A keystone species is a species that exerts great influence on an ecosystem. This influence can take one of several forms.
A keystone predator may prevent a particular prey species from overrunning an ecosystem. Some sea stars may perform this function by preying on mussels and other shellfish that have no other natural predators. If the sea star is removed from the ecosystem, the mussel population explodes uncontrollably, driving out most other species.
The grizzly bears of the northwestern United States are also keystone species, but in a different way. They transfer nutrients from the oceanic ecosystem to the forest ecosystem. The first stage of the transfer is performed by salmon who swim up rivers, sometimes for hundreds of miles. The bears then capture the salmon and carry them onto dry land, dispersing nutrient-rich feces and partially-eaten carcasses.