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How Wolves Change Rivers

How Wolves Change Rivers

Jennifer Scarlott explains the phenomenon of trophic cascades using the example of wolves that were reintroduced in the Yellowstone National Park.

Dear Cub kids,

Ready for a journey of scientific discovery and hope? In the last half century, one of the most exciting findings in the field of biology is the phenomenon of trophic cascades. Think of it as a waterfall – a trophic cascade is a process in nature that starts at the top of a food chain and tumbles all the way to the bottom. Scientists have found that if an apex predator is removed or restored at the top of the chain, each level of the chain beneath it is affected. Here’s an example:

Photo Courtesy: Doug Smith/Wikimedia Commons.

Before 1995, wolves had been absent from Yellowstone National Park in the American West for 70 years. The entire wolf population had been killed. In 1995, scientists were allowed to re-introduce a small population of wolves to the park. What happened next took the world by surprise.

During the decades that wolves had been absent, the deer population in Yellowstone had grown out of control, putting increasing pressure on the vegetation of the park. In some places, the deer had browsed the vegetation down to the barest scrub. With the return of just a small wolf pack, the deer were suddenly on a predator’s menu again.

The scientists began to see a striking change in the behaviour of the deer as a result of the mere presence of the wolves. The deer quickly learned that there were parts of the park where they were more vulnerable to wolf attacks, and they began to avoid those places. And in the places that the deer avoided, what do you think happened? The plants and trees and other vegetation began to bounce back – forests re-grew, and the rate of tree growth increased rapidly. Many bird species suddenly returned, encouraged by the growth of available habitat. As forest area increased, so did numbers of beavers, and of course, beavers are famous “ecosystem engineers” whose activities slow the flow of streams and rivers, creating improved habitat for countless creatures like muskrats, otters, and all sorts of fish and amphibians. The wolves killed some of their competitors, the coyotes, which led to an increase in the mouse population. This brought more predators of mice, in the form of raptors. Bears and carrion-eating birds returned, to feast on the kills left by the wolves. The bears also found more berries on more shrubs than there had been before the wolves returned, and they reinforced the wolves’ effect on the deer by occasionally preying on young deer.

The final part of the cascade is the most startling: can you guess what else might have happened as the vegetation and forests of Yellowstone began to regenerate? Many of the plants and trees that re-grew in large areas of the park returned along the river and streambeds. This led to more stable river and stream banks, to less soil erosion, and ultimately, to slower, more meandering rivers and streams, which was in turn beneficial to many types of animals.

What was fascinating to scientists was that the presence of the wolves not only led to the predictable death of some prey animals, but to life for many other species of flora and fauna. And the re-introduction of Yellowstone’s apex predator transformed not just the plant and animal life of the Yellowstone ecosystem, but its physical geography as well – the wolves changed the rivers, in ways that improved conditions for yet more life!

But then, this isn’t news to Cub kids, who have grown up learning that tigers and trees and water are all part of the same beautiful natural system. Take tigers away, and eventually, the deer population will explode, the forests will suffer, and the rivers will dry up or erode the landscape. Leave tigers in place, and nature’s bounty knows no bounds.


Your friend,


P.S. Thank you to British naturalist and educator George Monbiot for this lesson in trophic cascades! If you have a computer at home or school, you can enjoy his tale of the wolves and rivers of Yellowstone here: https://www.youtube.com/watch?v=ysa5OBhXz-Q

Author: Jennifer Scarlott, First appeared in: Sanctuary Cub, May, 2014.


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