How wolves change rivers

Yes, that's right – wolves can change rivers! Watch this video and find out how.

Do the preparation task first. Then watch the video and do the exercise. Remember you can read the transcript at any time.


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 70 years, but 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 of the vegetation 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 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 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 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.

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 were 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.

© Sustainable Human


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Advanced: C1

Submitted by Bruno2020 on Fri, 05/03/2021 - 01:02

Let's just start being kind with each other.

Submitted by OlaIELTS on Sat, 09/05/2020 - 19:31

This kind of small change could be introduction of new idea or policy on a particular task, thus it can generates a big effects of results to the organization.

Submitted by Alexander Togulew on Sun, 24/02/2019 - 22:16

A really great story. I knew that many things are connected in the nature but wasn't aware that absense or introduction of only one specie can have such consequences. Tropical cascade is really interesting theory. I would be interested for other examples which can confirm it in such convincing way. One thing that I can't agree with is that the main impact of wolves was that they ate many deers - there is such statement among the tasks. It could be output or outcome. Impact was the regeneration of valleys and gorges, restoration of plant and animal diversity, improving of the behaviour of the rivers. And what small changes caused by humans can make with nature? This question we must deal with I guess more often in the future. For example now many species of insects are disappearing or decreasing in number substantially. If they will disappear completely the consequences can by just creepy: no food for birds, no pollination of plants. In result of this degeneration of agriculfure and destruction of many food chains including those ones of humans.

Submitted by LAL85 on Fri, 28/09/2018 - 01:43

Nature is amazing and so perfect. Each organism is located where it has to be and I think that one of the reasons for this is because everything is connected. For example, if we remove the rabbits, what would happen? First of all, the population of coyotes would be reduced, if we continue with the food chain, the wolves would be affected too as well as the deers. About the main subject of the question, everybody can help to make the Earth a better place, it is just a matter of becoming conscious. In theory, the only way to become sustainable is cultivating our own vegetables and food, also having a small farm with the animals that you would eat. In addition, if we talk about the electricity the best way could be having panels for solar and wind electricity instead of thousands of connections all over the house. In conclusion, there are several ways to make small and big changes but sometimes is easier to keep with this crazy and illogical consumption that the humans have.

Submitted by Juan R. Velazq… on Tue, 11/09/2018 - 23:02

One of the small changes that we can make and it can surely have a huge impact on the mother nature, but most significantly on our population is perhaps changing radically the production and consumption of our aliments. For example, the majority of the food that we intake day after day is processed (not natural) and even when it appears to be natural (like chicken) it contains chemicals or substances that has transformed or altered their natural growing and subsequently they will affect us after being ingested; obviously it is a long term process that as you can see is well related to many human diseases like CAD, stroke, diabetes, obesity, etc. In other words we are poisoning ourselves. Not to mention that, the most the companies produce this products and the most we continue to contribute to this wave of production to satisfy our demands, is the most that our ecosystem will bear the brunt of it. So i think that if we, individually start changing our feeding habits, educate, teach and help other people that are exposed to the same harm, we can have an increasingly percentage of health in our population in years to come and as it grows higher by extension we can lead the food industry to reform their procedures and somehow life itself might start to look up.

Submitted by L.Michel on Sun, 12/08/2018 - 04:24

We can make a big effect on the environment by simply changing our consumption habits. We can buy long-lasting clothing, appliances, or even re-use our bags whenever we go to the grocery store, instead of getting a new plastic bag.

Submitted by Adilson P Lima on Thu, 05/07/2018 - 16:46

This is an amazing history about the nature power. And how can it teach us.

Submitted by Waseem Akhtar Faizi on Thu, 31/05/2018 - 00:13

There is a lot of knowledge in a short video, surprisingly effects of wolves are discovered here.

Submitted by mariaidabertocchi on Fri, 11/05/2018 - 19:19

Using technological devices more wisely would be a minor change in our habits that could have a dramatic, positive impact on our ecosytem, not to mention the beneficial effects on our lifestyle

Submitted by johnflawed on Fri, 04/05/2018 - 16:50

How interesting! Wolves has became more intricate to me

Submitted by smctaimoorali on Sat, 21/04/2018 - 10:23

This is amazing. I am surprised to know these facts. Unbelievable!

Submitted by Ali.safa on Mon, 09/04/2018 - 14:40

This is undeniable that everything in the Universe are connected to each other and a slight change in one element can lead to the big changes. For instance,if you turn off a light bulb that you don`t use and if this action continues among all people around the world,we will save Energy which requires precious natural resources such as oil and gas to be generated.Not wasting Energy equals not wasting money and this means we will be able to allocate more funds to environment.
Moreover, for what electric power is concerned, I would point out that its saving means mainly reducing the use of nuclear power

Submitted by thefaulty on Thu, 05/04/2018 - 16:16

It's a very interesting video!

Submitted by anbbomfim on Wed, 21/03/2018 - 17:13

Very nice video! Specially for me who likes biology!!!

Submitted by Vadim V on Tue, 20/03/2018 - 12:20

Use of aerosol cans reduces the ozone layer of the planet.
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