They seem invincible, the largest species in the deer family, 6-feet tall at the shoulders with mature bulls averaging 1,000 pounds.
Yet the moose subspecies found in the lower 48 states have met a formidable foe of some sort in a wide swath of their range, from Idaho east through the northern tier of states.
Only Washington in the West and New England on the East are reporting stable or increasing moose populations.
The explanation could include the revival of wolves, infestations of parasites, changes in fire management and timber markets or the impacts of climate change.
Scientists say they don’t know for sure.
Minnesota’s moose population is dropping so fast that conservation officials have suspended moose hunting indefinitely starting this year.
In Montana, where moose have been declining since the mid 1990s and moose hunting permits have been reduced, the Fish, Wildlife and Parks Department has launched a 10-year moose study involving portions of the state. A dozen cow moose were tranquilized, captured and radio-collared in the East Cabinet area south of Libby in February. Biologists plan to study the health, reproduction rate and calf survival for the moose in each area while also studying their habitat.
Wyoming has documented serious declines of Shiras moose in the northwestern portion of the state over the past 30 years.
The Wyoming Game and Fish Department has considerably reduced its annual moose harvest in the region, no longer allowing cow moose hunting in the northwest and significantly limiting bull moose tags.
The big forest fires of 1988 that burned over a third of Yellowstone National Park were a setback for moose, said Kevin Monteith, who works with the Cooperative Fish & Wildlife Research Unit in Laramie, Wyo.
Loss of old-growth spruce-fir forests, where moose like to winter, could be a factor in the animals’ decline.
Increases in the number of large predators, like grizzly bears, black bears and wolves, may also be contributing to the drop.
But more recently Wyoming is seeing the moose population fall in other regions where those factors aren’t at play.
“Throughout the West, moose populations are declining in most herds,” Monteith said.
Nowhere in the lower 48 states has the decline in moose populations been worse than in Minnesota, which is home to the southernmost populations of Northwestern moose. Shiras moose, the ones found in Wyoming and Montana, are the smallest member of the moose family.
Alaskan moose are the largest, standing up to 7 feet tall at the shoulder and weighing up to 1,300 pounds. Northwestern moose fall between Alaskan and Shiras moose in size.
Northeastern Minnesota moose declined 35 percent from last year, state Department of Natural Resources officials report. Since 2010, the population has declined 52 percent.
“The state’s moose population has been in decline for years but never at the precipitous rate documented this winter,” said DNR Commissioner Tom Landwehr. “This is further and definitive evidence the population is not healthy. It reaffirms the conservation community’s need to better understand why this iconic species of the north is disappearing from our state.”
Based on the aerial survey conducted in January, the new population estimate is 2,760 animals, down from 4,230 in 2012. The population estimate was as high as 8,840 as recently as 2006.
Minnesota’s limited hunts were not behind the population decline, officials said, but “it’s the only tool we have to control mortality of moose,” Landwehr said.
Minnesota’s DNR is leading the largest and most high-tech multi-partner moose research effort ever initiated. Cost is estimated at $1.6 million over two years, with more research expected.
Starting in January, wildlife researchers began fitting 100 moose in northeastern Minnesota with GPS tracking and data collection collars. This multi-year research project will investigate the causes of adult moose mortality, calf mortality, calf survival, moose use of existing habitat and habitat quality.
The reasons for the decline are not entirely clear. Biologists say it could be parasites such as ticks and liver fluke that weaken the moose, making them vulnerable to disease or predators. It could be overall warmer temperatures from climate change - the giant animals are extremely heat sensitive and during hot summers may not eat enough to survive the winters. Or it could be a combination of these factors.
Previous surveys indicate nearly three-quarters of the moose calves born each spring are dying, a death rate too high to sustain the population. In many cases, dead moose are found that haven’t been touched by wolves or bear.
“We really think everything points to a health problem, but we haven’t been able to pinpoint anything with the data we’ve had up to now,” said Erika Butler, DNR wildlife veterinarian.
The trouble in northeastern Minnesota comes just a few years after northwestern Minnesota’s moose population crashed from 4,000 to just a few dozen in just 25 years. The phenomenon is creeping north into Ontario as well.
Research papers identify higher summertime temperatures in recent decades as an underlying issue. But that’s not what’s actually killing the animals. Scientists believe it’s a combination of higher temperatures, parasites such as brain worm and ticks, disease and increased deer numbers.
But a higher density of wolves - more wolves in the moose range - may also be killing more young moose than 20 years ago.
And a reduction in logging spurred by lower demand and shuttered mills has caused the forest to age in the past decade beyond the young aspen stands that moose thrive on.
“Were finding that moose really like to use small clearings, cuttings, as small as 20 acres or less, rather than the big openings that a lot of people had assumed they wanted,” said Ron Moen, a Natural Resources Research Institute moose researcher who has used similar collars the past two winters to track where moose eat, sleep, mate and have calves.
“We’ve got three things going on that are limiting moose right now - adult moose are dying, calves are dying and their habitat is diminishing. That’s not a good combination.”
In Wyoming, a recently published study showed that about 50 percent of 168 moose killed during the 2009 hunting season were infected with Elaeophora schneideri, a worm that takes up residence in the animal’s arteries.
Although Wyoming researchers haven’t made a direct link between infection in moose and the population decline, they’re trying to find out if it’s contributing to their poor condition, or if their poor condition makes them more susceptible to Elaeophora, Monteith said.
Montana has focused more resources on moose, including the hiring last year of a full-time moose biologist, Rich Deceasre, who will be in charge of the decade-long study of moose in the Cabinet Mountains, the Big Hole region and along the Rocky Mountain Front.
Colorado also is gearing up for a moose study and Utah has shown an interest in Wyoming’s work.