Wolves in Yellowstone – the impact of wildlife on physical geography

Written By Isabel Rummell, ESLLC 2015-2016


Wolves were reintroduced into Yellowstone National Park from 1995 to 1997 as a response to issues of elk overpopulation and the loss of healthy vegetation and forests. With no natural predator to keep the elk population in check, their amounts reached unsustainable numbers and were overgrazing much of the grasses and baby trees in the area. Introducing wolves fixed this problem by limiting the elk population and frightening them away from open grasslands (where they are easily preyed upon).


While the changes in the biota of the area are widely known, it is lesser known that bringing in new species can change physical geography. After the reintroduction of wolves, the streams at Yellowstone changed drastically and the soils became more fertile and strong.

Check out this video, called How Wolves Change Rivers.

Today there are approximately 500 wolves in the Yellowstone ecosystem and at least 95 within the park (about 10 packs within the official park area). In the history of the park, there have not been as many. When the area was named a national park in 1892, the wolves were already at a decline. With predator control programs to create a safer environment for ranchers and their livestock, wolves were hunted expeditiously. By 1926, all sustainable populations of wolves were eradicated in the Yellowstone area.

Since reintroduction, wolf populations have flourished, yet are held in check by the constraints of the wild area. The biggest complaint so far is that many of the wolves leave the national park and hunt in livestock grazing areas, which was the reason they started. This puts them at risk of being shot by ranchers, and today, it is the single largest cause of wolf deaths outside of the official wolf-protected area.

However, the changing rivers are the most astonishing change for the park. With more grasses and tree roots holding the soil back, there was less erosion along the banks, streams meandered less, and they became much more narrow – harkening back to the pre-human landscape. Vegetation and forests regenerated after years of degradation as well. Since grazing animals like elk were staying away from exposed river beds and tender areas like grasslands, vegetation was able to grow back, including woody shrubs that hold the banks intact. River ecosystems were restored as well, with more vegetation creating a selection advantage for native species that need shadows.

All in all wolves were able to restore balance not seen in Yellowstone for years, and help some of its struggling ecosystems and changing topography for the better. We will see more of this in the coming years, as the wolves continue to stabilize their population and find sustainable niches. For more information, see the links below and the video.










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