Wolves were once native to the US’ Yellowstone National Park — until hunting wiped them out. But when in 1995 the wolves returned, something interesting happened: the most remarkable “trophic cascade” occurred.

The presence of wolves substantially changed the behaviour of large grazers. Wapiti deer stopped munching their way through the valleys and gorges where wolves could easily hunt them. The vegetation was able to re-establish and re-grow, thereby increasing biodiversity by providing food and shelter to a larger variety of plants and animals.

Remarkably, the presence of wolves also changed the rivers. Riverbank erosion decreased so the rivers meandered less, the channels deepened and small pools formed. The recovering vegetation stabilised the riverbanks, which in turn changed the geography and microclimate in the park.

Have a look at this amazing video showing how one species can have a massive cascading effect on the entire ecosystem in which it lives. 
What is a trophic cascade and how exactly do wolves change rivers? George Monbiot explains in this Youtube mashup.

National Geographic reports this may be one of the most important conservation concepts to come out of natural science in the last half century. Remarkable about this case study is that the same can be applied to apex predators around the world: lions in Africa, tigers in Asia. Sharks, bears, and wild dogs are all species sitting at the top of their respective food chains, creating stability amongst the species they prey on and maintaining the health of plants and animals right down the trophic ladder.

You can find Monbiot’s full TED Talk on ‘rewilding’ here.