In 2018, scientists moved wolves to isolated Isle Royale island to rebuild its ecosystem; five years later, their population had grown to at least 27

What began as a carefully planned wildlife restoration experiment on a remote island in Lake Superior is showing promising signs of success.

Between 2018 and 2019, wildlife managers released 19 wolves from Minnesota, Michigan and Ontario into Isle Royale National Park after the island's wolf population had collapsed to just one non-breeding pair.

Five years later, researchers found that the reintroduced wolves had established packs, reproduced every year and occupied territories similar to those used by earlier generations of wolves.


By April 2023, researchers counted a minimum of 27 wolves, compared with just 10 during the first winter they monitored.

The findings, published in Conservation Science and Practice in a study titled “Five-year assessment of wolf translocation to Isle Royale National Park, 2018-2023,” suggest the restoration has met its initial goal — although an old problem could still threaten the population's future: genetic diversity.

A population rebuilt from 19 wolves

Isle Royale has long been an unusual natural laboratory for studying the relationship between wolves and moose.

But by 2016, the island's wolf population had dwindled to a single non-breeding pair, raising concerns about whether wolves could continue to survive there at all.

The National Park Service responded by relocating 19 adult wolves from four nearby source populations between September 2018 and September 2019.

The animals were selected for good physical condition, while managers also tried to preserve social relationships by moving some wolves from the same packs together.

The goal wasn't simply to put wolves back on the island. Researchers wanted to create a population capable of surviving for at least 20 years and restore the predation pressure that wolves provide in the island's ecosystem.

To see whether that was happening, researchers used GPS collars, remote cameras, field observations and howl surveys. They also tracked reproduction, survival, pack formation and territory use and compared the results with historical wolf populations on Isle Royale.

The wolves started reproducing — and forming packs

The numbers steadily moved in the right direction.

The minimum population reached:

• 10 wolves in winter 2019-2020
• 23 wolves in each of the following two winters
• 27 wolves in winter 2022-2023

Researchers recorded at least 10 litters and 29 pups during the five-year study.

The young made up a proportion of the population similar to earlier estimates from Isle Royale, suggesting that reproduction was occurring at a level consistent with the island's historical wolf population.

The animals also established social groups.

Researchers identified three distinct packs, with their territories remaining relatively stable from 2019 through 2023. Wolves originally relocated from Michipicoten Island had the highest survival rate among the translocated animals.

That matters because simply surviving after release is not enough for a reintroduction project. The animals have to reproduce, form functioning social groups and establish territories without continued intervention.

By those measures, the wolves were beginning to behave like an established population rather than a collection of recently relocated animals.

Most wolves survived — but aggression was a major risk

The reintroduced population didn't grow without losses.

Researchers documented 11 deaths among 28 collared wolves during the study period. Aggression between wolves was the most prominent identified cause, confirmed or suspected in five cases.

Four deaths had no specific cause assigned, while two other wolves died from apparent injuries or stress associated with capture or translocation and were excluded from the survival analysis.

The estimated annual survival rate was 0.74.

The island's abundant prey base and the absence of human-caused mortality may have helped the wolves survive. But there is an interesting trade-off: as the population grows, competition between wolves could also increase.

In other words, the very success of the restoration could eventually create new pressures within the population.

Why the return of wolves matters

Wolves aren't simply another species living on Isle Royale.

They are a major predator in an ecosystem famous for its long-running wolf-moose relationship. Their return is therefore expected to help restore a more natural balance between predators and prey.

But the recovery is also valuable for scientists.

Because the reintroduction was carefully documented from the beginning, Isle Royale provides a rare opportunity to watch a predator population rebuild almost from scratch — from the survival of individual animals to the formation of packs and the birth of new generations.

The researchers can essentially follow the population as it moves through the difficult transition from a handful of relocated animals to a self-sustaining wild population.

The biggest question may be hidden in the genes

The numbers are encouraging, but there is a catch.

Genetic health remains a concern.

Computer simulations showed that the population has more than an 80% probability of surviving for 20 years, even under scenarios involving high levels of inbreeding.

That sounds reassuring, but survival as a population doesn't necessarily mean the wolves will have ideal genetic diversity.

A family group from Michipicoten Island played a major role in rebuilding the population, and its numerical and social dominance could eventually narrow the genetic diversity of the island's wolves.

That problem has historical precedent. The previous Isle Royale wolf population experienced significant inbreeding.

There is another complication, too: climate change.

Isle Royale is separated from the mainland, and winter ice bridges can provide opportunities for wolves to move between the island and the mainland, allowing genes to enter the population naturally.

As winters become warmer, those ice bridges may form less often.

That could make genetic isolation an even bigger challenge in the years ahead.

What happens next?

The five-year assessment suggests that the reintroduction achieved what it set out to accomplish: the wolves survived, reproduced, formed packs and established territories, while the population grew from a minimum of 10 to at least 27.

But the experiment isn't really over.

The next chapter will be about whether the population can maintain that growth without losing genetic diversity.

For now, though, Isle Royale offers a striking example of what can happen when a missing predator is deliberately returned to an ecosystem — and how restoring a species can be only the beginning of the conservation story.