How Wolves Change Rivers

When wolves were reintroduced to Yellowstone National Park in the United States after being absent nearly 70 years, the most remarkable "trophic cascade" occurred. What is a trophic cascade and how exactly do wolves change rivers? George Monbiot explains.


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How Wolves Change Rivers

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How Wolves Change Rivers

One of the most exciting scientific f___________ of the past half century has been the discovery of widespread trophic cascades. A trophic cascade is an ecological process which starts at the t_____ of the food chain and tumbles a___ 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 r______________ in 1995.

Now, we – we all know that wolves kill various s___________ of animals, but perhaps we’re slightly less aware that they g______ life to many others.

Before the wolves t________ 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 d_________ efforts by humans to control them they’d managed to reduce much the v____________ there to almost nothing. They had just g_________ it away.

But as soon as the wolves arrived, even though they were few in number they started to have the most remarkable e_______.

First, of course, they killed some of the deer but that wasn’t the major thing. Much more significantly, they r___________ 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 g________ and immediately those places started to r_____________. In some areas, the height of the trees quintupled in j____ six years. Bare valley sides quickly became forests of aspen and willow and cottonwood. And as soon as that happened, the birds started m________ in. The number of songbirds and migratory birds started to increase g_________. The number of beavers started to increase because beavers like to eat the trees. And beavers, like wolves, are ecosystem engineers. They c_______ niches for other species. And the dams they built in the rivers provided h__________ for otters and muscrats and ducks and fish and reptiles and amphibians.

The wolves killed coyotes and as a r________ of that, the number rabbits and mice began to r_____ 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 f___ 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 r__________ the impact of the wolves by killing some of the calves of the deer.

But h_____ where it gets really interesting.

The wolves changed the behavior of the rivers. They began to meander less. There was less e_______. The channels n_________. More pools f_________. More riffle sections. All of which were g______ for wildlife habitats. The rivers changed in r_________ to the wolves. And the reason was that the regenerating forests stabilized the b______ so that they collapsed less o_____. So the rivers became more fixed in their c_______.

S___________, by driving the deer o____ of some places, and the vegetation recovering on the valley side, there was less s____ erosion because the vegetation stabilized that as well. So the wolves, s______ in number, transformed n___ just the ecosystem of the Yellowstone National Park – This huge area of land… b___ also, its physical geography.

Pronunciation focus

ly

particularly

similarly

immediately

slightly

dipthongs

stabilized

trophic

erosion

Yellowstone

v

vegetation

valley

wolves

recovering

Second syllable stress

effects

regenerating

transformed

erosion

immediately

particularly

ju:

reduced

reintroduced

huge

population