There used to be a hard line at the bottom of the boreal forest. Above it, white-tailed deer struggled, and populations were unable to sustain themselves.
But the changing climate has led to milder winters — fewer days of extreme cold and less snow cover —and that’s allowed deer to expand hundreds of kilometres northward. They also benefit from human development and reforestation, which promotes the growth of new young plants for them to forage.
“Traditionally, across the continent, deer were a southern Canada species, and we didn't see them in the boreal forest at all,” says Kim Dawe, whose PhD research established climate change as the primary driver of deer migration.
Dawe’s analysis of data gathered in northwestern Alberta in the latter half of the 20th century suggests that winter severity — a measure of the number of days the temperature is below -17.7 C and snow cover is above 38 centimetres — is the best predictor of whether deer will be found in an area. Below -17.7 C, deer begin to shiver to keep warm, burning more energy than they may be able to restore. And deer have trouble walking through snow more than 38 centimetres deep.
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“You can think of it as an energetic story. These deer are trying to find enough food to get through the winter, and most ungulates, including deer, lose body weight through the winter, so they're just on a sustain mode rather than trying to put on any weight or maintain it,” says Stan Boutin, a professor at the University of Alberta and Dawe’s co-author on her paper about deer and climate change. “As soon as you have a series of years where those temperatures or snow depths aren't so severe, they seem to be able to push northward.”
This northward migration has the potential to dramatically affect the boreal forest. When deer populations grow, so do wolf populations. And wolves prey on the boreal caribou, a species that is threatened in Ontario.
“More deer on the landscape changes the predatory relationship between wolves and caribou,” says Dawe. Normally, the wolf population in the boreal forest is sustained by moose, which live in upland areas, Dawe says — caribou, though, live in the peat lands and so have historically been protected.
But deer are now found in both areas.
“The only way you can get more wolves in the system is you have to have more food than they've had in the past,” says Boutin. “Suddenly, there's a new level of prey biomass represented by the deer as they move northward. The wolves take full advantage of that, so their numbers go up.”
Deer also carry the brainworm parasite — found in their feces, it can be picked up by snails and slugs and deposited on leaves, which may then be eaten by other animals. Although brainworm doesn’t harm deer, it’s lethal to moose and caribou.
The deer will also almost certainly have an impact on local flora: voracious herbivores, they eat small plants and young trees. Point Pelee National Park closed for two weeks in January for a deer cull because the population there had grown, becoming a threat to Carolinian forest and the Lake Erie Sandspit Savannah ecosystems.
“We're going to see widespread effects over time as these become a dominant herbivore in the system,” says Boutin. “You might, in fact, see changes in vegetation in these systems.”