Rapid Climate Changes Turn Minn.’s North Woods into a Moose Graveyard

But the animals that inspired Bullwinkle are not what they were. Here, even healthy bulls — whose size, strength and rutting prowess make them the undisputed kings of the North Woods — are dying from what appear to be a combination of exhaustion, exposure, wasting disease triggered by parasites and other maladies.

The biologists are baffled and also helpless.

Mark Lenarz, who retired in March from the Minnesota Department of Natural Resources (DNR), where he led moose research efforts, said it’s not like the TV show “CSI: Crime Scene Investigation.”

“Unlike ‘CSI,’ it’s very hard to identify in the field exactly what an animal is dying from,” he said. “We know something about the symptoms” of distressed moose, he added, “but we don’t necessarily know the exact causes of mortality.”

What Lenarz and other experts do know is that a variety of climate stressors — including higher average annual temperatures, a long string of very mild winters, and increasingly favorable conditions for ticks, parasites and other invasive species — are conspiring to make northern Minnesota a moose graveyard.

Since 2002, Minnesota DNR specialists have put radio collars on 150 healthy adult moose; 119 subsequently died, most of them from unknown causes, according to wildlife officials. Car and train collisions accounted for 12 mortalities, while wolves were culpable in just 11 deaths.

Sudden collapse of herds

Meanwhile, annual surveys taken from helicopter overflights show that the state’s primary moose population, in the state’s northeastern Arrowhead region, has been halved in just six years, dropping from 8,840 animals in 2006 to just 4,230 this year. The decline mirrors a similar collapse a decade ago in the state’s northwest corner, where moose plummeted from an estimated 4,000 animals in the mid-1980s to less than 100 by the mid-2000s.

While some monitoring of moose had occurred in the 1990s, most of the animals were gone before scientists could examine cause-and-effect relationships. In the Arrowhead, however, experts are watching mass mortality, discovering multiple moose carcasses in the same area, including animals that appeared relatively healthy only a few years before.

It’s not just the occasional sickly moose succumbing to common causes of mortality, said Lenarz. “We’re out in the field collecting dead radio-collared moose, and we were finding other moose that had died along with them.”

Similar mysterious deaths of one or more moose have been documented in Voyageurs National Park, where the National Park Service had launched its own radio-collar study of the animals, and in the Boundary Waters Canoe Area Wilderness, where moose sightings used to be routine for visitors but are increasingly rare.

Bob Baker, regarded as one of the best wildlife trackers on the Gunflint Trail, believes the region’s gray wolves are preying on moose, especially young calves, whose survival is essential for the herd to maintain itself. The wolves were only recently removed from the endangered species list. “You can radio-collar as many adult moose as you want. But if wolves are eating the calves, you’re still going to lose your herd,” he said.

Scientists acknowledge wolves are a factor in the spiking moose mortality, but not significant enough to explain a 50 percent decline.

On the Grand Portage Indian Reservation, at the Minnesota-Canada border, Seth Moore, the Chippewa band’s chief biologist, has documented similarly stark declines, including a 40 percent mortality rate among radio-collared animals. The 70-square-mile reservation’s once-robust population of up to 80 moose has dwindled to fewer than two dozen. This forces Moore to plot a future without the animal that has provided subsistence to the tribe since the early 18th century.

Moose are part of the religious and cultural life of the tribe, and their disappearance would mark a “profound” change in the economic lifeblood of northern Minnesota. “I don’t want to think of a world where moose don’t exist here,” Moore said. “I’m too much of an optimist for that.”

Subzero survivor can’t take the heat

It will take more than optimism, however, to help moose adapt to what meteorological data show to be one of the most pronounced warming trends in the Upper Great Lakes since the end of the last ice age, when retreating glaciers raked across the landscape creating a “Land of 10,000 Lakes.”

Findings from Minnesota’s Interagency Climate Adaptation Team show the state has experienced a 1.5- to 2-degree-Fahrenheit average surface temperature increase over the last 100 years, with the greatest rise in the northern reaches of the state, including primary moose habitat.

Such changes are minimally felt by humans and other native mammals such as black bear, lynx and gray wolves, but scientists note that even minor temperature shifts can affect moose, which are adapted to live in the harshest cold and deepest snow.

Although northern Minnesota is on the southernmost edge of the Northwest moose’s range, which extends into Canada’s vast Nunavut territory, it traditionally provided an abundance of what moose need to thrive. That includes large, contiguous forest tracts with dense stands of fir, spruce and pine trees that provide cover against harsh weather conditions, along with a healthy mix of birch and aspen trees whose bark and tender saplings are the original “moose munch.”

