How Wolves Change Rivers

One of the most exciting findings of the past half-century has been the discovery of widespread trophic cascades. A Trophic cascade is an ecological process that starts at the top of the food chain and tumbles all the way down to the bottom. And the classic example is what happened in Yellowstone National Park in the United States when wolves were reintroduced in 1995. Now, we all see that wolves kill various species of animals, but perhaps we are less aware that they give life to many others. Before the wolves turned up, they’d been absent for 70 years – the numbers of deer because there was nothing to hunt them had built up and built up in the Yellowstone Park and despite efforts of humans to control them, they’d managed to reduce much of the vegetation there to almost nothing. They had just grazed it away. But as soon as the wolves had arrived, even though they were few in numbers, they started to have the most remarkable effects. First, of course, they killed some of the deer but that wasn’t the major thing. Much more significantly, they changed the behavior of the deer. The deer started avoiding certain parts of the park - the places where they could be trapped most easily – particularly the valleys and gorges and immediately those places started to regenerate. In some areas, the height of the trees quintupled in just six years. Bare valley sides quickly became forests of Aspen, Willow and Cottonwood. And as soon as it happened, the birds started moving in. The number of songbirds and migratory birds started to increase greatly. The number of Beavers started to increase because Beavers like to eat trees. And Beavers like wolves are ecosystem engineers. They create niches for other species. And the dams they built in the rivers provided habitats for otters, muskrats, ducks, fish, reptiles, and amphibians. The wolves killed coyotes and as a result of that, the number of rabbits and mice began to increase, which meant more hawks, more weasels, more foxes, and more badgers. Ravens and bald eagles came down to feed on the carrions that the wolves had left. Bears fed on it too and their population began to rise as well partly also because there were more berries growing on the regenerating shrubs. And the bears reinforced the impact of wolves by killing some of the calves of the deer. But here’s where it gets really interesting. The wolves changed the behavior of the rivers. They began to meander less. There was less erosion. The channels narrowed. More pools formed. More rifle sections. All of which were great for wildlife habitats. The rivers changed in response to the wolves. And the season was that the regenerating forests stabilized the banks so that they collapsed less often, so the rivers became more fixed in their course. Similarly by driving the deer out of some places, and the vegetation recovering on the valley sides, there was less soil erosion because the vegetation stabilized that as well. So the wolves, small in number, transformed not just the ecosystem of Yellowstone National Park, that huge area of land but also, its physical geography.