Wolves Keep Brain Worm–Spreading Deer Away From Moose Populations in Minnesota

Wildlife managers now face the challenge of creating conservation plans for all three species while maintaining balance between predator and prey animals

An image of a male moose laying down in a forest
Warmer winters and shrinking snow depths have made it easier for white-tailed deer to migrate further north. Ryan Hagerty via Wikicommons under Public Domain

In Minnesota, moose used to roam the boreal forests by the thousands. The population had 8,800 individuals in 2006, and since then, numbers in the northeastern part of the state alone have fallen by 64 percent, reports Liz Scheltens for Vox. Warmer, shorter winters, tick infestations, liver issues, wolves, and parasites all contribute to declines in remnant Minnesota moose populations, reports Dennis Anderson for the Star Tribune.

However, the biggest threat may be migrating white-tailed deer. As deer entered moose habitats, they brought brainworm, a fatal parasite to moose. While harmless to white-tailed deer, the parasite (Parelaphostrongylus tenuis) causes disorientation, extreme weakness, and the inability to stand in moose. It may be a critical factor as to why Minnesota's northern moose populations have declined significantly.

A study published in Science Advances in December 2021 shows grey wolves may help moose populations by keeping infected deer at bay. More wolves were linked to less overlap between deer and moose, reducing the risk of parasite transmission, reports John Myers for Twin Cities' Pioneer Press.

"We often think of wolves as bad news for moose because they kill a lot of calves," said study co-author Tiffany Wolf, an expert in veterinary population medicine at the University of Minnesota, in a statement. "But this suggests that wolves may provide a protective benefit to adult moose from a parasite-transmission perspective. Because brainworm is such an important cause of adult moose mortality in Minnesota, we can now see that the impact of wolves on moose is a bit more nuanced."

Warm winters with less snow have made it easier for white-tailed deer to migrate further north. Over the past three decades, grey wolf numbers have exploded due to white-tailed deer ranges expanding into moose territory. Moose catch brainworm from deer poop after snails and slugs on the forest floor consume the deer droppings and then climb up trees and shrubs where moose tend to forage for food. Once the moose consumes the leaves, eggs hatch inside the moose's brain, and worms tunnel in and out, causing neurological damage, per Vox.

After capturing and tracking 94 adult moose, 86 deer, and 47 adult wolves, they found the most significant risk of brainworm transmission occurred when deer and moose overlapped during the spring and summer migrations, a statement explains. They also found that deer and moose populations were less likely to overlap when the presence of wolves increased.

The study's implications may help state and tribal managers draft up wolf management plans in Minnesota. Still, increasing the number of wolves requires a balance. Every eight out of ten moose calves born in northeastern Minnesota are killed by the predators in their first two weeks of life, meaning fewer calves survive into adulthood to breed, Vox reports.

"I think if we can agree on an area in the core moose range where we are going to work to benefit moose, and we include deer management and maybe some wolf management to start, along with targeted habitat work, we might succeed," study author Seth Moore, a wildlife biologist at the Grand Portage Band of Lake Superior Chippewa, explains to Pioneer Press. "We might be able to keep moose in Minnesota."