We Reintroduced Wolves To Yellowstone 20 Years Ago And It Profoundly Changed The Ecosystem


Tom Hale


Tom Hale

Senior Journalist

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist


A wolf, with population-tracking tag and collar, runs through snow in Yellowstone National Park. Barry O'Neill/Creative Commons CC 2.0

The ecosystem of Yellowstone National Park has been to home a giant real-world experiment over the past few decades. Finally, the results are in and they’re pretty incredible.

Between 1995 and 1996, researchers reintroduced over 30 wolves back to Yellowstone and essentially left them to do their own thing with minimal human intervention. Not only did the wolf population quickly flourish and stabilize, but the reintroduction of an apex predator also sparked a chain of complex interactions that affected everything from the lives of other predators to the ground vegetation, making the ecosystem richer and more intricate in very unexpected ways.


“Yellowstone has benefited from the reintroduction of wolves in ways that we did not anticipate, especially the complexity of biological interactions in the park,” Professor Mark Boyce, ecologist and study author from the University of Alberta, said in a statement“How the vegetation in one valley responded to wolf recovery can be very different than in the next valley.”

As reported in the Journal of Mammalogy this week, the population of the wolves has remained relatively stable for over a decade at around 100 animals. Just as their models predicted, the rise of the wolves initially saw a decline in elk, which previously had no natural predators, but numbers are evening out now. This has opened up an opportunity for other big animals – both herbivores and carnivores – to become more prominent in the park's complex system.

Bison, a more formidable and dangerous prey, have now replaced elk as the dominant herbivore on Yellowstone’s Northern Range, and their numbers continue to steadily increase. The shift has also led to grizzly bears have an increasing role in elk calf mortality.

Elks also tend to avoid certain risky areas of the park where there’s a high threat of predation from wolves, areas known as “landscapes of fear”. However, this is only true during the day. When night falls and the wolf activity decreases, the landscape of fear opens up to the elk once again.


A change in foraging patterns by herbivores, such as elk and bison, has caused a substantial shift in Yellowstone’s vegetation with the recovery of willow, cottonwood, and aspen trees, which are constantly munched on by the resident herbivores.

“We would have never seen these responses if the park hadn't followed an ecological-process management paradigm—allowing natural ecological processes to take place with minimal human intervention,” added Professor Boyce.

Wolf reintroduction continues to be an appealing project for many wildernesses in North America and Europe. However, Boyce notes that these results might not necessarily be repeated in other systems, namely due to the influence of humans.

“Human-dominated systems are very different and wolf recovery will not produce the same results because agriculture, livestock, and hunting overwhelm the effects caused by large carnivores," he added. "We already have viable populations of wolves, bears, and cougars across much of Alberta but their influence varies depending on the extent of human alterations to the system.”


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  • biodiversity,

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  • Yellowstone National Park,

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  • environment,

  • predator prey,

  • elk