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New research published in Nature Scientific Reports reveals how gray wolves in Isle Royale National Park seasonally alter their habitat preferences to align with beavers’ habitat preferences, a shift that might have implications for the island’s ecosystem.
Isle Royale is a remote national park in Lake Superior between Michigan’s Upper Peninsula and Minnesota and home to a plethora of animal species, including one of nature’s renowned architects, the American beaver.
“Beavers are ecosystem engineers by building dams that are creating impoundments. Large ponds that used to be moving water are now standing water, so that can actually affect the entire ecosystem that is now flooded,” said Adia Sovie, the study’s lead author and a Michigan State University researcher.
The team gathered pre-existing data on the number and locations of active beaver lodges in the park since the early 2000s and also used GPS trackers placed on gray wolves re-introduced into the park in 2018.
This data allowed researchers to look at the location of the wolves relative to the location of known active beaver lodges.
“We found that wolves are really dynamic in how they’re looking for prey and selecting prey throughout our study. In about March through November, they are hunting moose or looking for moose calves, and then during other times of the year they are looking for beavers,” Sovie said.
“They’re looking for beavers, and we think that relates to the difficulty of hunting moose later in the summer,” she said.
Prior to the reintroduction of wolves into the national park, the wolf population reached an all-time low primarily due to extreme inbreeding resulting in genetic defects.
Without the presence of large predators, the beaver population boomed.
Since the translocation of wolves onto the island, the beaver population has dropped drastically and the number of active lodges dropped from around 400 to 100.
Because of the high density of beavers in the past, resulting in fewer aspens, the environment had become less suitable for the lodges that are still active.
“The park has been used by beavers for several decades, so all the nice, easy aspens have been cut down, so it is a relatively poor beaver environment. Beavers are forced to eat marginal foods because the aspens are too far away from their ponds – they can’t get there safely,” said Rolf Peterson, a professor at Michigan Tech University and an expert in predator-prey interactions who was not involved in the study.
According to his own research, Peterson said he believes that American beavers are the preferred prey for wolves as they are easier to catch and kill than bigger prey like moose.
Milder winters may also be a factor in the wolves’ choice of prey, he said.
“As winters are shorter and warmer, I think that extends the open water season for beavers. They start building their food caches pretty later in October than I think they used to,” Peterson said.
“Beavers have to come out of the water to cut wood stands and trees for their winter food cache, which they put under the ice. That’s primarily for their young, the kits,” he said.
Another important factor in the increase of beaver-predation is the fluctuation in moose vulnerability throughout the seasons.
Moose are vulnerable to winter ticks, which benefit from warmer temperatures.
“The tick numbers seemed to fluctuate with the previous summer weather. When it’s warmer and dryer in the summer and the next winter, there’ll be more ticks on moose, causing more hair loss and nutritional stress for moose,” Peterson said.
The weakened moose are easier for wolves to hunt. As the moose grow stronger during summer and fall, wolves appear to shift their attention to beavers.
Wolves and beavers are keystone species in the island’s ecosystem, the study said.
The interplay between species may drive ripple effects throughout Isle Royale’s unique ecosystem, with potential consequences for other species like red foxes that may alter their behavior in the presence of wolves, according to the study.
“There’s a renewed appreciation for how important beavers are to wolves and a better understanding of how wolves are important to both [moose and beaver] these prey species,” Peterson said.
“There is, in many ways, a strongly top-down influence of apex carnivores on both moose density and beaver density, but beaver populations fluctuate in relation to their environment quality as well,” he said.
Akia Thrower writes for Great Lakes Echo.
