There’s no doubt that bees have had their share of the spotlight lately. The pervasive collapse of honeybee colonies, in particular, has captured national attention. But other bee species are starting to get their share of recognition, too. Wild bees, which are not managed by humans like honeybees are, are also declining, due to factors such as habitat loss and the effects of pesticides.
Now, there’s evidence that bumblebees, at least, are suffering at the hands of yet another environmental crisis: climate change. A study, published Thursday in the journal Science, found that climate change is causing the geographic range of bumblebees — the area the bees live in — to shrink in both North America and Europe.
The study’s authors analyzed data on 67 bumblebee species in Europe and North America, examining changes in their northern and southern range limits. They found that bumblebees on both continents were experiencing dramatic losses in the southern parts of their ranges. In other words, climate change has been driving the southern edge of the bumblebees’ range northward.
According to the study, bumblebees have lost up to 300 kilometers (more than 180 miles) of their southern range in both continents over the past 110 years. “This is a huge loss, and it has happened very quickly,” Jeremy Kerr, lead author and a professor of biology at the University of Ottawa, said at a press conference on Wednesday.
Climate change has been known to affect other organisms in similar ways, so in one sense the loss of the bees’ southern ranges makes sense. Bumblebees evolved in mild, temperate climates, so as things heat up near the equator, it’s logical that conditions might get too warm for them to survive, driving them northward.
“They may not have the ability to adapt to changing climate in the way that some organisms that have a tropical evolutionary history are able to adapt,” said Leif Richardson, a postdoctoral research fellow at the University of Vermont’s Gund Institute for Ecological Economics and another one of the paper’s authors.
In many cases, if an animal’s southern range moves up, so does its northern range, so that its entire geographic range shifts. But the researchers in this study found this was not the case for bumblebees. While the southernmost edges of their ranges have been creeping northward, their northernmost edges have not moved. This means that the total geographic area the bees occupy has been shrinking.
Shrinking geographic ranges could put major pressure on already vulnerable bee populations, driving them closer to the edge of endangerment or extinction. And the shifting ranges could also be a problem for both wild plants and crops at the bees’ southernmost limits if the bumblebees aren’t around in the future to pollinate them, said Richardson.
“There certainly is a threat to human agriculture,” he said. “What we know from research is that this can lead to lower crop yields, higher food costs, and there are all these consequences for the human food supply and the economy.”
Wild plants tend to be important components of their ecosystems. If they start to decline because of a lack of pollination, other species that depend on them for food or habitat could suffer, too. It’s unclear whether other pollinators, such as other wild bee species or butterflies, could adequately step up to fill the void left behind by bumblebees in these cases.
Since bumblebees are known to be vulnerable to other environmental influences, the researchers wanted to be sure that climate change was the culprit behind their geographic shifts. So they analyzed the bumblebees’ range shifts with data on land use changes (essentially the alternation or destruction of bee habitat by humans) and pesticide use. They found that neither of these factors correlated to the changes in the bees’ geographic ranges.
This is not to say that these factors don’t have negative impacts on bumblebee populations, Richardson said. They can still hurt bees’ local populations — it’s just that they don’t appear to be responsible for the bees’ larger geographic shifts.
The scientists aren’t sure why bumblebees haven’t been able to expand their northern edges. According to Kerr, there are two reasons a species might not shift well in response to climate change: Either it has problems actually moving from one place to another, or it has problems building its population up once it gets to a new place. “Clearly bumblebees are pretty good at getting around,” Kerr said at the press conference, since they can fly. Rather, the scientists suspect that they’re having trouble growing their populations at their northern range limits. The reason remains unclear.
More research is needed into the bees’ behavior and the biological and environmental factors that may be causing them problems. In the meantime, there are some actions humans can take to try to help out their struggling populations.
The obvious course of action is to keep up with efforts to mitigate climate change so that rising temperatures don’t continue to drive the bees out of their historical southern ranges, Richardson said. But there are some other strategies conservationists can try in the meantime.
One fairly drastic solution is assisted migration, which is when humans physically relocate bumblebee populations to a new area. “It’s certainly a possible solution on a case-by-case basis or a site-specific basis for bumblebees,” Richardson said. “However, it will be expensive to do this, and it’s not really a solution to what is probably a global problem for many hundreds of species.”
Simply trying to protect local populations is another option. Stricter policies on pesticide use can help keep minimize declines. And citizens can even help out by diversifying the plants they grow in their own back yards to attract more pollinators, said Laurence Packer, another author on the study and a professor of biology at York University, at Wednesday’s press conference. “The way you plant your garden can make a very big difference,” he said.
With concern over the health of pollinators at an all-time high, this study highlights the impact of climate change on an already vulnerable (and economically important) organism — and underscores the fact that its consequences are already noticeable.
“The impacts are large, and they are underway,” said Kerr.
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