One of the most exciting findings of the past half-century has been the discovery of widespread trophic cascades. A trophic cascade occurs when predators in a food web suppress the abundance or alter the behaviour of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore).
A classic example of this is what happened in the Yellowstone National Park, USA when wolves were reintroduced in 1995. We know that wolves kill various species of animals, but we are less aware that they allow diversity of life where they exist. Before the wolves were reintroduced, they had been absent for over seventy years as a consequence the numbers of deer increased significantly as they were no longer threatened by hunting. Despite human efforts to control them they had managed to reduce much of the vegetation in the National Park to almost nothing.
As soon as the wolves were reintroduced the numbers started to change, they radically changed the behaviour of the deer. Firstly, the deer started to avoid certain parts of the park such as the valleys and the gorges where they could be trapped easily. Immediately these areas started to regenerate, in some areas the height of the trees quintupled by 2001. Bare valley sides quickly became forests, Aspen, Willow and Cottonwood, this then attracted the birds. The number of songbirds and migratory birds increased greatly. The number of beavers started to increase as they are attracted by the trees as a source of food. Beavers create niches for other species to prosper, and the dams they built in the rivers attracted otters, muskrats, ducks and fish, reptiles and amphibians. The wolves killed coyotes, and as a consequence, the number of rabbits and mice began to rise which meant more hawks, foxes, weasels, badgers, ravens and bald eagles, which fed on the remains left by wolves. The bear population also began to grow as more berries began to grow on trees and shrubs. The bears also reinforced the cycle by killing cubs of the deer.
The wolves also changed the physical geography of the area by changing how the rivers moved. The rivers began to meander less; there was less erosion as the regenerated forests stabilised the banks, so they collapsed less often. The rivers became more fixed course; there was also less soil erosion at the valley sides had become more vegetated and therefore stabilised. The wolves not only changed the ecosystem but also the physical geography of Yellowstone National Park.