For most of the 20th century Yellowstone National Park had no wolves. Park rangers killed the last known pack members, two pups near Soda Butte Creek, in 1926. In January 1995 the animals came back: 14 grey wolves captured near Hinton, Alberta, were flown in, held in acclimation pens in the Lamar Valley and released that March. What happened next became one of the most famous stories in modern ecology.
How wolves returned to Yellowstone
Federal protection made the comeback possible. The grey wolf was listed as endangered in 1967, and the Endangered Species Act of 1973 gave the U.S. Fish and Wildlife Service a legal route to restore the species to the Northern Rockies.
The National Park Service sums up the project in a few numbers:
- Wolves were released in Yellowstone and central Idaho in 1995 and 1996, and in Yellowstone alone in 1997: 76 animals in total.
- Yellowstone received 41 of them: 14 from Alberta, 17 from British Columbia and 10 from north-west Montana.
- The regional recovery goal was 30 breeding pairs.
The population grew quickly. During the colonisation years up to about 2008, the park held as many as 174 wolves in 16 packs. Since then numbers have settled at around 100 wolves in roughly 10 packs, a level shaped by disease outbreaks such as canine distemper in 1999, 2005 and 2008 and by competition between packs. Pack size averages about nine or ten animals; the Druid Peak pack reached 37 in 2001.
What is a trophic cascade?
A trophic cascade is a chain reaction that runs down a food web. When a top predator reduces the number of its prey, or changes how the prey behaves, the plants or animals the prey would have eaten get a reprieve, and the effects can spread through the whole ecosystem.
The idea is older than Yellowstone’s wolves. Aldo Leopold described it after watching deer strip mountainsides once wolves had been exterminated. Another classic case comes from the Pacific kelp forests: when sea otters were hunted out, sea urchins multiplied and grazed the kelp away.
How Wolves Change Rivers: the video
In February 2014 the channel Sustainable Human set an extract from a TED talk by the British writer George Monbiot to music and footage. How Wolves Change Rivers runs about five minutes and tells the Yellowstone story as a single sweeping cascade: wolves push elk out of valleys, trees and willows return, songbirds and beavers follow, and the stabilised riverbanks change the course of the rivers themselves. It became one of the most shared nature videos of the decade.
What changed in Yellowstone after 1995
Several links in that chain are well documented:
- Elk. Elk make up as much as 92% of the wolves’ winter diet. The northern Yellowstone herd, once above 20,000 animals, fell from 16,791 in the winter of 1995 to 8,335 in 2004 and has been roughly stable since about 2010. Bears, human hunting outside the park, hard winters and drought all played a part alongside wolves.
- Bison. As elk declined, bison numbers rose.
- Trees and shrubs. Willows, cottonwoods and aspens have recovered in places, especially at the edges of heavily timbered areas.
- Beavers. The number of beaver colonies on the northern range grew from one in 2001 to nine in 2011.
- Coyotes and scavengers. Coyote numbers dropped sharply where wolves settled, while wolf kills now feed ravens, eagles, magpies, bears and even wolverines.
How far does the cascade go?
Ecologists agree that wolves reshaped Yellowstone; they still argue about how much. The Greater Yellowstone Ecosystem is huge and open to many other influences: cougars recolonising the park, grizzly and black bears, drought, hunting beyond the boundary and a long history of earlier wildlife management. Much of the northern range still looks like elk grassland. In one field experiment, willows only re-established where researchers both fenced out grazing animals and built artificial beaver-style dams.
That makes Yellowstone a richer story than the video can tell in five minutes. Wolves were the missing piece that set change in motion, and the park is now a living laboratory for watching an ecosystem rebuild itself.
The Lamar Valley, where the first wolves were released, is the park’s classic wolf-watching ground. Bring binoculars or a spotting scope and go at dawn or dusk, when wolves are most active and stand out against winter snow.
Related reading
- How Wolves Change Rivers: Reading the Links in a Trophic Cascade
- A Wolverine Digs Through Snow in America the Wild
- life in glacier-surface habitats
- the bear-and-barrel photo sequence
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