A creek running through an old beaver meadow (a beaver pond that has drained and become a meadow) near Junction Bay in Voyageurs National Park. Photo by Tom Gable.
This story originally appeared in Quetico Superior Wilderness News
Imagine a wolf kill, and the remains of an ungulate—a hoofed animal like a moose—probably come to mind. Yet scientists have known for some time that wolves also eat smaller prey, like deer fawns and beavers. This is particularly true during summer, when wolf packs primarily hunt and travel as individuals or pairs.
What hasn’t been well understood is how wolves kill those smaller mammals, or how their presence affects wolf predation of larger mammals. An ongoing study in Voyageurs National Park (VNP), however, is uncovering summer wolf predation on beavers. It’s leading to new insights about the ways wolves hunt—breakthrough information helping scientists create a fuller picture of how wolves impact the larger ecosystem.
“Wolves and beavers are important ecological species. Beavers totally transform landscapes and wolves can too in direct and indirect ways. Understanding factors that cause those populations to change and fluctuate has implications for Voyageurs and beyond,” said Tom Gable, researcher and PhD student at the University of Minnesota. He’s been working with VNP and park wildlife biologist Dr. Steve Windels for the past several years, studying wolf-beaver interactions.
Gable likens it to untangling the “secret lives” of wolves, and he isn’t exaggerating. Until recently, studying wolves during the summer has been difficult.
The mysteries of summer
Dr. David Mech is a senior research scientist with the U.S. Geological Survey and a professor at the University of Minnesota. He’s been studying wolves since 1958, and knows these challenges first hand.
“It’s been quite hard for science to learn very much about how wolves function in the summer,” Mech explained. “We know a lot about their movements and things like their denning behavior, but we don’t know much about their hunting in summer.”
Scat collection has shown that they eat primarily deer, moose and beavers, but understanding how they hunt has been another story. Mech himself has tried, attempting a study of summer wolf-beaver interactions in northeastern Minnesota about a decade ago. Of the 200 localized areas of wolf activity that Mech and the team investigated, they found only one beaver kill.
He attributes some of that to the particular pack he studied, but wolves are also hard to study during the summer.
University of Minnesota PhD student Tom Gable holds a yearling wolf’s leg so Northern Michigan University graduate student Austin Homkes can draw blood from the wolf. Holding the leg causes the vein in the leg to be more obvious which makes it easier to get a sample. Photo courtesy Tom Gable.
As Gable explained, “In the Minnesota boreal system, the vegetation is thick.” It’s nearly impossible to see the wolves, and when they kill smaller animals like beaver, there are fewer if any remains. Fortunately, advances in GPS technology have made it possible to track wolves more closely. And with the support of VNP, Gable has been able to overcome these challenges.
Going where the wolf goes
Gable started working with VNP in 2014, after Windels reached out to Dr. John Bruggink at Northern Michigan University looking for partners to study wolf predation. As a wildlife biologist, Windels had been monitoring wolf and beaver populations in the park since the mid-2000s and wanted to understand more about predation. Gable was looking for opportunities for his masters’ research, and the resulting partnership has continued into Gable’s PhD research at the University of Minnesota.
According to Windels, Voyageurs presents a unique opportunity to understand wolf-beaver dynamics. “Beaver population density here is likely higher than anywhere else in the United States. We also have very high wolf densities, and both deer and moose are present in the park. The densities of beavers in the park likely reflect densities of beavers that were present here before European settlement.”
That offers a unique window into how southern boreal ecosystems evolved to function. And according to Gable, “We are trying to gobble up as much information as we possibly can to have better idea of what’s going on out there.”
To do that, Gable and the team fit wolves with GPS collars that transmit the wolf’s location every 20 minutes, or 72 locations per day. When consecutive locations are within 200 meters of each other, Gable and a crew of four to six research assistants and volunteers investigate those clusters as possible kill sites.
“We do this every day,” Gable said. “From eight until five, we’re bushwhacking and going where the wolf goes.”
