A keystone species is a species that plays a critical role in maintaining the structure of an ecological community and whose impact on the community is greater than would be expected based on its relative abundance or total biomass.
The concept of a keystone species was first introduced by University of Washington professor, Robert T. Paine in 1969. Paine studied a community of organisms that inhabited the intertidal zone along Washington's Pacific coast. He found that one species, the carnivorous starfish Pisaster ochracceus, played a key role in maintaining the balance of all other species in the community. Paine observed that if Pisaster ochracceus was removed from the community, the populations of two mussel species within the community grew unchecked. Without a predator to control their numbers, the mussels soon took over the community and crowded out other species, greatly reduced the community's diversity.
Keystone species, because of their proportionately large influence on species diversity and community structure, have become a popular target for conservation efforts. The reasoning is sound: protect one, key species and in doing so stabilize an entire community. But the keystone species theory remains a young theory and the underlying concepts are still being developped. For instance, the term was originally applied to a predator species (Pisaster ochracceus), but now the term 'keystone' has been extended to include prey species, plants, and even habitat resources.