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.