JWM: Caribou adjust their migrations in response to snow

Caribou that migrate across northern Ontario adjust the way they move in a given winter based on the amount of snowfall.

This new discovery—the result of years of GPS tracking collar analysis—shows that the northern ungulates may be somewhat resilient to some of the effects of climate change.

“It’s possibly a positive to show that they have this flexibility,” said Alexis Pereira, a wildlife research analyst with the Ontario Ministry of Natural Resources.

Ontario has two ecotypes of woodland caribou (Rangifer tarandus caribou). The forest-tundra group typically moves from the peatlands around the coast of Hudson Bay, where they calve in the summer, to forested areas in the winter. These animals typically make their seasonal treks together in large herds.

The boreal, or forest-dwelling, ecotype is found farther south around James Bay. These individuals are more sedentary, not migrating as far when they do move. The females typically remain more isolated from other individuals in the summer when they calve rather than sticking to large communal calving grounds.

Pereira, who was a master’s student at the University of Guelph at the time of this research, and her colleagues wanted to see how weather conditions and other factors affected caribou migrations each year. They gathered tracking data from GPS collars placed on 109 caribou from both ecotypes between 2011 and 2018, with some years missing in the middle of that period.

Snowfall affects migration behavior

In a study published recently in the Journal of Wildlife Management, the team found that individual caribou in both the northern tundra-forest ecotype and the more southerly forest-dwelling ecotype were flexible in their yearly migrations. The average snowfall during the winter seemed to be the main factor dictating their migratory behavior.

For both ecotypes, caribou migrated longer distances when there was heavier snowfall. When there was less snowfall, the forest-dwelling caribou in the south either migrated less or didn’t migrate at all, while the forest-tundra caribou moved shorter distances. A few animals in the more nomadic group—the tundra-forest ecotype—in the north didn’t migrate some years, sticking around the coast of Hudson Bay in the winter.

This correlation between movement and snow surprised Pereira and her colleagues, since there’s a common idea that deep snow makes it harder to move. The team isn’t entirely sure why snow affects caribou migratory behavior—they only had yearly snowfall averages for this study rather than specific data from different regions. But Pereira speculated that heavy snowfall might affect their access to food, forcing them to travel larger distances to sustain themselves.

For example, for the forest-tundra ecotype, those individuals may move to wooded areas that offer more accessible lichen. While forests often do have deeper snow, lichen doesn’t only grow on the ground but also grows on tree trunks, where it might be less buried by snow and easier for caribou to reach.

Other factors that the team examined, such as vegetation estimates, didn’t seem to correlate with migration distance.

Neither did human disturbance. While some research in other parts of the North has found that caribou avoid linear features like roads, seismic lines or powerlines, since wolves (Canis lupus) use these to hunt moose (Alces alces) and occasionally caribou, Pereira’s team didn’t find such a relationship in northern Ontario.

Overall, she said that the discovery that woodland caribou migration is flexible is good news—as climate changes, this behavior suggests a degree of resilience. However, Pereira is concerned that if these changes in behavior occur for too long, they may forget how to go back to the former ways when conditions require it.  

“This relationship of their migratory behavior with their environment is something that exists and should be taken into consideration in general in research,” she said.

This article features research that was published in a TWS peer-reviewed journal. Individual online access to all TWS journal articles is a benefit of membership. Join TWS now to read the latest in wildlife research.