In 1995, 31 grey wolves returned to Wyoming's Yellowstone Park; decades later, 9 beaver colonies are thriving as willow forests recover and erosion risks fall

One single predator can change the fate of an entire ecosystem. When grey wolves were reintroduced to Yellowstone Park, Wyoming, in 1995, wildlife experts expected their comeback to have an impact on the local deer and elk populations. However, their arrival did not just change the animal population but also the behaviour and movement of their prey and also had an impact on the vegetation as well. Vegetation bloomed back into life, waterways transformed, and species that had struggled for years found new opportunities to thrive. Decades later, the restoration of the grey wolf population in Yellowstone taught how restoring one species can have an impact beyond that animal population and also transform the ecosystem.

How Restoring The Grey Wolf Also Gave Beaver Colony A New Lease Of Life

According to the Yellowstone National Park website, there was just one beaver colony in the park when the grey wolf was reintroduced into the Greater Yellowstone Ecosystem in 1995. These wolves were taken from Jasper National Park in Alberta, Canada. After the last pack was murdered in the 1920s, they became the first wolves to roam Yellowstone. Thirty-one wolves were moved into the park by the end of 1996.

Today, there are nine beaver colonies in the park, and there may be more to come as the reintroduction of wolves continues to astound researchers with its direct and indirect effects on the ecology.


One of those outcomes, according to Smith, is a thriving beaver population.

How The Wolves' Arrival Also Changed The Deal For Vegetation

Doug Smith, a wildlife biologist leading the Yellowstone Wolf Project, said that when the wolves came, it also changed the plants in the area. Even though elk were still being hunted by black bears, grizzly bears, cougars, and to some extent by coyotes, the absence of wolves helped lessen the pressure on elk from predators. So, the number of elk increased a lot—perhaps too much.

Two things happened: the elk stayed within the number of animals Yellowstone can support, and they didn’t move much during the winter. They ate a lot of young willow, aspen, and cottonwood plants. That was tough for the beaver because they rely on willows to survive during the winter. In 1968, Smith mentioned that when the elk numbers were about a third of today's level, the willow trees by the streams were in bad shape. Now, with three times more elk, the willow trees are still strong and healthy. Why? Wolves follow elk, which makes the elk always run and not stay in one spot for very long, so the elk don't eat the willow plants too much.

Indeed, a research project led by the U.S. A study by the Geological Survey in Fort Collins found that when elk eat a lot of willows and when beavers cut the trees like they do, the willows and other trees such as alders grow into small and weak shapes. On the other hand, when beavers were simulated to cut willow without elk browsing, the willow plants grew thick and healthy. In a study that lasted three years, plants that were not eaten had ten times more willow stem biomass compared to plants that were eaten. Plants that weren't eaten grew back 84 percent of their body size in just two growing seasons, but plants that were eaten only grew back 6 percent.

As the elk moved around in the winter, the willows stayed strong even though they were eaten a lot, and the beavers found plenty of food to eat. The beavers, which are the second biggest rodents, built new dams and ponds, which changed the ecosystem even more.

change how water moves through streams. They help balance the changes in water flow during different seasons; they keep water stored to refill the underground water supply; and they provide cool, shaded water for the fish. At the same time, the strong willow trees currently standing there provide home and shelter for songbirds.

Not Just Beavers Benefited From Wolves' Presence

Since the wolves, in the absence of hard winters, were the one factor that increased mortality rates for elks, researchers from the University of California concluded that the elk’s population loss benefited not just beavers but also grizzly bears and ravens!

Chris Wilmers said in the online journal Public Library of Science Biology that instead of the old pattern where elk carcasses were only available in large amounts sometimes and not others, like before wolves came back or during tough winters, there's now a more even spread of carcasses throughout the winter and early spring. He said that scavengers which used to depend on elk that died in the winter now rely on elk that are killed by wolves. This helps ravens, eagles, magpies, coyotes, and bears—like grizzly and black bears—especially since the bears come out of hibernation hungry.

How The Wolves Return Healed The Ecosystem

A 2025 short communication in Global Ecology and Conservation explained that bringing wolves back to Yellowstone had many more benefits because it helped increase the number of beavers. Beavers help hold more water in the land, which makes ecosystems stay balanced during dry times and through different seasons. In this way, wolves help another ecosystem engineer do their job without directly doing it themselves.

Wolves help plants grow again, and those plants help hold the streambanks in place so they don't wash away. Strong plants like willow and alder grow deep into the ground, which helps hold the soil in place.This makes it less likely for the soil to wash away during floods or when there is a lot of water moving. When streams have these features, they can get narrower, more steady, and have more different kinds of structures.This helps create better pools and side channels that make it easier for aquatic animals to live.

Aquatic ecosystems also get some advantages from wolves in an indirect way. Tall plants growing next to the stream give shade, which helps keep the water cooler. This is a key point for many kinds of fish and other creatures that live under the water. Besides that, less erosion means there's less mud and dirt flowing into rivers and streams. This makes the water look clearer and helps keep the surroundings of the water healthier.

At the same time, when wolves hunt, it creates new opportunities for another group of animals: the scavengers. Wolf kills serve as temporary ecological hotspots. They feed ravens, eagles, foxes, bears, insects, and many small microbes with food. When wolves eat carrion, they spread nutrients into ecosystems in big, strong bursts. This helps animals that might struggle to survive during harsh winters or when there's not enough food around.

The website says the number of wolves in Yellowstone Park has gone up and down, staying between 83 and 123 since the year 2009.

A well-known zoologist says that bringing back a species to its original environment can affect the local ecosystem. Dr.Axl Moehrenschlager wrote an article called "Can Conservation Translocations Turn Desperation to Inspiration?" that was published on the official IUCN Crossroads Blog.The article looks at how moving animals for conservation can help save endangered species.

He said that even though these actions have brought some good results, they are often seen as too dangerous or not clear enough. Moving animals from one place to another can help fix broken environments and bring back people's belief in the future of nature, as long as it's done properly following the best practices.