Competition for Food between Two Intertidal Starfish Species and its Effect on Body Size and Feeding

Two predaceous intertidal starfish that overlap broadly with respect to food, space, and time were found to compete for a limited food supply in the San Juan Islands, Washington State, suggesting that competition is widespread and that the species are generally in competitive equilibrium.

Abstract

Two predaceous intertidal starfish that overlap broadly with respect to food, space, and time were found to compete for a limited food supply in the San Juan Islands, Washington State. The experiment involved complete removal of a larger (up to 600 g average wet weight) starfish (Pisaster ochraceus) from a small island—reef and addition of them to a second island—reef while a third reef served as a control. In response to Pisaster removal, the mean individual wet weight of the smaller (maximum size = 45 g wet weight) asteroid (Leptasterias hexactis) increased significantly in 15 months. Addition of Pisaster resulted in a significant decrease in Leptasterias size; no change in average Leptasterias size was observed on the control reef. A highly significant inverse correlation between the estimated biomass densities (wet weight/m 2 ) of the two species at 10 areas suggests that competition is widespread and that the species are generally in competitive equilibrium. A major consequence of the small size of Leptasterias is an apparent inability to capture larger prey. Coexistence seems based upon "specialization" by each predator on different—sized prey. Evidently, reduced competition stress results in an increase in Leptsterias's community role as a predator. Conversely, when competition is severe, the community role of Leptasterias appears unimportant.