Now You See Them
Tracking the diet of Minnesota's vanishing moose
By Sara Specht
People like to say that you are what you eat.
Some university researchers are hoping the same could be said for moose—because what moose are is disappearing in record numbers in northern Minnesota. CFANS ecologist James Forester is hoping the reasons for it can be found by looking at what, when and where moose eat.
The recent dwindling of the moose population in Minnesota has been dramatic. Since a 1990s study estimated 4,000 moose in the western part of the state, those animals have virtually vanished—a recent count found only about 100.
The focus now has turned to moose in northeastern Minnesota, where a similar decline has begun. In the past eight years, the moose population in the arrowhead region has dropped 52 percent—from 8,800 to about 4,300. That makes the yearly mortality rate more than double the historical average of 12 percent per year in the same region.
With numbers like that, several groups are coming together to gather data on moose survival in the northeast. Forester, an assistant professor in the Department of Fisheries, Wildlife and Conservation Biology, has teamed up with the Minnesota Department of Natural Resources (DNR) and the Grand Portage Band of the Lake Superior Chippewa to collar more than 130 moose in the area over the past year.
The radio collars track the movement of the individual moose across the landscape, sending regular location emails, including mortality notifications. DNR and Grand Portage Band representatives in the area collect the fallen animals to perform necropsies to determine cause of death.
The DNR’s goal is to identify specific causes of death—like predation from wolves or health threats from parasites like brain worms. Forester plans to take a step back to look at changes in the moose’s food supply that might prove to be a larger source of troubles—and maybe reveal some steps land managers can take to save Minnesota’s moose.
“In order to survive and reproduce, an animal has to avoid predation, avoid heat stress in summer and wintertime, find enough energy in the summer and carry it through winter when food quality is low, and it has to avoid parasites,” Forester says. “All of that can be affected by its diet and the quality and composition of the forage. What we expect to find is that the distribution of resources in the landscape will be important in predicting how likely animals are to survive to the next year.”
The first task is to find out what food is available and what exactly the collared moose choose to eat. In the past such determinations have mostly been done by sight—walk into moose habitat and note which plants had been browsed at about moose-height. While this can be useful for population-wide estimates, Forester says, it doesn't identify individual animal behaviors.
“If we look at areas where known animals live and measure the composition of the forage that’s available to them, we can relate what they chose to eat to what they could have eaten,” Forester says. “Until we know what food is available to them on the ground, it’s hard for us to identify any kind of preference.”
To document the moose’s menu, Forester sent graduate student John Berini (’07–B.S., fisheries and wildlife) into the woods with a team of students to collect plant samples. Those samples will be subjected to a stable isotope analysis in paleontologist David Fox’s lab. The isotopic composition will vary depending on a variety of environmental factors to map a plant’s individual signature, Fox says.
To find out what each moose eats, Fox and Berini apply the same analysis to hairs collected from the collared animal at capture. Each spring, moose shed their hair and regrow a new coat by fall. So a long hair collected from the shoulders in early winter will hold a record of the previous year’s diet.
“You are what you eat,” Fox says. “If you section off the hair and compare the isotopic traits there to the signature of plants in localized parts of the landscape, you can trace when and where the moose is foraging. That isotope analysis is a tracer that links movement to how the moose is using the landscape.”
The team also is analyzing the chemical composition of the plant samples, based on the ground temperature where it was harvested. As a plant grows, it develops plant secondary metabolites (PSMs) as defense against being eaten. As the climate warms and extends the growing season, some plants develop more or more concentrated PSMs.
“Maybe there’s some effect of temperature that’s playing a role in this arms race with herbivores,” Forester says. “Maybe the quality of the forage is changing and some of these protective secondary compounds are making plants taste bad or become harder to digest.”
The team distributed temperature loggers across the region, and also is taking advantage of CFANS’ B4Warmed project, where forest researchers are maintaining forest plots at warmer temperatures. Using clippings from the B4Warmed plots of paper birch and balsam fir—common moose foods—analysis shows that as the temperature increased, more PSMs were created in the plants.
Now the team must identify the implications of those new compounds in a moose’s diet and expand the sample pool as other plant foods are identified.
“Things are definitely changing in the stuff that moose eat,” Forester says. “So we’ll have to look at how the production of these PSMs change across the landscape. Do we see a change in selection, based on their hair, that reflects the change in the biochemistry in the plants, and how does it affect an animal’s survival?”
A changing landscape
If the plants may be benefiting from warmer temperatures, the moose decidedly are not. They are well adapted to cold temperatures, and they have to expel substantial amounts of energy to cool off. This, too, can have an effect on their diet and the landscapes they inhabit.
Using hourly location signals from the collars, Forester is tracking the moose through space over the course of days and seasons, looking at the different land cover types the animals choose. For instance, when the temperature rises, a moose will look for cover or for wetlands to keep cool. But old forests with a lot of cover may not have as much low-slung forage as a younger forest.
Forester will use the tracking data to show whether moose respond to a landscape by changing their behavior so they can get all the things they need—food, cover and protection from predators. He hopes that if landscape patterns and plant composition can be linked to how a moose can survive, changes in land management practices could have a positive impact on moose survival.
“It may be that we can’t control what’s causing the decline in moose, that moose are going to leave Minnesota,” Forester says. “As an outdoorsman, that would be tragic to me. I wonder if my daughter and son will ever float the Boundary Waters with me and experience that connection with the wilderness, and I hope that we can figure out some management tools that will ensure that they'll be able to make that trip and see a moose in the wild.”