A vanishing icon? Minnesota moose are in trouble
ISABELLA -- Minnesota's moose are in trouble. The population of this iconic mammal that symbolizes the northern Minnesota landscape is in decline. If the decline continues at current rates, Northeastern Minnesota moose could be down to a remnant population within 50 years, biologists say.
And the state's leading moose research biologist thinks a warming climate may be the most significant factor in that decline.
"There's a strong correlation between the number of 'heat days' in a summer and the fall and winter mortality of our moose," said Mark Lenarz, leader of the Forest Wildlife Populations and Research Group for the Minnesota Department of Natural Resources at Grand Rapids.
Lenarz's conclusions are based on analysis he has made in the past nine months. He analyzed data collected over the past six years in an ongoing study of Northeastern Minnesota moose. The study is being conducted by the DNR, the Fond du Lac Band of Lake Superior Chippewa and the 1854 Treaty Authority. Biologists and helicopter crews radio-collared another 34 moose cows near Isabella this past week as part of that continuing study.
Minnesota's moose roam at the southern fringe of North America's moose range, and Lenarz said he is concerned about the implications of a warming climate on that population.
"It would be real nice to find something else that's the smoking gun that we could fix, but I'm not confident that's going to happen," Lenarz said.
Other factors -- winter ticks, liver flukes and brainworm among them -- may be the final cause of a moose's death. But those parasites and other diseases are always prevalent in a moose population, Lenarz said. He believes heat stress may weaken moose enough that their resistance is lowered, eventually allowing disease and parasites to kill them.
Mike Schrage, wildlife biologist with the Fond du Lac Band, cautions that a correlation between warmer summers and moose mortality does not imply a cause-and-effect relationship.
But, he adds, "I believe that climate is certainly impacting moose and is stressing moose."
Schrage also is concerned that Minnesota's expanding deer population may be adversely affecting moose. Deer carry some of the parasites that can kill moose, but deer are unaffected by the parasites.
Natural mortality rates for Northeastern Minnesota moose are much higher than the North American average. If those rates continue, Schrage said, "we've got about 50 years left and then we'll have very few moose left in Minnesota."
Researchers, working under a grant secured by Schrage and the Fond du Lac band, hoped to capture and radio-collar another 35 moose this past week in the ongoing study. Since the study began in 2002, the agencies have radio-collared 116 Northeastern Minnesota moose and gathered data from many of the 87 that have died.
PROBLEMS IN THE NORTHWEST
Moose problems in Minnesota are not new. The moose population in northwestern Minnesota has fallen dramatically since the early 1990s. Today an estimated 84 remain from a population that once numbered about 4,000, Lenarz said.
A DNR and U.S. Fish and Wildlife Service study said liver flukes, brainworm and a warming climate were all contributing factors to that decline. Brainworm is a parasite carried by deer and snails. It can be fatal to moose but usually does not kill deer.
About 6,500 moose still roamed Northeastern Minnesota in 2007, but that number was down from 8,400 in 2006. This year's survey results haven't been released, but Lenarz said they are not significantly different from last year.
Last fall, the DNR imposed a bulls-only restriction on the state's annual moose hunt, although 80 percent of hunters took bulls before the restriction was put in place. State hunters took 115 bulls in last fall's hunt, down from 159 in 2006 and 163 in 2005. Tribal hunters took another 47 moose last fall.
Hunting mortality is not a factor in the moose decline, biologists say. Plenty of bulls remain to breed all of the receptive cows, Lenarz said.
Calving rates still are relatively good among Northeastern Minnesota moose, Lenarz and Schrage say, but natural (non-human-caused) mortality rates have averaged 21 percent annually during the ongoing study. That compares to a North American average of 8 percent to 12 percent. For 46 of the 87 moose that have died during the current study, the cause of death was officially listed as "unknown."
"It's potentially disease or parasite-related," Schrage said. "What we have is moose that should otherwise be in the prime of life that look like they laid down and died. Many died in the summer to early winter period when moose should not be nutritionally stressed. A number of our bulls had very poor antler development for their age or shed their antlers very early, suggesting to me that they had long-term chronic problems..."
Autopsy results often are inconclusive, Schrage said, but "it has often pointed towards brainworm, liver flukes or winter ticks."
Lenarz was asked if biologists are seeing the Northeastern Minnesota moose population following the pattern that occurred among northwestern Minnesota moose.
"Basically, we are," he said.
HOW WARM IS TOO WARM?
Moose begin to feel heat stress when the temperature reaches 57 degrees, Lenarz said. At that temperature, moose must increase their metabolism -- burn more energy -- to dissipate heat. At 68 degrees, moose resort to open-mouthed panting to relieve stress, he said. That inhibits their ability to feed.
Studies on cattle, another large ungulate, show that heat stress leads to reduced hours of feeding, reduced weight gain, reduced production of milk and reduced efficiency in the immune system, Lenarz said. It is not unreasonable to think that moose might react in the same way, he said.
"We've seen an increase over the past 20 years of temperatures over 68 degrees," Lenarz said.
But heat stress is unlikely to be the direct cause of a moose's death.
"I don't think we'll ever walk up to a moose in the heat of an August day and say it died of heat," Schrage said. "But by continuing this study, we're going to be able to better predict how climate impacts moose."
A warming climate also may contribute to a moose decline in other ways, he said. Warmer temperatures may create more favorable conditions for deer, which harbor the parasites that can be fatal to moose. And a warmer climate may increase the survival of those parasites and their hosts, Schrage said.
THE DEER CONNECTION
Schrage thinks there should be more diligent research on the relationship between deer and moose and their habitat needs.
"I don't think the public agencies that have land management and wildlife management responsibilities are working together on this issue as much as they should," he said.
Lenarz, with the DNR, believes that current levels of research are sufficient.
"It's not a matter of throwing money at it. I'm not sure there is a way to solve it," Lenarz said.
Schrage thinks the DNR should be working harder to control the deer population, especially in areas where it overlaps the moose range.
"I don't think we're as aggressive in going after deer as we need to be," Schrage said. "It [deer herd reduction] is not going to just happen through hunting. I think we need to look at deer hunting, deer feeding and deer habitat."
Schrage believes deer feeding is artificially supporting a higher deer herd, making it harder to reduce deer numbers through hunting. He's trying to remain optimistic in tough times for moose.
"To be 100 percent fair," Schrage said, "given climate change, we could put a lot more research into moose and the answer will be that no matter what we do, we are not going to stop the moose decline. But I'm not convinced that's what's going to happen."