Keystone species
Some sea stars (e.g. Pisaster ochraceus) prey on sea urchins, mussels, and other shellfish that have no other natural predators. If the sea star is removed from the ecosystem, the mussel population explodes uncontrollably. This drives out most other species. The urchin population grows likewise, and destroys coral reefs. Living coral reefs are the habitat for many hundreds of other species.