MSN: There are certain animals worldwide that hold a heavier sway in the balance of ecosystems, and they were dubbed "keystone species" in 1969 by American zoology professor Robert T. Paine.
The theory is that like the wedge-shaped keystone (or headstone) that locks together all the pieces used in an architectural arch, there are species that keep certain ecosystems in together. Removal of the species can cause the eventual collapse of the ecosystem.
Just a drop in the bucket of the keystone species...