Case Study

Re-introduction of wolves in Yellowstone National Park


In the United States, the wolf was considered to be extinct from Yellowstone National Park by the mid-1930s. Wolves had been exterminated from Yellowstone National Park, for fear and threat to humans and a predator pest to livestock. Yet, their absence resulted in negative cascading effects.

Without the wolves present in Yellowstone to hunt and kill prey, the elk population increased dramatically, resulting in overgrazing and ecological degradation. This in turn affected the habitat of many other animals and plants in harmful ways resulting in an unbalanced ecosystem. By removing an apex predator like the wolf from the food web, we caused a harmful “top-down trophic cascade”.

Having been absent for over 70 years, it was believed that returning the wolf as an apex predator may restore ecosystem balance. So, in 1995, around 30 Canadian wolves were reintroduced back in the park.


Within six years, existing trees had quadrupled in size and trees had begun to re-grow in valleys, which had previously become bare. Birds and beavers began to flourish, beaver dams created habitats for otters, fish, frogs and reptiles. The return of the trees altered the course of the rivers themselves, reducing the rate of erosion, narrowing the width of streams creating more pools and rapids. Vegetation began to recover on the hillsides. The reintroduction of a single species, the wolf, transformed an entire ecosystem.


Following the reintroduction of wolves, both vegetation cover and height increased in the park due to the wolves’ increased predation of elk. In addition, elk behaviour changed. Herds are now more mobile and avoid areas where they may be vulnerable to predation, which has reduced the intensity of grazing, resulting in a number of benefits to other species such as


  • beaver through improved habitat
  • grizzly bear through improved food availability
  • birds of prey through increased carrion associated with wolf kills
  • benefits to the physical environment have also been recorded including reduced erosion of river and stream banks


Wolves became an ecosystem engineer – creating a niche for other species.


The wolves restored and transformed the eco-system and physical geography. These documented cascade effects highlight what happens when a species is removed from an ecosystem, by local extirpation or even extinction. In Yellowstone, biologists have the rare, almost unique, opportunity to document what happens when an ecosystem becomes whole again, what happens when a key species is added back into the ecosystem equation. And this is what we hope to achieve at Aussie Ark – to restore an unbalanced ecosystem and let nature take its course!