The pine needle-covered forest floors under Minnesota's deep conifer woods may be a key to the survival of moose in the state.
A scourge of parasitic brainworms has been weakening, sickening and killing moose throughout much of their traditional territory. But the moose that spend springtime under the tall pines and spruce trees of the state's remaining pockets of conifer woods seem to avoid the bug, according to a new study from the University of Minnesota and the Minnesota Zoo.
Biologists say whitetail deer are largely responsible for spreading the parasites across moose habitats. But the findings show that infection rates are not simply tied to the presence of deer, said Mark Ditmer, a researcher with the U and the study's lead author.
"That means increasing hunting or thinning the deer herd is not the only thing we can do," Ditmer said. "We also might have to find ways to manage the landscape for certain forest types."
Scientists have been racing to learn more about the brainworm and other threats to the state's moose population since numbers have fallen to just a third of what they were in the mid-2000s.
Studies have shown that up to 45% of individual and scattered moose populations in the state are infected with the brainworm. Some of them are killed outright by the parasite. Many more are made sick, weak or compromised to the point where they become easy prey for wolves or succumb to other diseases.
It's unclear exactly why moose are faring better in woods dominated by pine and spruce trees, Ditmer said. It may have to do with the way moose get infected in the first place — their food.
The brainworms may have always been present in Minnesota woods, but they only recently started causing havoc. The parasite likely evolved over untold years to coexist harmlessly with whitetail deer, said Tiffany Wolf, a researcher at the University of Minnesota's College of Veterinary Medicine.
It lives in slugs and snails, which are accidentally eaten by deer while they are foraging. Once eaten, the brainworm travels through the deer's nervous system to the tissue that surrounds its brain, where it can grow and reproduce without harming the deer. The deer eventually passes the parasite through scat, where it is once again picked up by snails and slugs, completing its life cycle.
The problem, Wolf said, is that warmer winters have allowed more and more whitetail deer to creep into moose range and leave behind more infected snails and slugs.
Moose, just like deer, accidentally eat the slugs. And moose are similar enough to deer that the parasite can still find its way to a moose's nervous system. But when the worm gets to the tissue that surrounds a moose's brain, it gets lost, Wolf said. It keeps migrating into the brain, itself, or into the spinal cord, where it can be deadly. Moose that aren't killed stop behaving like moose, Wolf said. They can starve. They're more likely to get hit by vehicles or be killed by other accidents. They become easy prey.
"Wolves may actually be doing what they should be doing — they're improving herd health by taking out the sick and the old," Wolf said. "But if we have a lot of sick moose, that becomes a problem."
Conifer forests hold just about the same densities of deer as other woods where moose are getting infected at higher rates, Ditmer said.
The difference could simply be that conifer forest floors are covered in needles, leaving less plants and food on the ground for moose to eat, Ditmer said. The moose, instead, need to forage for plants a few feet off the ground, where scat-eating slugs and snails are less likely to be.
The findings could help emphasize the importance of forest management as the state and other researchers hurry to find a formula to help moose numbers rebound.
It will likely take some combination of habitat reconstruction, forest protections as well as thinning deer herds in certain areas, Wolf said.
It's anyone's guess as to how much time Minnesota moose may have. The latest findings from the Department of Natural Resource's annual moose survey show that while the numbers are not getting better, they are not getting any worse.
For the past nine years, the DNR has estimated the wild moose population to be in the range of 3,000 to 4,000 animals, a third of what it was in 2006. That relative stability provides some hope that the population could increase, said Glenn DelGiudice, DNR moose project leader.
The survival rates for both juveniles and adults are close to where wildlife managers want them, DelGiudice said. If the survival rate of adult females could be raised by a few percentage points, the population might start to grow. But turning that corner has proved to be difficult.
It wasn't long ago that Minnesota had two healthy moose populations. In the late 1980s, more than 4,000 of the animals roamed the northwestern corner of the state. By 2007, that number was down to fewer than 100, DelGiudice said.
The population would suffer a sharp decline, followed by three or four years of stability, followed by another sharp decline, until the moose were all but wiped out from that part of the state, DelGiudice said.
For years, it looked like northeastern Minnesota was following the same trajectory.
"But now it seems like — knock on wood — this may stay stable for a while," DelGiudice said. "If it does, depending on a lot of factors, maybe we can get it to turn around."