Opportunities to study carnivores before and after wolves are reintroduced to their ranges are rare. So researchers from CALS thanked their lucky stars when they were given the chance to do exactly that at Isle Royale, an island in the northwest part of Lake Superior. Their unusual and fortuitous study found that the effects of new wolves on the island were short-lived. And even in this remote location — one of the least-visited national parks — humans still had a significant impact on the lives of resident carnivores.

The study uses DNA from the scat and hair of foxes and martens to understand where these animals were and what they ate before and after wolves were reintroduced. While many studies have examined the effects of a carnivore reintroduction on their prey, far fewer have looked at how reintroduction affects other carnivores in the same food web — in this case, foxes and martens.

“We had this really amazing opportunity in Isle Royale, where we had data before this large carnivore reintroduction and then following the reintroduction of wolves, where we could look at how these effects within carnivores are taking place, and how they shift,” says Mauriel Rodriguez Curras MS’19, PhD’24, who completed this work as a graduate student in the lab of forest and wildlife ecology professor Jonathan Pauli.

Isle Royale’s isolated geography and limited variety of animals — including moose, beavers, and squirrels — make the island a relatively simple ecosystem in which to study the complexities of carnivore reintroductions.

Wolves first came to Isle Royale in the 1940s, likely by means of an ice bridge that formed naturally across 15 miles of Lake Superior from Minnesota or Canada. Recently, climate change has kept ice bridges from forming as often, meaning new wolves can’t cross over to Isle Royale.

The island once had 50 wolves across several packs, but by 2018 there were just two wolves left: a father-daughter duo that, due to inbreeding, were also half siblings. With the goal of restoring the natural apex predator to the island and rebalancing the ecosystem, the National Park Service introduced 19 wolves in 2019.

For this study, a typical field day involved hiking between 15 and 20 miles of trail to check traps — open PVC tubes with little brushes inside them — for hair samples and looking for scat to swab and collect. Once back at UW, Rodriguez Curras and Pauli extracted DNA from the samples and for each one determined the individual fox or marten that had left it behind. By measuring ratios of carbon and nitrogen present in the samples, they could also reconstruct the animals’ diets.

From their analysis, Rodriguez Curras and Pauli categorized the effects from wolves on other carnivores into three phases: absence, establishment, and coalescence. The absence phase is represented by data the lab had collected on foxes and martens the year before wolves were reintroduced to the island.

During establishment, which included the first year of the wolves’ reintroduction, no clear territories or packs had formed, and the wolves were wandering the island mostly as individuals. Foxes altered their island hangouts in this phase, moving away from the dense forest and closer to campgrounds.

Since foxes compete with martens for food (and have been known to kill the weasel-like animals), martens normally stick to the densely forested areas of the island where it’s easier to hide. But, with foxes shifting to other areas after wolf reintroduction, martens were able to expand their distribution on the island and increase their population.

Meanwhile, foxes were facing greater risk. Foxes hunt small prey, but they often rely on scavenging. Theoretically, scavenging off wolf kills is beneficial to foxes, who can’t easily kill prey as large as a beaver or a moose calf. But scavenging large kills also puts foxes in more frequent contact with wolves and elevates their risk of being killed. Rather than contend with wolves, foxes supplemented their meals by sticking close to campgrounds. They leveraged their cuteness and begging and raiding skills to target more accessible fare: handouts from human visitors.

By 2020, the wolves had coalesced into packs with defined territories. The effects of wolves on the other carnivores disappeared, and foxes and martens occupied areas and ate food similar to the absence phase.

“The rewilding of these species is an important move that conservation biologists are making to try and reweave the fabric of ecosystem function,” says Pauli, who’s been studying the island for eight years. “But I think the point is that when we do this reweaving of communities, unexpected things happen. I don’t think these are bad things, but they’re not necessarily things that we’d immediately predict.”

Another unexpected consequence was how strongly the island’s human visitors could affect these species interactions. Even though Isle Royale is considered one of the most pristine wilderness areas in the country and has relatively few visitors, Rodriguez Curras and Pauli found that humans, and the food they bring with them, have a significant effect on the relationships among the carnivores, where they live, what they eat, and how they then interact.

Rodriguez Curras and Pauli credit their partnership with the National Park Service for providing the opportunity to conduct research that can guide ongoing and future carnivore reintroduction efforts in other areas. Their work revealing the ways species interact with one another and with humans also provides Isle Royale National Park with the best available science to potentially improve visitors’ experiences while preserving
the island’s wilderness.

This study was published in May 2024 in Frontiers in Ecology and the Environment. The research was supported by grants from the U.S. National Park Service (P20AC00057,
P22AC01601), the National Park Foundation, and a fellowship from University of Wisconsin–Madison SciMed Graduate Research Scholars.