[Music] [Music] 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 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' been absent for 70 years that the numbers of deer because there was nothing to hunt them had built up and built up in the Yellowstone Park and despite efforts by humans to control them they' managed to reduce much of the vegetation there to almost nothing they 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 song birds of migratory Birds started to increase greatly the number of beavers started to increase because beavers like to 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 um provided habitats for Otter 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 Carion that the Wolves had left Bears fed in 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 Imp of the Wolves by killing some of the cves 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 stabilize the banks so that they collapse 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 so the Wolves small in number transform not just the ecosystem of the Yellowstone National Park this huge area of land but also its physical geography [Music] [Music] oh [Music]