Resource Overlap, Prey Dynamics, and The Strength of Competition

Abstract

Patterns of resource use and the strength of interspecific competition were explored for two temperate marine reef fishes, black surfperch (Embiotoca jacksoni) and striped surfperch (E. lateralis). These species occupy a depth—related gradient in abundance of food (crustaceans) and preferred foraging microhabitats (species of foliose algae that contain food items). Field experiments revealed that the intensity of competition varied greatly along the resource gradient. At the shallow end of the gradient, which contains seasonally high food levels and large amounts of the most preferred foraging microhabitat (the red alga Gelidium), interspecific competition was intense during seasons when food was scarce. This was accompanied by low overlap in use of feeding microhabitats. We detected no competition in deep areas that lacked Gelidium. In this portion of the resource gradient, low overlap occurred because there was no preferred microhabitat that both fishes could share. Low overlap in the deep habitat was not a consequence of competition but rather of the slightly different foraging adaptations of each species. In the surfperch system, static, among—habitat patterns of resource overlap provided little insight into the existence or strength of competition. High overlap occurred when competition was dampened by abundant food (in the shallow zone during food—rich seasons). Low overlap was either associated with strong competition (during periods of relative scarcity of food in the shallow zone) or its absence (deep areas which lacked the shared, preferred microhabitat). Spatial and temporal variation in the strength of interspecific competition was better predicted by dynamic (seasonal) patterns of microhabitat overlap, but only when coupled with knowledge of food dynamics, foraging adaptations of the competitors, and availability of preferred shared microhabitat.