A species whose presence and role within an ecosystem has a disproportionate effect on other organisms within the system. A keystone species is often a dominant predator whose removal allows a prey population to explode and often decreases overall diversity. Other kinds of keystone species are those, such as coral or beavers, that significantly alter the habitat around them and thus affect large numbers of other organisms.
The definition of a keystone species is a group of living organisms that has a drastic effect on their overall ecosystem.
An example of a keystone species is the grizzly bear, because as a preditor it keeps down the number of moose and elk.