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Every spring, herds of caribou make their move from the boreal forests of northern Canada to calving grounds hundreds of kilometers away in the barren Arctic tundra. Much remains unknown about their annual trek, including how the caribou—a keystone species experiencing precipitous population declines—know when to migrate. Recent research suggests snowmelt may be their cue.
Scientists compared 10 years of data on the spring migration timing of 117 female caribou with satellite data tracking when snow started to melt in the animals’ habitat.
This relationship adds “a piece of the puzzle” that helps us understand how environmental changes, such as subtle changes in the snowpack, are affecting caribou herds.
The study revealed a strong relationship between snowmelt onset and the beginning of the caribou migration, said Joan Ramage, a study coauthor and environmental scientist at Lehigh University. This relationship adds “a piece of the puzzle” that helps us understand how environmental changes, such as subtle changes in the snowpack, are affecting caribou herds and the ecosystem they support, she said.
Ramage presented the research on 10 December at AGU’s Annual Meeting 2024.
Microwaves and Migratory Movements
To determine when snow began to melt in the caribou’s boreal wintering grounds, Ramage and her colleagues used satellite observations of microwave radiation emitted from Earth’s surface. Microwaves are sensitive to liquid water, so satellites can detect when water begins to appear within the compressed winter snowpack.
The researchers compared these satellite data to data from 2006 to 2017 on the migratory movement of the female caribou, part of the Bathurst herd, that were tracked using GPS collars as they traveled hundreds of kilometers to the tundra to give birth to calves. The GPS data are collected by the Government of Northwest Territories’ Department of Environment and Climate Change, which monitors the herd as part of a plan to recover the region’s declining caribou population.
From its analysis, the team found that the Bathurst caribou began their migration, on average, just 2.6 days after the onset of the main snowpack melting event.
The findings from this work, published in Remote Sensing, are contributing new understanding “that comes from this really interesting technological ability to see factors about the snow that more traditional kinds of imaging can’t,” Ramage said.
A Keystone Species
Research into caribou behavior is taking on greater urgency as the Arctic warms nearly 4 times faster than the rest of the planet. The warming temperatures are altering snow patterns and other conditions in caribou’s habitat, adding pressure on a species whose numbers are dwindling. The Bathurst herd’s population, for example, has plunged more than 98% in the past 30 years because of environmental changes, human disturbances, and other factors.
The caribou’s decline is especially concerning because of the species’ important role in its ecosystem.
“Caribou are the lifeblood of the tundra…they circulate all across the ecosystem, touching all facets of the ecology.”
“Caribou are the lifeblood of the tundra,” said Kyle Joly, a wildlife biologist with the U.S. National Park Service who was not involved in the recent study. “Like blood, they circulate all across the ecosystem, touching all facets of the ecology.”
In addition to serving as the primary prey for Arctic predators such as wolves, grizzly bears, and wolverines, caribou also modify their habitat by trampling the landscape, eating local vegetation, and depositing nutrients.
“If you removed [caribou] from the ecosystem, the ecosystem would change dramatically,” said Steeve Côté, an ecologist at Université Laval in Quebec who was not involved in the study.
Studying Snow Conditions
Just as caribou affect their environment, environmental changes—like shifting snow cover and snow quality—affect them.
Lichens that grow under the snow layer make up 70%–90% of caribou’s diets in the winter, according to Joly. But if the snow melts and refreezes, it becomes harder for caribou to dig through to reach their food supply.
On the other hand, when snow is less dense—either because of especially dry or especially warm conditions—it is easier for caribou to sink into it. The extra energy caribou expend when they wade through deep snow makes an already taxing migration even harder. “It’s like you’re at mile 25 of a marathon and suddenly they add 8 or 9 miles,” Côté said.
A caribou can walk more than 44,000 kilometers in its lifetime thanks to its long migrations, Joly noted. Focusing on how changing snow conditions affect the migration, like the study authors did, “makes a lot of intuitive sense,” he said, because of the species’ important symbiotic relationship with its ecosystem.
Ramage and her colleagues are now analyzing GPS data from other caribou herds and comparing them to satellite observations to establish whether the relationship they found between snowmelt and migration timing among the Bathurst herd is typical of other caribou. This question, Ramage said, is one of many for future interdisciplinary research to address that may “be really valuable for understanding this species and what’s going on in their world.”
—K. R. Callaway (@KR_Callaway), Science Writer
Citation: Callaway, K. R. (2024), Snowmelt sends caribou packing, Eos, 105, https://doi.org/10.1029/2024EO240542. Published on 10 December 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
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