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Predator-Prey Relationships on Isle Royale

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Why is Isle Royale a Unique Research Location?

Ecosystem comparison of Isle Royale and Yellowstone National Parks. Isle Royale has a simpler ecosystem.
Isle Royale's ecosystem is simple compared to other national parks.

NPS

Isolation: Isle Royale is an island in Lake Superior. The closest mainland location is Canada, located about 20 kilometers to the north.

Limited Human Influence: Isle Royale National Park was established in 1940. The park closes annually for public use from October 30th to April 15th in order to protect wildlife.

Simple Ecosystem: the island’s remoteness restricts the amount organisms that populate the island.

Field researchers have conducted surveys of wolves, moose, beaver, and vegetation since the mid-1900s. Isle Royale predator-prey relationships have been continuously investigated since 1958- making this study the longest of its kind in the world.

Isolation and Predator-Prey Relationships

Five gray wolves hunt a moose at Isle Royale National Park in the winter.
Wolves are the island's apex predator. They prey on moose, beaver, and snowshoe hare at Isle Royale.

Photo Courtesy of J. Vucetich, Michigan Technological University

What if an ecosystem was unreachable for some animals and plants?

Not all mainland species are found on remote Isle Royale. Large mammals like wolves, moose, and beavers have made the journey. Some animals that hibernate in the winter do not inhabit the island, like bears. These sleeping animals snooze on their island link when an ice bridge sometimes forms between the island and mainland.

What would happen if only one predator inhabited an isolated ecosystem?

Wolves have been the single island apex predator of moose and beavers since the late 1940s. Other large predators are absent from the island, such as bears, coyotes, and mountain lions. Wolves directly impact how many moose and beavers there are at Isle Royale National Park.

What if an island lacked an apex predator, allowing prey to roam fearless on a secluded landscape?

Scientists have been asking similar questions about the predator-prey relationships on Isle Royale for over sixty years. The populations of wolves, moose, and beavers have fluctuated over time and one thing is certain- their island existence is interconnected.

Cow moose eating an aquatic plant on Isle Royale.
Aquatic plants are a great snack and rich in sodium.

Photo Courtesy of Jonathan Ringdahl

Wolves and Moose

Island Predator: Wolves

Lynx and coyotes have been absent from Isle Royale since the early-mid 1900s. Island presence of bears and mountain lions has not been recorded. Wolves became the single island apex predator upon their late 1940s arrival.

Island Prey: Moose

Moose were found on island in the early 1900s. Caribou and moose existed together for a brief period before caribou were wiped out in the early 1900s. Moose became a food source for wolves upon wolf arrival to the island.

Beaver swims in an inland lake. Reflected shoreline scenery can be seen on this Isle Royale lake.
Beaver are ecosystem engineers that create aquatic habitat for other animals and plants.

NPS / Jacob Frank

Wolves and Beavers

Island Predator: Wolves

Wolves utilize beavers as a secondary food source to moose. Wolves prey on beavers during the summer primarly, but will campout beside a beaver lodge in winter in hopes of an ambush. However, most of the time beavers are hidden in their lodges under the safety of ice during the winter.

Island Prey: Beavers

It is uncertain when beavers first inhabited Isle Royale. They are ecosystem engineers who construct dams that provide important aquatic habitats for other animals. Beavers are vegetarian and influence island vegetation, similar to moose.

Predator-Prey Relationships: Wolves-Moose & Beaver

Although considered a relatively simple ecosystem when compared to the mainland, the relationships between moose, beavers, and wolves on Isle Royale are quite complicated. Many internal and external factors influence the populations of island predator and prey.

Internal Factors: disease and the abundance of vegetation, prey, and predators

External Factors: climate change, weather, and disease

Abundance of prey on Isle Royale is undoubtedly influenced by the number of wolves, however, many other factors impact both moose and beaver populations. In one study, wolves were demonstrated to have a larger impact on the moose population of Isle Royale prior to a disease outbreak within the wolf population than after. In combination with decreasing populations of both moose and beaver, a likely increased amount of wolf on wolf mortality related to food shortages, and the canine parvovirus (CPV2) disease outbreak which greatly reduced the number of pups surviving into adulthood, the Isle Royale population of wolves, naïve to this strain of the virus, was significantly impacted. Dropping from 50 to 12 in just two years, this change shifted the ecosystem from one where wolves had a greater influence on the prey population, to one where climate, available forage and other undetermined factors were more influential. The relationships between prey and predator are constantly changing and that dynamic can be influenced by any number of environmental factors.

The island ecosystem may seem simple at first glance, but relationships between island predator and prey are more complex then is readily apparent.

Predator-Prey Research

Graph comparing populations of wolves, moose, and beavers over time at Isle Royale National Park.
Estimated abundance of wolves, moose (NPS and MTU, unpublished data) and beaver (NPS, unpublished data), Isle Royale National Park, Michigan, USA, 1958-2018. Graph retrieved from Wolves and the Isle Royale Environment 2018-2020, NPS and SUNY-ESF.

NPS/SUNY-ESF/MTU

This graph displays how Isle Royale predator and prey populations have varied through time. The National Park Service and Michigan Technological University have collaborated to conduct the longest consecutive predator-prey study that focuses on the relationship between wolves, moose, beaver, and vegetation. Relationships among wolves, their prey, and the environment have been affected by:

  • fluctuating population numbers

  • moose and beaver effects on plant browse

  • wolf inbreeding depression

  • disease

  • vegetation dynamics

  • climate change

Due to the wolf population decline in recent years, Isle Royale National Park decided to relocate wolves to the park in hopes to restore an apex predator in island predator-prey relationships.


The State University of New York College of Environmental Science and Forestry (SUNY-ESF) began partnering with the predator-prey research and wolf relocation in 2019.

Last updated: March 19, 2021