The wolves of Yellowstone

Yellowstone National Park, the majority of which is in Wyoming in the USA, was established on the 1st of March in 1872. In the late 1800’s wolves were a common sight in what was to become the National Park, however, as settlers travelled further North the wolves presented the issue of predation of livestock and threats to human lives. This lead to an anthropogenically caused decline in wolf populations. This was further supported by US Congress which presented funding for the removal of large predators from federal land in 1914. Wolves were then trapped, shot or poisoned until the last packs were removed in the 1920’s. After these events, Yellowstone’s ecosystem saw some dramatic changes to the landscape and the wildlife. The Elk saw rapid increases in their populations into 20,000 in the 1990’s; nearly double of what they once were.

After grey wolves (Canis lupus) made it onto the endangered species list the proposal to reintroduce them was brought forward and finalised in 1987. Three main groups were strongly against this decision. First of all tourists and locals were concerned about the risk that the wolves might injure or kill someone. Hunters were also worried that the wolves would drop the elk and deer populations causing there to be a lack of game over the following seasons of hunting. The main group that was strongly adverse to the reintroduction were the ranchers in the area. They feared that the wolves would cause large losses to their livestock, damaging their incomes. Despite the number of opponents, the wolf reintroduction went forward.

The wolves to be reintroduced were taken from the Canadian provinces as researchers thought this would yield the highest number of surviving wolves, as in Canada their main food source would be elk, similar to the National Park, the wolves in Montana primarily hunt deer. Fourteen wolves from several different packs were captured and transported. Following several weeks in acclimatisation pens, the wolves were finally released in the winter of 1995. Wolf numbers continued to grow as there were ten pups from that year and seventeen more wolves shipped in. On top of this, the mortality rate was less than half the expected value, however, some were killed illegally, and others died of natural causes. The damage to livestock and game expected by the wolves was largely unfounded. A non-profit organization was set up to reimburse ranchers for livestock lost to wolves.

Two grey wolves like those introduced to Yellowstone following their endangerment. Image: Ronnie Macdonald [Flickr]

The reintroduction of wolves had indirect consequences on the populations of the other organisms living in the ecosystem, also known as a trophic cascade. This phenomenon occurs when the predators of a food chain are able to alter the number or behaviour of the animals further down the chain. The wolves presented a rare opportunity to study the effects that a top predator can have on the lower trophic levels.

The main effect the wolves had on the deer population was not the amount they reduced it by but how they altered their behaviour, who quickly learned to avoid certain areas of the park such as gorges and valley bottoms. These areas made them vulnerable to being trapped. Changes to the grazing patterns allowed rejuvenation in the areas where the deer avoided. Within six years of the reintroduction, some of the trees had quintupled in height. Woodlands were able to grow on the once bare valley sides. This lead to an increase in the number of songbirds in the area. The impact the wolves created on the surrounding woodland allowed beavers to move back into the rivers, where they had recently been extinct. The Beavers’ dams created niches for other animals such as muskrats, otters, and ducks allowing their populations to grow and thrive. The wolves also killed the coyote in the area, which led to more rabbits and mice, allowing their predatory species, such as the hawk, fox, and weasels, to grow. Ravens and bald eagle numbers grew in the area as they fed on the carcasses left behind by the wolves, as did the bears living in the area. The increased number of bears reinforced the work done by the wolves as they would kill deer calves.

The most extraordinary impact the wolves had on Yellowstone National Park was their actions were able to change the course of the rivers. The deer stopped feeding in areas around the rivers and so vegetation was able to grow. This increased the stability of the riverbanks and reduced the amount of erosion and collapse. This caused the rivers to meander less and pools were able to form, which in turn formed habitats for different species. This combined with the other species that are living in the rivers increased the diversity of the area further and gave rise to additions to the food chains allowing predators and prey to prosper.


  1. 15 Feb ’18 at 9:25 pm

    dean shamblen

    I enjoyed reading and learning more about the wolves and their impact on the Park.
    Great article!!

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  2. I am going to Yellowstone this May for the first time. I plan on getting up at dawn in hopes of seeing a wolf or a pack of wolves. Thank you for your article.

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  3. I really enjoyed this article-amazing to think how much one species can impact the entire system!

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