Foraging Strategy of a Starfish in Relation to Actual Prey Availability and Environmental Predictability

The foraging ecology of the asteroid Leptasterias hexactis was studied at five rocky intertidal areas in the San Juan Islands, Washington, for 2—2¾ years. Feeding activity during high tides reaches a high point in mid— to late summer (July and August) and a low point in the winter (January). Three probable factors selecting for this cycle are (1) reproduction, virtually immobilizing the females, during the winter; (2) low food availability in winter compared to summer; and (3) violent winter storms. Persistently reduced low tide feeding during all but autumnal night low tides indicates that some light—related factor(s) select for a tidal cycle of feeding also. Factors believed responsible are desiccation stress and possibly predation by visual predators. A graphic model of foraging behavior is presented. Analysis of the diet of Leptasterias by both numbers and calories of prey consumed indicates that this starfish is a food generalist. Moreover, numbers of prey consumed, when compared to prey available, indicate those prey chosen selectively and those which are avoided. Lastly, calories consumed compared to calories available indicate why particular prey are chosen. Many factors are involved in prey choice including ease of capture, relative sizes of predator and prey, and caloric yield of the prey. Actual availability is defined as that portion of available prey (i.e., prey physically present) which could be consumed by a predator. The factors affecting prey choice, coupled with prey and predator distribution, abundance, and behavior must be considered when determining actual availability of prey. Actual availability coefficients are computed and suggest that (1) hard—to—capture prey are least available; (2) Leptasterias is more selective in summer and autumn, when food is more abundant; and (3) prey are more available to large starfish. Calculations suggest that, under normal conditions, Leptasterias would be severely energy—limited in winter if it foraged at rates similar to those of spring or summer, or both. Thus, food limitation may be partially responsible for reduced feeding in winter. Comparison of numerical patterns of consumption of energy—rich and energy—poor prey indicates that energy—rich prey are most important except in summer when energy—poor prey become the most important prey, presumably because of their tremendous numerical increase and domination. Comparisons between the foraging strategy of this starfish and theoretical models previously proposed support some predictions but not others, indicating that complex natural systems require more comprehensive and precise models. Predictability of resources in time and space probably determines the degree of resource specialization or generalization in animals.