Once or twice a week, the crew heads back into the field after dinner. They drive around and howl for wolves until three or four in the morning. The late nights enable them to locate packs they haven’t been able to collar. They also set up cameras near beaver ponds and lodges. The semiaquatic rodents don’t have the necks for GPS collars so the cameras provide insight into animal behavior in these areas.
The efforts are yielding what Gable calls “insight no one else has.”
New insights into wolf-beaver dynamics
When wolves hunt deer and moose, Gable says they primarily outrun and outlast their prey. Wolves are not known for secretive behavior or waiting out their prey. However, Gable and his colleagues determined that wolves appear to hunt beavers by bedding down next to feeding trails and lodges, and then ambushing the beaver when it comes close. They published these findings in 2016.
A beaver lodge at the west end of Jorgens Lake in the interior of Voyageurs National Park. Pictured: Northern Michigan University graduate student Austin Homkes. Photo by Tom Gable.
Interestingly, this behavior isn’t consistent from wolf to wolf; some wolves appear to specialize in hunting beaver. Dr. Joseph Bump, Gable’s co-advisor with Windels, is a University of Minnesota associate professor and Gordon W. Gullion Endowed Chair in Forest Wildlife Research and Education. He noted that this behavior is “not only opportunistic, but very deliberate in some cases. And it is not uniform across the wolf population or even within packs.”
Understanding this type of individual variation in predatory behavior is what Bump calls a cutting edge area of predator-prey ecology.
The research is also revealing some interesting insights about the effects of this predation on the beaver population. In 2017, Windels and Gable published a paper outlining that impact, Where and How Wolves (Canis lupus Kill Beavers (Castor canadensis), in PLOS ONE.
“We found that one pack removed around 40 percent of the beaver population in its territory,” Gable said, or about 80-88 beavers. “Previous research has shown that once hunters and trappers remove around 25 to 30 percent of a population, we start seeing the population decrease. But we saw that even though wolves removed 40 percent, the next year the beaver population increased by 43 percent. It shot way up.”
There are so many beavers in Voyageurs that as wolves remove them, beavers from other areas move in. And understanding this type of relationship is completely new territory. According to Gable, theirs was the first such estimate published in the scientific literature.
The significance of the research
The interior of Voyageurs National Park where Tom Gable is conducting research on wolves and beavers as part of a study cosponsored by the Park and the University of Minnesota.
At this point, it’s difficult to extrapolate the impacts of this research. It could have implications for understanding wolf-prey interactions in parts of Canada and Europe where beaver are also prevalent. Parts of Canada, for example, are experiencing a decline in caribou and wolves have been implicated as a cause. Understanding how the presence of other species, like beaver, affects wolf predation could influence how land managers respond to the situation.
“One thing we’re really interested in is what is the relationship between wolf predation of beavers and wolf predation on deer and moose. We want to see, do beavers negatively impact a population of, say, moose or caribou, or do they actually benefit them?” Gable said.
“Another question we’re interested in at Voyageurs is that we have a low density of moose but haven’t seen the decline in our moose population that people have seen elsewhere in Minnesota. The question is why. What’s different about Voyageurs compared to other places, and is it possible that deals with beavers?” he continued.
The answers to these questions will only become clear over time. For now, the goal is to collect as much information as possible. And in the meantime, the significance of the findings can’t be underestimated.
In a 2015 book, Wolves on the Hunt: Behavior of Wolves Hunting Wild Prey, published by Mech and his colleagues, the section on beavers is just a few paragraphs. So little was known about wolf predation of beavers that there wasn’t much to share. The research at Voyageurs is changing that.
As Mech himself said, “It represents a breakthrough.”
About the Author
Alissa Johnson moved from Minnesota to Crested Butte, Colorado, where she works as a freelance writer and writing coach. As the founder of WritingStrides, she helps writers overcome obstacles and create meaningful stories. Her work has been published in the Wall Street Journal, Dirt Rag Magazine, and The Master’s Review among other publications.
Further conversation with researcher Tom Gable
Agate was so intrigued with Alissa Johnson’s story that we reached out to researcher Tom Gable for more information. Here’s what we learned:
Agate: You published research that showed that wolves actually hide and ambush beavers, and that was not known before?
