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North American and European bumblebee species are shrinking, a new study shows. These insects are failing to migrate northward, the study reveals, and in their southern territories, their ranges are compressing -- with range losses up to 300 kilometres in both North America and Europe.Jeremy T. Kerr

A detailed study of bumblebees across two continents has yielded a troubling message about the impact of climate change on vulnerable species.

While the bees are losing ground at the southern limits of their ranges and at lower elevations as temperatures rise, they are not compensating by shifting northward or expanding into new territories that were formerly too cold.

The finding suggests that human intervention may be required to physically transport colonies to new environments to avoid the catastrophic collapse of many bumblebee species.

"I think we need to have a thoughtful conversation about assisted migration at continental scales," said Jeremy Kerr, a biologist at the University of Ottawa and lead author in the study, which was published Thursday in the journal Science.

The study's appearance coincides with the meeting in Toronto this week of regional representatives from across the Western Hemisphere for the Climate Summit of the Americas. It offers what is likely be the most comprehensive look ever at how climate change is affecting an entire group of species.

The results suggest that more environmental monitoring is needed urgently or the ongoing impacts of climate change on ecosystems around the world will be difficult to predict and potentially more damaging than anticipated.

Although their role is not as widely appreciated as domestic honeybees, bumblebees pollinate a significant fraction of wild plants at northern latitudes as well as commercially important agricultural crops.

Larger, rounder and fuzzier than other bee species, bumblebees "are kind of the polar bears of the insect world." said Sheila Colla, an assistant professor at York University in Toronto and a co-author on the study. "They are really adapted to cold weather."

For the study, researchers assembled a database consisting of 423,000 documented specimens and sightings of 67 bumblebee species across Europe and North America, some dating back more than a century. Only observations that firmly located a species in a specific time and place were included in the database.

The researchers then analyzed the data to shows how the ranges of the various bumblebee species have changed through time. By separating out effects that are not related to temperature, such as pesticide and land use, they were able to track the impact of climate change on the bees – an impact they say is clearly visible in the data.

They found that rather than shifting with warming temperatures, many bumblebee species are simply being compressed into ever-narrowing ranges.

"If these patterns don't change it's a question of when, not if, we lose these charismatic and essential bees," said Nigel Raine, a professor and expert in pollinator conservation at the University of Guelph who was not involved in the study.

Exactly why the bumblebees aren't moving is hard to pinpoint, the authors say.

Bumblebees have an elaborate life cycle with colonies dying off every fall and queens hibernating under snow-covered ground to emerge and start new colonies in the spring.

For some species, these steps are precisely timed to the flowering of specific plant species and other environmental cues which may not be able to shift in synchronization as the climate changes. The number of queens available to reboot the population every summer may also be a factor that limits bumblebees' ability to adapt to climate change.

"For a bumblebee species to establish in an area where a bumblebee species has never been found is a chancy affair," said Dr. Kerr. "You're going to need a lot of individuals."

If the bumblebees can't move on their own, they risk being trapped by warming temperatures unless humans step in. A continental scale effort to move bees into cooler regions could buy them time, Dr. Kerr said.

"We can manage around this for a little while… but ultimately we have to fix this climate change problem," he added. "We're really, really out of time on climate change."

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