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

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. The classic example is what happened in the Yellowstone National Park in the United States when wolves were reintroduced in 1995. 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 70 years. The numbers of deer, because there were nothing to hunt them, had built up and built up in the Yellowstone Park, and despite efforts by humans to control them they had managed to reduce much of the vegetation there to almost nothing. They just grazed it away. 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 behavior of the deer. The deer started avoiding certain parts of the park, the places where they could be tracked 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. As soon as that happened, the birds started moving in. The number of song birds and migratory birds started to increase greatly.

The number of beavers started to increase because beavers like to eat the trees. Beavers, like wolves, are ecosystem engineers. They create niches for other species. The dams they built in the rivers provided habitats for otters and muskrats 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.

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 riffle sections, all of which were great for wildlife habitats. The rivers changed in response to the wolves and the reason was that the regenerating forests stabilized 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 a soil erosion because the vegetation stabilized that as well. The wolves, small in number, transformed not just the ecosystem of the Yellowstone National Park, this huge area of land, but also it’s physical geography.

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