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 70 years -

the numbers of deer (because there had been nothing to hunt them) had built up and built

up in the Yellowstone Park

and despite efforts by humans to control them

they'd managed to reduce much the vegitation there to almost nothing. They had 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 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 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 like 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 muscrats and ducks

and fish and reptiles and amphibians.

The wolves killed coyotes and as a result of that, the number 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 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 the rivers became more fixed in their course.

Similarly, by driving the deer out of some places,

and the vegetation recovering on the valley side,

there was less soil erosion because the vegitation stabilized 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.