What effect do keystone species have on an ecosystem?

The Keystone Species is a species that has a large effect on the environment when it comes to its abundance. In an ecological community, these species are needed for they are the ones which are described as the ones playing the critical role of maintaining the structure of the ecosystem. They are the predators, mutualists and engineers in the ecosystem. Without the keystone species, the ecosystem will not function well and may result to dramatic changes and may cease to exist altogether.

They keep populations under control.