How the Wolves Changed a River…

Gray wolfThree years ago I watched a short movie by Chris and Dawn Agnos about the reintroduction of the gray wolf (Canis lupus) to Yellowstone National Park. It was as amazing as it was revealing.1

When forty-one wolves from Canada and northwest Montana were reintroduced to the park in 1995—after wolves had been absent for seventy years—something incredible happened. The wolf helped provide balance, complexity and stability to an ecosystem that had been mismanaged and lacked a balanced functionality1,2. In the absence of a major predator (like the wolf), the elk population had exploded (to over 20,000) and, despite efforts by the park to reduce their populations through hunting, the elk massed along the river banks and managed to over-graze much of the lower valleys of the park. Mice and rabbits, unable to use vegetation for cover, fell prey to coyotes. Grizzlies suffered, having no berries to feed on for their long hibernation.

gray wolf in yellowstone

Grey wolf in Yellowstone National Park

The first major change of the wolf reintroduction was an alteration of the behaviour of the elk. The elk started avoiding certain parts of the park where they could be trapped, particularly the valleys and gorges. As a result, these areas rapidly regenerated. In some areas, tree height quintupled in six years. Aspen, willow and cottonwood recovered from severe browsing on bare valley sides and lowlands.1,2 With increased flowers, shrubs and trees, the song birds and migratory birds increased as did bees and other pollinators. Beavers also came back; these ecosystem engineers created more complex habitats for other life forms such as amphibians, fish, and reptiles.2 Researchers at the University of California at Berkeley determined that the combination of less snow and more wolves has benefited scavengers both big and small, from ravens to grizzly bears.3


Beaver ponds in Yellowstone

“Instead of a boom and bust cycle of elk carrion availability—as existed before wolves and when winters were harder—there’s now a more equitable distribution of carrion throughout winter and early spring,” writes Brodie Farquhar on the Yellowstone Park site4. Chris Wilmers in the on-line journal Public Library of Science Biology added that, “scavengers that once relied on winter-killed elk for food now depend on wolf-killed elk. That benefits ravens, eagles, magpies, coyotes and bears (grizzly and black), especially as the bears emerge hungry from hibernation.”4

“I call it food for the masses,” said Ed Bangs, wolf recovery coordinator for the U.S. Fish and Wildlife Service. He was genuinely surprised by the vast web of life that is linked to wolf kills: “Beetles, wolverine, lynx and more,” he said. “It turns out that the Indian legends of ravens following wolves are true—they do follow them because wolves mean food.”4

Wolves as Eco-Engineers

Yellowstone River2

Yellowstone River

The wolf population ultimately changed the behaviour and nature of the river. Prior to the wolves presence, the elk population gathered in large herds along the river banks, grazing away the stabilizing vegetation and trampling the river banks with their massive hooves; banks eroded and clouded the river with silt, impacting all aquatic species such as fish and aquatic insects. Following the introduction of the wolf population and the elk avoiding the open valley sides and banks, vegetation root structures stabilized the soil, preventing erosion. With less erosion from hoof-trampling and loss of vegetation, river morphology changed and stabilized; sections narrowed, pools formed, riffles developed. Increased complexity and stability provided more habitat for a richer and more diverse community.1,2

Wolves & Trophic Cascades

wolf pack

The wolf fulfills a required role of keystone predator by creating a trophic cascade. “A trophic cascade is an ecological process which starts at the top of the food chain and tumbles all the way down to the bottom,” says George Monbiot in “How Wolves Change Rivers.”1 Trophic cascades provide a key mechanism in maintaining an ecosystem’s health. Trophic cascades occur when predators in a food web suppress the abundance or alter the behaviour of their prey; this releases the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore). In a previous article, “How Whales Change Climate”, I describe how the krill-eating grey whale serves as keystone predator in maintaining—oddly enough—the krill population itself.5

Castor canadensis: Beaver chewing on branch.  Grand Teton N.P. WY

Yellowstone beaver

Insight into a predator’s role in ecosystem health is only gained when you look at the entire ecosystem and how the behaviour of the top predator affects the functioning parts of that entire ecosystem. The whale, once it feeds at depth, migrates to the surface, releasing fecal plumes at the surface and providing nutrients for the algae to grow and a means for them to travel down (through vertical mixing) to where the krill will feed on them.


Yellowstone River

The wolf fulfills a very similar role through the trophic levels of its ecosystem by helping to balance monopolizing lower trophic levels, such as the elk and coyote, to create a more diverse and balanced assemblage of interacting life from beetles and beavers to bears. The wolf and the whale are just two examples not only of “trophic cascading” but of the interconnectedness of all life, the natural role of all things and ultimately the great wisdom of Nature.

The benefits of the wolves presence in the park have even cascaded to the downstream residents of Billings Montana, who now enjoy more clean water from the Yellowstone River.2

Combatting Climage Change

Just as the whale’s engineered trophic cascade combats climate change by increasing the ocean’s carbon sink (through more phytoplankton), the wolf combats climate change by increasing terrestrial carbon sink (through more trees and vegetation).

Debunking Myths of the Wolf

wolf pups

Wolf pups

William Graham of Nature’s Web of Life cited several studies which showed that “about 75% of the wolf’s diet is elk, 11% of its diet is small mammals, 10% is deer, and only 4% is livestock. With only 4% of the wolf’s predatory diet being domestic livestock, “one must wonder why the US government has been so strongly responsive to the arguments of the farmers and ranchers ( who wish to see the extinction of the wolf ) and so reluctant to proceed with strong protection of the wolf in a wider geographic area,” Writes Graham. On the strong lobbying of the agricultural industry to extirpate the wolf, Graham writes: “The relationship between Nature and mankind is often defined by the agricultural industry and their powerful influence on government agencies like BLM, the National Park gray wolf socializationService, the US Forest Service, and the US Fish and Wildlife Service. A cursory search of the Internet reveals the very loud voice of agriculture calling for the destruction of the wolf. The current fury concerns the protection of the Gray Wolf and the Mexican Gray Wolf. These populations have been decimated by the agriculture industry with full support from our government agencies who are supposed to be stewards of Nature. The rise in the elk population is a strong testament to the fact that our stewards of Nature are either ignorant of or choose to ignore Nature’s connections.

The sad part about all of this is that there are non-lethal ways for the agricultural industry to minimize or prevent predation of their livestock.  Big dogs, llamas and predator fences are viable ways to cut livestock losses and negate the practice of killing wolves and their cubs.”



  1. “How the Wolves Change Rivers”, a film by Chris and Dawn Agnos, narrated by George Monbiot; Yellowstone Park:
  2. National Geographic: “Wolves of Yellowstone”:
  3. Laundre, John. W. Cougar biologist, State University of New York at Oswego:
  4. Farquhar, Brodie: Yellowstone Park site:
  5. Munteanu, Nina: “How Whales Change Climate”:

nina-2014aaaNina Munteanu is an ecologist, limnologist and internationally published author of award-nominated speculative novels, short stories and non-fiction. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit for the latest on her books.

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