Maria Stenzel/National Geographic Creative

Out of the frying pan, into the fire. Caribou mothers move their calves to habitats with fewer wolves – only to put them at much greater risk of being eaten by bears instead.

Woodland caribou populations in the boreal forests of northern Canada have declined sharply in recent decades, and the government now lists the subspecies as threatened.

Logging and oil extraction may be to blame. Human activities have fragmented the caribou’s habitat, shifting the ecological balance. Where once wolves were the main danger to young caribou, black bears now seem to be their greatest threat.


To understand this shift, Mathieu Leblond at Laval University in Quebec City, Canada, and his colleagues used GPS collars to track the movements of 26 adult female caribou for up to four years. They also tracked all 44 of the calves these females had over this period, as well as 12 black bears and 9 wolves.

The data allowed the team to determine the location choices made by each caribou mother and see how that may have affected whether a calf survived to adulthood. They found that mothers that were best at avoiding wolf habitats – patches of mature forest and older tree regrowth – were more likely to lose their calves.

Wolves are the main predators of the adults and were probably also the main threat to their calves until recently. But bears are now thriving in the human-altered woodland, and the GPS records indicate that habitats that are bad for wolves are good for bears, and vice versa.

A rock and a hard place

“By being very good at avoiding wolves, they fall victim to the black bears,” says Leblond. “Mothers that are not as good at avoiding wolves in fact have better survival for their calves, because they are in regions where there’s not as many bears.” In effect, the caribou have failed to adapt to the new, bear-rich environment.

Efforts to protect caribou have focused on trying to prevent them from encountering wolves. Leblond’s findings suggest this won’t be enough.

One option currently being considered in Canada’s oil sands region would be to introduce fenced enclosures, allowing caribou mothers to begin raising their young calves without predation from bears, says Stan Boutin at the University of Alberta in Edmonton.

The new finding also adds urgency to such efforts, underscoring that caribou mothers seemingly have no escape. “The clock is ticking here,” says Mark Hebblewhite at the University of Montana in Missoula. “Studies like this drive home the point that it’s worse than we thought.”

Journal reference: Journal of Applied Ecology, DOI: 10.1111/1365-2664.12658