Minnesota moose decline stems from spread of deer

GRAPHIC: @MinnesotaBrown

Scientists say they’ve solved the mystery of the missing moose. The brain-eating bacteria that kill so many moose, and leave others more vulnerable to wolf predation, is carried by the spread of whitetail deer into Northeastern Minnesota.

For the past several years, we’ve been wondering why Minnesota’s moose population — once a strong feature of the northern pine forest — now dwindles to precarious lows. From increased predation to a bacterial infection that causes moose to act erratically, scientists explored many possible answers.

In a Sunday feature in the Star Tribune, Josephine Marcotty interviews state officials, hunters, scientists and others to explain how the spread of whitetail deer now predicts the spread of the brain-eating disease in moose. The disease is spread by snails who contract the disease from deer feces and spread it to moose.

Writes Marcotty:

Deer and moose weren’t made to share the forest. Deer are native to the southern part of the North American continent, and over hundreds of thousands of years, they evolved to withstand the parasites they carry. Two of those parasites are problematic — brainworm and liver flukes — and both spend part of their lives in slugs and snails. Deer eat the snails and slugs while browsing, and the microscopic brainworms make their way to the outside of the deer’s brain, where they lay the eggs. After hatching, the larvae move through the digestive tract and out in feces, where they are picked up again by snails. Liver flukes start in the liver, but end up back in snails by the same route.

Moose pick them up the same way — by eating snails while browsing. But moose, which are believed to have crossed over from Siberia just 70,000 years ago, have not developed the defense mechanisms that deer have, and the parasites can damage their brains, nervous systems and livers.

With brainworm, moose often develop a characteristic head tilt, and the infected moose are often the ones seen wandering aimlessly where they shouldn’t be — like roads.

“You can’t have moose and whitetails together,” said Margo Pybus, an expert on deer parasites at the Fish & Wildlife Division in Alberta, Canada. “Unless we want to wait 60,000 years until moose get the parasite relationship figured out.”

Unfortunately, knowing the the answer to *why* the moose mortality is so high offers only complicated questions of *how* to solve it.

Essentially, Minnesotans have two choices. They may either dramatically increase the deer harvest in moose habitat, to the point of diminishing the population (and hunting opportunities) over time. Or they must prepare for a Minnesota that no longer contains moose, driving the animals further north and west to areas where whitetail deer aren’t as prevalent.

Living in Northern Minnesota, deer become an ever-present reality of life in the woods. A simple drive to town might reveal as many deer as you would see crows, and your local body shop will happily report the results of too many deer on the roads.

This wasn’t always the case. Even 100 years ago, moose and elk dominated the northern forests. Whitetails resided along the edge of the prairies in west central and southern Minnesota. However, deer love forest land that’s been cleared by humans the way humans love disrupting ecosystems in the first place. The expansion of human settlement in previous wilderness creates bountiful resources for deer, and it shows.

However, while Minnesotans happily agree that deer are too plentiful (and delicious), the idea of trying to drive down the deer population in Northeastern Minnesota doesn’t sit well with hunters, especially those who own shacks or businesses that serve hunters.

That leaves the snails. And as gardeners know, keeping snails out of a six-by-six-foot garden plot can be challenging enough.

This becomes another example of how climate and biological changes play out in real time. Can the moose be saved in Minnesota? At this point, it would seem to require a miracle.


  1. So deer are an invasive species? I’m on Team Moose and Team White Pine.

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