Population Responses of Surfperch Released from Competition

Abstract

Previous short—term (behavioral) field experiments revealed that two temperate marine reef fish, black surfperch (Embiotoca jacksoni) and striped surfperch (E. lateralis), compete with one another along a depth gradient. Here we report abundance responses of these viviparous species following 4 yr of sustained release from competition with each other. We tested predictions derived from the short—term experiments concerning the symmetry of interspecific competitive effects on abundance. We also explored among—habitat variation in the occurrence and magnitude of density compensation. Populations of both surfperch species increased on the order of 40% at experimental (competitor—removal) sites; no change in density was detected for either species at unmanipulated control sites. These strong, reciprocal effects on density conformed with our expectations based on behavioral shifts in microhabitat use immediately following competitive release and on correlation analyses between population densities and feeding—associated resources, but not with predictions from habitat shifts (e.g., changes in depth distribution) that occurred shortly after manipulation. Density compensation was detected only in habitats (depths) where behavioral responses to competitive release had occurred (i.e., in shallow reef habitats). The correlation analyses accurately predicted variation in the relative magnitude of density compensation among habitats and among populations; predictions of the absolute changes in density based on the analyses were less accurate. Take together, these results suggest that behavioral experiments may be useful in identifying potential spatial variability in competitive effects on density. “Scaling—up” the results of behavioral experiments to project relative interspecific effects on density will be more problematic, in part because the relative demographic costs of the interaction may not be accurately portrayed.