Seasonally fluctuating resources and temporal variability of interspecific competition.

The influence of seasonal availability of two critical resources (food and substrates from which food was harvested) on interspecific competition between striped surfperch (Embiotoca lateralis) and black surfperch (Embiotoca jacksoni) was examined. There was a strong depth-related gradient in density of prey and in cover of foliose algae; both declined with increasing bottom depth. Density of prey was reduced 5-10 fold during the winter season, but cover of substrates remained constant throughout the year. Although both fishes co-occurred throughout the same depth range, striped surfperch were more common in shallow habitats and black surfperch were more abundant deeper. Local abundance and distribution patterns of both surfperch species did not change seasonally. Stepwise regression analyses suggested that availability of favored substrates was a proximate influence on local patterns of surfperch distribution and abundance, and that interspecific competition depressed abundance of the two species to the same degree. Removal experiments conducted during the cold-water season revealed that interspecific competition influenced depth distribution of black surfperch but not striped surfperch. Seasonal change in density of prey was accompanied by marked changes in overlap in use of foraging substrates by the surfperches. The pattern of change in interspecific overlap suggested that surfperch competed for food only when prey were seasonally scarce. There was no difference in the agonistic tendencies of the two fishes, and the absolute and relative frequency of interspecific chases was independent of food level. These results have important implications regarding the impact of temporal variability of interspecific competition in natural communities. In the surfperch system, competition was characterized by constant and time-varying elements that had symmetrical and asymmetrical effects and involved both interference and exploitation mechanisms.