TG: That’s right. We usually think about wolves as predators that outrun and outlast their prey, rather than sneaking up and ambushing prey. But beavers are totally different prey from deer or moose. If you want to catch a beaver, you’ve probably got to change your tactics because beavers spend very little time on land—intentionally—to avoid predators like wolves, or even bears or cougars. It does seem that wolves have figured out that the best way to catch beavers is to wait at the areas where beavers will predictably come on land, for example on trails where beavers are feeding, or by dams. If the lodge is on the edge of a pond, wolves will wait next to the lodge. So they do appear to be ambushing beavers, but we’re still trying to figure out some of the specifics about how are they figuring out how to do this, and where are they choosing to wait in ambush. We’re starting to get to the really interesting questions about what wolves are thinking when they’re going through this process.
Agate: It must be interesting to try to get inside a wolf’s brain.
TG: It’s interesting and frustrating at the same time. Unfortunately, they don’t tell us what they think, so we have to infer as much as we can from the data we get from GPS collars and the wolves’ behavior or movements, and try to deduce what the wolf is actually trying to do. And there are patterns that become discernable over time, but it takes a long time to get that data, and a lot of work as well.
Agate: If they’re like dogs, they can learn.
TG: Oh, yes, they’re very intelligent predators and we do think there’s a certain element of learning involved, but what that is or when do they figure it out, we don’t know. That’s one of the things we’re interested in, seeing when wolves actually learn that “this is how I catch a beaver.” I’m sure it’s probably a combination of a lot of things. Some individuals are just more adept at it, and they probably learn from running around their territory and seeing a beaver on land and trying to catch it but not succeeding, or just by watching beavers and making a connection like, “oh, I only see a beaver on a feeding trail at these times.” That’s speculation on my part but I’m guessing that something has to happen for them to learn to ambush beavers.
Agate: What are some of the patterns you’ve noticed?
TG: Wolves are bedding down really close to where they’re trying to hunt and kill beavers: they’ll wait maybe two or three meters from a lodge or a dam. Why are they waiting so close instead of farther away? We think they probably have a very short window to catch a beaver. Once that beaver comes on land the wolf has to attack quickly before the beaver goes back into the pond.
Agate: There must be a special barrier between the pond and land, because a beaver under water would have no way of knowing there’s a wolf out there.
TG: That’s another part of the dynamic: how does a wolf know that a beaver is going to come on land if they can’t even see the beaver? Somehow the wolves know that “If I wait here long enough, a beaver will probably come on land.” And on the beaver’s side of things, it’s also fascinating because they have to balance between pretty intense pressures: they have to get food, they need to forage on land, but they also know there may be wolves waiting for them and they’ve got to avoid getting caught and killed. We’re also trying to understand how beavers go about feeding in a way that reduces the chance of getting killed by wolves, and in Voyageurs National Park it’s not a small risk. They’ve probably adapted their strategies to minimize that, but we don’t know what those adaptations are yet. We’re trying to look at it from both sides.
Agate: I suppose your big question is, are beavers benefiting moose and are moose doing well in the Park because there are so many beavers?
Wolf V026, a two-year-old female collared in summer 2015. She was fitted with a GPS collar that could transmit locations every 20 minutes, allowing researchers to track movement and predation behavior. This photograph was taken as the wolf recovered from being sedated, which researchers did to put on the collar, collect samples, and take several measurements. Photo courtesy Tom Gable.
TG: Compared to parts of northeastern Minnesota, Voyageurs National Park has a fairly low density of moose: we have about 50 moose in the middle of the park, but that population has stayed relatively stable over time. Moose are declining throughout the rest of the state, but we haven’t seen a similar decline in Voyageurs. We’re not sure why: we have all the same issues—disease, predators, and all the other things that contribute to losses of moose populations in other places in Minnesota. So we are wondering about the role beavers may be playing. Some research has suggested that by killing moose calves, wolves have a big impact on moose because the calves aren’t surviving to become adults. Maybe beavers are a summer food source for wolves and when there are lots of beavers around, the wolves are not so interested in going after a moose calf and fighting its mother to get the calf, and instead they’re going after beavers because it’s easier. The question is, are beavers buffering moose from wolf predation? Does it come down to: the more beavers there are in a wolf’s territory, the fewer moose and deer the wolf will kill?
