How Wolves Change Rivers

“When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” — John Muir

When wolves were reintroduced to Yellowstone National Park in the United States after being absent for nearly 70 years, the most remarkable ‘trophic cascade‘ occurred. In this short film, George Monbiot explains what a trophic cascade is, and how wolves do actually change rivers.

I found this so remarkable that I took the time to transcribe George’s words:

One of the most exciting scientific findings of the past half century has been the discovery of widespread ‘trophic cascades’. 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, and the classic example is what happened in the Yellowstone National Park in the United States when wolves were reintroduced in 1995. Now, we all know that wolves kill various species of animals, but perhaps we’re slightly less aware that they give life to many others.

Before the wolves turned up, they’d been absent for seventy years, but the numbers of deer — because there’d been nothing to hunt them — had built up and built up in the Yellowstone Park and despite the efforts by humans to control them, they’d reduced much of the vegetation there to almost nothing; they’d just grazed it away. But as soon as the wolves arrived, even though they were few in number, 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 radically changed the behaviour 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 the 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, and willow, and cottonwood.

And as soon as that 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 liked to eat the 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 and musk-rats and ducks and fish and reptiles and amphibians.

The wolves killed coyotes, and as a result of that, the number of rabbits and mice began to rise, which meant more hawks, more weasels, more foxes, more badgers. Ravens and bald eagles came down to feed on the carrion 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 the wolves by killing some of the calves of the deer.

But here’s where it gets really interesting: the wolves changed the behaviour of the rivers. They began to meander less, there was less erosion, the channels narrowed, more pools formed, more riffle sections, all of which was great for wildlife habitats. The rivers changed in response to the wolves. And the reason was that the regenerating forests stabilised the banks so that they collapsed less often, so that 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 stabilised that as well.

So the wolves, small in number, transformed not just the ecosystem of the Yellowstone National Park, this huge area of land, but also its physical geography.

Note from the video’s publisher (Sustainable Human): “There are ‘elk’ pictured in this video when the narrator is referring to ‘deer.’ This is because the narrator is British and the British word for ‘elk’ is ‘red deer’, or ‘deer’ for short. The scientific report this is based on refers to elk so we wanted to be accurate with the truth of the story.”