While there were once tolerable numbers of the kinds of pests that drive wild mammals crazy — ticks, black flies and humans — these pests are proliferating.

Take ticks, for instance. While moose are well adapted to host some native winter ticks, their tolerance for the blood-sucking arachnids is being challenged as the tick population has surged under warming conditions.

Biologists are now documenting individual adult moose with tick burdens of 50,000 to 70,000, a ten- to twentyfold increase over what used to be a normal load. In addition to transmitting diseases, the ticks are irritating the moose, causing them to rub off large patches of hair and even skin, and leaving them greatly weakened from blood loss.

Lenarz said biologists have encountered moose in February and March, both deep winter months in northern Minnesota, with as little as 10 percent fur coverage on their bodies. “The ticks are giving them plenty of grief already,” Lenarz said, “And with no hair, if you’re trying to survive in a cold climate, you’re basically going to die from exposure. So it’s a double whammy.”

Deer-borne parasites

Minnesota‘s moose are also seeing increased incidence of brain worm and liver fluke, parasites whose transmission is aided by warmer temperatures because the forest is more hospitable to deer, the primary carriers and distributors of the parasites.

Scientists and locals say deer have thrived in the North Woods over the last several decades, in part because of milder winters, but also because their primary forage food, oak and maple leaves and saplings, is migrating northward from the state’s more temperate zones.

Moore, the Grand Portage biologist, noted that deer, while major carriers of brain worm and flukes, are not affected by the diseases. Rather, they aid in their distribution by dropping larvae-laden feces throughout the forest. The larvae in the feces are absorbed by snails and slugs, which attach themselves to plants and are ingested by grazing moose.

While not always fatal to moose, brain worm and flukes can have serious consequences on their well-being. Brain-worm-infested moose, for example, often show weakness in their hindquarters, have difficulty standing and turning their heads, and experience lethargy, blindness and even aggression.

The transmission cycle for liver flukes is similar, though it originates in a deer’s liver and is spread by eggs excreted in feces and absorbed by an aquatic variety of snail. The fluke causes liver fibrosis in moose. While not fatal, it can weaken the animals, making them more vulnerable to other threats, including predators.

Ron Moen, a biologist at the University of Minnesota, Duluth, and an associate in the school’s Natural Resources Research Institute, said the general consensus among scientists is that moose are taking a beating from ticks, brain worm and liver flukes. But individually or in tandem, the parasites may not be enough to cause such extensive mortality.

Increasingly, however, an additional factor is coming into play — heat stress.

Just as a tick-infested moose that rubs off all its fur in February will die of cold exposure, the same animal is equally threatened in August by 70-degree heat, scientists say. Even a balmy January, when temperatures climb into the 20s and 30s, can trigger behavioral changes in moose.

As ruminants, moose have a complicated digestion system that affects their ability to self-cool, and their internal thermostats are triggered with even the smallest temperature shifts.

So when temperatures creep upward in spring, moose immediately begin to feel the effects, scientists say, increasing activities that cool their bodies — like wading in lakes or lying in alder swamps — while cutting out heat-producing activities like walking and eating.

No ‘prescription’ for rescue

Lenarz, the retired DNR scientist, said such behaviors can increase moose’s vulnerability to other threats, especially if warm conditions persist into the fall, when the animals need to be building fat stores to maintain winter warmth and provide extra calories when foraging plants are buried under snow.

In fact, recent studies show the strongest correlation between moose mortality and warming temperatures in late January, when winter food supplies are running out, and again during the late spring months, when temperatures can rise quickly before moose have shed their winter coats.

Yet even as the evidence points strongly toward a climate-induced mortality, the state of Minnesota remains largely without policy solutions.

In December, the state released its “Minnesota Moose Research and Management Plan,” the most extensive report to date on the status of the state’s moose, threats to their well-being and prospects for recovery.

The report draws upon the latest scientific information about factors affecting moose. The 18-member brain trust behind the report made several recommendations to stem the population slide, but offered no solution.

In summary, the report states, “there is no cookbook or prescription for reversing a declining moose population in Minnesota. The issue is decidedly complex, and the research needed to answer critical questions will take time and will be very expensive.”

But the evidence is mounting against climate change as the North Woods shift from colder to warmer, from boreal to temperate, from moose-friendly to moose-intolerant.

Those trends will be difficult, if not impossible, to turn around, said Lenarz.

“If there is, in fact, a relationship between climate change and moose mortality, then we’re going to see it speed up even more in the next 20 years,” he said. “We may see little clusters hanging on in some areas, but it won’t return to where we were before. Those days are gone.”