Keystone species.

The sea star Pisaster ochraceous feeds on mussels.

When Pisaster was removed from an intertidal zone, mussels eventually took over the habitat and eliminated most other invertebrates and algae.

In a control area from which Pisaster was not removed, species diversity remained rich.

Thus sea star is a keystone species in the intertidal community: its presence promotes species diversity.

In the absence of the sea star, mussels become a dominant species (the species with the most biomass) in this community.