Agate: How will you answer that question?
TG: We use GPS collars to look for kills all summer long, so we can estimate how frequently wolves are killing a certain prey. We can see if there’s an inverse relationship between how frequently a wolf kills a beaver and how frequently it kills a deer. We’ll factor in the number of deer and beavers living in each of the packs’ territories, and we’ll compare packs with different prey densities to see what trends we may identify. We’ve also collected about 5,000 wolf scats over the past three years, which tell us a lot about what they’re eating.
It’s possible beavers don’t have any effect at all. Other researchers have suggested that beavers might not be helpful for deer, moose, or caribou populations because perhaps the wolves eat beavers during the summertime, and fewer moose or caribou, but because of this extra food from the beavers, the wolf population might reman higher than it would be if there were no beavers around. Then, in the winter, when all the beavers are trapped below the ice, you have this sort of inflated wolf population that’s hungry, and the only thing available are deer, moose, or caribou, so now the predation pressure increases on those species because the wolves don’t have anything else to eat. That’s kind of the flip side of what we’re thinking in Voyageurs. We don’t know for sure, but it’s an interesting question and it’s relevant for large parts of the boreal ecosystem, especially in North America.
Agate: Will there be any implication for management in your results? Can you see wildlife managers bringing beavers in to protect the moose population, if that’s how it turns out?
TG: It’s hard to say. If there is an effect, how significant is it—is it negligible or substantial? There is always the possibly of management implications but they’re got to be well thought-out ahead of time. If we know beavers are reducing predation on moose, you could do things to try to encourage a larger beaver population—reducing harvest of beavers or making better beaver habitat—that could reduce predation pressure. But if beavers are actually not benefitting moose, then you might keep beaver populations lower in areas where you want moose to recover. In Canada they’ve had issues with caribou populations declining dramatically, somewhat similar to our moose populations, and they think wolf predation is a big factor. The caribou weren’t the main prey; it was moose that was the main prey; the wolves would just kill caribou incidentally as wolves moved through their territory. So the government started having larger moose hunts that actually culled a lot of the moose, and by reducing the moose population, they indirectly reduced the wolf population, which then reduced predation on the caribou. So there are ways you can manage these things to get the result you want. But there’s a lot of complex things that go into management; we’re just providing the research that might show how the system operates, and we’ll let the managers use it for what they think is the best fit for their area.
Agate: When do you hope to have an answer?
TG: Hopefully by the end of my PhD, in about 2 or 3 years, when we’ll have collected most of the data. So far we’ve only followed two or three wolves each summer, but this year we plan to collar and follow more. One of the first questions we want to answer is, do beavers actually increase the size of the wolf population? That would be an insightful piece of information to start with. Then we can ask what effect beavers could be having. Doing wolf research, you just never know. There’s a lot of logistical challenges: you can get a collar on a wolf but it might not work or the collar might fail or stop working; it’s possible we might not be able to catch enough wolves to put the collars on for a large enough sample for statistical analysis, so there’s a lot of factors outside our control.
Agate: And you haven’t even mentioned the mosquitoes.
TG: Oh, yes, field conditions are another thing. Just keeping the sanity of everyone working on this project is another challenge. The questions we are trying to answer are difficult questions to get at, which is probably why it hasn’t been answered yet. Wolves travel vast distances—a typical pack’s home range is 150 or 200 square kilometers—and to understand what they’re eating we have to travel those distances as well. It’s challenging enough to keep track of one or two wolves, but when you have to follow eight or ten wolves, trying to see everywhere they go, it’s an all-consuming life; you’re basically married to those animals during the season. It’s very rewarding in a sense because you get to see the intimate lives of these animals—indirectly— but it’s also very challenging.
