Foxes on Isle Royale Rely on Humans in Summer, Wolves in Winter, Study Finds

Photo: Red fox trotting along the pathway in Rock Harbor on Isle Royale. Photo by Doug Marrin

Author’s Note: At the end of a week-long backpacking trip on Isle Royale, we were resting at a picnic table outside a Rock Harbor shelter when a red fox suddenly leapt up, snatched a protein bar just inches away from us, and vanished into the brush, leaving us all gaping in astonishment. Encounters like this may seem surprising in such a remote wilderness, but new research reveals a bigger story about how foxes survive on the island.

A new study out of Isle Royale National Park highlights how red foxes survive Michigan’s harsh seasonal shifts by turning to two very different food sources, park visitors and wolves.

Researchers from the University of Wisconsin–Madison and the National Park Service tracked the diets of foxes between 2021 and 2024 using stable isotope analysis. They found that foxes leaned heavily on human foods during the summer months, when campers and hikers were present. In fact, nearly a third of their diet came from human sources such as discarded food.

When the park closed each fall and visitors left, foxes switched strategies. Through the winter, their diets became far more specialized, relying on carrion left behind by wolves, particularly moose carcasses from wolf kills.

The findings surprised researchers who expected the 2018–2019 wolf reintroduction to suppress smaller predators like foxes. Instead, the study found that the complementary food subsidies, human foods in summer and wolf carrion in winter, kept fox populations stable.

“This helps explain why we’re not always seeing mid-sized predators decline when wolves return,” said lead author Mauriel Rodriguez Curras.

Isle Royale, located in Lake Superior off Michigan’s Upper Peninsula, is known for the world’s longest-running predator-prey study of wolves and moose. The park is also the least-visited national park in the United States, yet even limited human presence appears to alter wildlife diets.

Researchers say the findings carry broader lessons for wildlife management across Michigan and beyond. In landscapes where people and predators overlap, mid-sized carnivores may continue to thrive by adapting their diets, sometimes in unexpected ways.