Figure 18.15 Sea stars as a keystone species. (a) Although they are not very abundant, sea stars act as keystone species in intertidal communities along the coast of Washington State because they remove competitively dominant mussels. (b) In areas where sea stars were removed, the competitively superior mussels came to dominate the community and caused a decline in the diversity of the competitively inferior species. (c) In areas where the sea stars remained, the community retained a high diversity of intertidal species.

Data from R. T. Paine, Intertidal community structure: Experimental studies on the relationship between a dominant competitor and its principal predator, Oecologia 15 (1974): 93–120. Photos by (b) Jonathan Hucke; (c) Gary Luhm/DanitaDelimont.com/Newscom.