Why are starfish considered a keystone species?

The starfish Pisaster ochraceus is a keystone species in the rocky marine intertidal communities off the northwest coast of North America. This predatory starfish feeds on the mussel Mytilus californianus and is responsible for maintaining much of the local diversity of species within certain communities. read more

So in the Pacific Northwest, pisaster starfish are a keystone species in tide pools and intertidal zones. They keep mussels from completely taking over everything. Further south, in California, a different species is the keystone that keeps mussels under control: sea otters. They use tools to eat mussels. read more

In other communities in which Pisaster occurs, however, the starfish has little overall effect on the structure of the community. Therefore, a species can be a keystone species in some communities but not in others. read more