bumblebees to find new ones at different elevations.

The Inheritance of Intolerance

In an attempt to explain the uniquely negative impact climate changes continues to exert on bumblebees, Kerr and Richardson reached even deeper into bee history. They speculate that modern species could inherit their upper thermal tolerances – that is, how much heat they can handle – from their distant evolutionary progenitors.

“The common ancestor of bumblebees comes from a kind of cool, temperate region somewhere in Eurasia,” Kerr says. “They don’t like it when it gets hot because their evolutionary ancestry comes from a cool place.”

Butterfly species, by contrast, evolved in tropical climates. According to Kerr, that could explain why butterflies are handling climate change more effectively than bumblebees.

“Bumblebees have a Palearctic, or temperate, evolutionary history,” Richardson says. “So it is possible that they lack the plasticity to adapt to a warming world.”

Richardson is quick to point out that this is merely an idea, not a tested theory. But both authors agree that the proposal warrants future research, which could help identify at-risk organisms.

“I guess the basic prediction we would make is that for groups that evolved under cool or temperate conditions, climate change is a risk right now,” Kerr says. “But we’ve got to try to identify what those groups are. We haven’t tried to do that at all. We need to think about the evolutionary origins of species more than we do when we evaluate climate risk.”

From the Farm to the Fridge

As important pollinators of wild plants, bumblebees represent a major ecological concern. They are also essential to human crops, and by extension, economies. Blueberries, pumpkins, tomatoes, peppers, squash, and apples are all pollinated mainly by bumblebees. A decline in bumblebee species could mean lower crop yields, higher food costs, and less variety in our diets.

Luckily, bees are generalists – most crops are pollinated by several different species. But if you lose even one species, you also lose robustness. One year of bad weather, Kerr says, could result in an entire crop going un-pollinated.

“If you take that to extreme levels, like we see in some decimated areas of China, you have to have hoards of people out there with paintbrushes hand-pollinating crops,” Kerr says. “That’s just not something you can do on a continental scale. That’s a pretty dark sort of world, and I don’t think we want to live in that place.”

Ethical Questions

In their study, Kerr and Richardson offer up some potential, if not temporary solutions to the problem of bumblebee range compression. By way of assisted migration, researchers could relocate dwindling bee populations further north. They might be better equipped to persist there and stave off the effects of climate changes for a little longer.

“An advantage of assisted migration is that maybe we can give them a hand to catch up with climate change,” Kerr says. “They’re just not doing it by themselves. And bumblebee species, in a practical sense, are not really a group you want to try to do without.”

But assisted migration is a controversial measure, especially when it means introducing foreign species to new ecosystems. But in Kerr’s view, these ethical questions are small when compared to the ethical and practical implications of extinction.

“Is it ethically correct of us to introduce species to places they were never historically present in? In a sense, it’s kind of like creating an invasive species,” Kerr says. “But these are areas that are adjacent to the places where these species are found normally. So you’re extending their range. It’s not like we’re taking species from Europe and introducing them to Hawaii, which would be crazy.”

Richardson agrees that assisted migration could have some merits, particularly when applied on a case-by-case basis. But it isn’t the only solution, or even the best one, to the problem at hand.

“To me, there are several other, more important things we should be concerned with based on these results,” Richardson says. “Our study suggests that we need to focus our attention on reducing anthropogenic causes – the emission of carbon dioxide and other greenhouse gases. This is a large problem, and one not easily solved, but this is just one more reason why we need to tackle this problem.”

“There are smaller things we can do to help bees in this context,” Richardson adds. “If climate warming is putting stress on bee populations, I think we can still do something positive for bees by reducing or eliminating our use of pesticides; by promoting and creating habitats for nesting, foraging, and overwintering.”