The heat is beginning to sting for bumblebees. As the Earth warms, they are being driven out of their habitats in North America and Europe, according to a new study published in Science.
"They have disappeared from places they used to be found," said Jeremy Kerr, an ecologist and one of the lead authors of the paper. "If these species are losing range at the rate at which we are observing here, that cannot go on for long before many of these species go extinct."
By studying the distribution of 67 bumblebee species on the two continents over a 110-year period the authors concluded that human-induced climate change was a "significant cause of rapid declines in bumblebee populations." The results are a grim reminder that not all species are adapting to climate change, experts said.
Warming temperatures are forcing bumblebees to retreat from the southern boundaries of their range while being unable to settle in regions farther north, the research found, effectively trapping them in what the authors called a "climate vise."
Compared to a base line period of 1901 to 1974, the bees have lost almost 300 kilometers (186 miles) of vital habitat in southern belts that are experiencing rising temperatures. There are "dramatic losses of bumblebee species from the hottest areas across two continents," Kerr said. A study of 31 species in North America and 36 in Europe showed that they have "run up against a kind of wall that most bumblebee species show no signs of passing."
Bad news for many crops
Scientists concur that a shrinking habitat means the threat of extinction is real for many of these plump, fuzzy pollinators, whose generic name Bombus is derived from the Latin word for buzz.
Population sizes over such a large area and time period were difficult to determine. "No one in the past has tried to measure the species populations across continents, which has made it almost impossible to actually compare population sizes," Kerr explained.
"If their range is shrinking at one end and not increasing on the other end, then one would assume that the numbers of those bees is declining," said David Inouye, a zoologist and bumblebee expert at the University of Maryland, College Park, who reviewed the research.
The loss of bumblebees from some regions could prove disastrous for ecosystems and for agriculture, as bumblebees play an important role in pollinating both crops and wildflowers. They can perform a feat called "buzz pollination," that other pollinators like honeybees cannot. Bumblebees power their way to pollen that is stubbornly attached to flower anthers, producing a distinctive buzzing noise as they get the job done. Crops like tomatoes, chili peppers, eggplants and cranberries are more deftly pollinated by bumblebees than by any other winged pollinator.
The anxiety about threats to bumblebees feeds into a wider concern about pollinator populations. The collapse of bumblebee populations in the southern reaches poses a threat to plant populations in this region, Inouye said.
Apart from their role in the wild, more than half of global food crops rely on pollination. A 2013 study in Science noted that falling populations of pollinators in the wild could adversely affect crop yields. Managed pollinators like honeybees that are mostly domesticated for pollination purposes and for honey cannot always fulfill their mandate. In fact, honeybee populations have also seen a decline in recent years.
The widespread use of a class of pesticides called neonicotinoids seems to have precipitated a rapid fall in their numbers. The chemicals in these pesticides find their way to the pollen and can prove fatal to pollinators like honeybees and bumblebees.
Why are butterflies heading north when bees are not?
While bumblebee populations are also under threat from other factors like habitat loss because of changing land use and use of toxic pesticides, the new research found that there was a stronger link to warming. "Bumblebee disappearances from warm, southern areas are just as likely when there is no pesticide use and little agriculture," Kerr said.
But this pattern of habitat contraction and population decline does not always hold with other insects. Butterflies, for instance, seem to be flying into cooler locations in response to warming.
Why bumblebees are failing to do so remains a puzzle.
Rich Hatfield, a conservation biologist at the Xerces Society for Invertebrate Conservation who has studied bumblebees for the past 17 years, said he was surprised by the results — especially the fact that bumblebees were not migrating to more habitable climates.
One theory proposed by the authors of the new study suggests it is because bumblebees — unlike butterflies — evolved in cooler climates and are biochemically unsuited for hotter conditions. "Imagine a car that starts running out of coolant and starts blowing steam out the front of the hood. That’s kind of like an analogy for what bumblebee species do when it gets too hot," the scientists said.
Inouye speculated on behavioral reasons why the bees were not moving, including their inability to disperse well. While bumblebees are generally mobile, his research showed that they do not venture too far from their hives. They may also not be able to grow their population quickly in the new environment.
Whatever the explanation, the loss of habitat raises concerns that some bumblebee species may already be extinct or be on the brink of extinction. Both Inouye and Kerr cited the case of Bombus affinis, or the rusty-patched bumblebee, which is usually found in the eastern and upper midwestern United States but may already have gone extinct.
"We are essentially hitting them with everything that we have in our global change arsenal," Kerr said. "We need to identify priority areas for conservation immediately."
As for the bumblebees, assisted migration may be one way to save them, according to the study. However, artificially relocating populations to what humans determine as more suitable sites for threatened species remains a controversial conservation strategy. It presents not only logistical challenges but also ethical concerns.
"It is an ethical issue — to what degree we want to manipulate these communities?" Inouye said. Hatfield suggested a more immediate response to the plight of bees: Take better care of their current habitats.
"I’d rather not imagine a world without bumblebees," he said. "It would mean that the world had taken a sad turn for the worse."