Size‐Limited Predation: An Observational and Experimental Approach with the Mytilus‐Pisaster Interaction

In many space—limited systems, for instance forests, coral reefs, and mussel beds, the main biological component of the spatial matrix is provided by large—bodied species. At some point during their growth these matrix species surpass the ability of any single individual of a predator species to consume them, although they still may be overwhelmed by concentrated group attacks (i.e., by aphids, fungi, internal parasites). These species thus surpass through continued growth a size threshold limiting their consumers, and thereby attain a refuge in size. The implications of these refuges are discussed, with special reference to a predatory starfish, Pisaster ochraces, and its primary prey, the mussel, Mytilus californianus. The mean wet weight of Pisaster varies between localities from ° 150 g to 2,640g. Long—term (12—yr) observation indicates little annual variation and generally no trends within areas, yet consistent differences between areas. Five transplant experiments demonstrate that Pisaster is an indeterminate grower, andis capable of adjusting its body size through growth to local conditions. Limited recovery of individually marked starfish suggest that exchange between areas is restricted. Observations on the natural diet of Pisaster show a pronounced maximum body size limit to the mussels that can be consumed by intertidal starfish of a given size. This limit was explored experimentally in two ways. First, Pisaster was allowed to reinvade an area from which it had been manually excluded for 5 yr. In this interval those mussels comprising the matrix of the clump had attained shell lengths > 8—10 cm. and could not be consumed by Piaster weighing 203 g. Second, artificial clumps of known—size mussels were protected by plastic screening, allowed to attach, and then exposed to predation. Survival was significantly greater in those mussels expected to be too large to be eaten. The significance of size—limited predation is that prey and predator can coexist in very intimate proximity, that the surviving prey often attain large sizes and probably make a reproductive contribution greater than their abundance, and that these same prey diversify the spatial structure of the environment, usually by adding a vertical dimension not otherwise present.