COMPENSATION IN RESOURCE USE BY FORAGERS RELEASED FROM INTERSPECIFIC COMPETITION

On shallow reefs where their populations co-occur, black surfperch (Embiotoca jacksoni Agazzi) and striped surfperch (Embiotoca lateralis Agazzi) compete strongly for a single preferred resource, dense patches of crustacean food located on the alga Gelidium robustum. Four-year-long competition experiments revealed a substantial degree of density compensation by each competitor; densities of each species increased on the order of 40% in the absence of the competitor. However, overall densities of surfperch on experimental reefs remained much lower at the end of the experiment than initial (combined) densities. Studies of behavioral compensation, following a reduction in competitor density, revealed that both black surfperch and striped surfperch were able to immediately utilize foraging opportunity on Gelidium, but the two species differed greatly in their ability to garner the freed resources. Striped surfperch compensated behaviorally for all of the new foraging opportunity, whereas black surfperch made use of only a small fraction of it. Although newly-available resources were not shared equitably among members of local populations of either species of surfperch, the unevenness in resource distribution during competitive release appeared to be more pronounced for black surfperch. An understanding of patterns of behavioral compensation sheds light on the determinants of the magnitude of density compensation by each competitor. The degree of density compensation realized by each species was influenced by different factors: the quantity of freed foraging opportunity for striped surfperch, and the ability of the forager to fully exploit the freed resource (black surfperch).