Competitor-mediated coexistence: Interactions among three species of benthic macroalgae

Manipulative field experiments were used to determine the mechanism of coexistence of three species of benthic macro-algae (Eisenia arborea Areschong, Halidrys dioica Gardner, and Pterocladiacapillacea (Gmelin)) which occur in a narrow band of substratum from 0.3 to 2.3 m below MLLW on Santa Catalina Island, California. Eisenia forms a canopy and excludes Halidrys from areas beneath this canopy. Pterocladia occurs in the space beneath the Eisenia canopy. Upon canopy removal, Halidrys grows adventitiously, excludes Pterocladia, and preempts all space on the substratum. However, Pterocladia is not physiologically restricted to being an understory plant. When the canopy is removed and Halidrys is prevented from entering the area, Pterocladia flourishes. Pterocladia persists in the community because of its ability to utilize the refuge from its superior competitor, Halidrys, afforded by the Eisenia canopy. Eisenia, the competitive dominant to Halidrys, therefore, mediates the coexistence of Halidrys and its inferior competitor Pterocladia in a manner similar to the role that predators and physical disturbance play in other marine communities.