The prevailing notion that interspecific competition has little impact on the abundances of tropica!reef fishes has been tested by few experiments, all of which examined its effects on recruitment and juvenile demography rather than on adult abundances. This study, in Caribbean Panama, examined whether a common territorial damselfish, Stegastes planifrons, limits the abundances of adults of four ecologically similar congeners in S. planifrons' primary habitat: S. partitus and S. variabilis, which commonly occur in that habitat, and S. diencaeus and S. leucostictus, which rarely do. When S. planifrons was nearly eliminated from 16 natural patch reefs for 4-6 yr, adult populations of S. partitus, which is less aggressive and half the size of S. planifrons, doubled and expanded their range into a microhabitat previously used almost exclusively by adults of S. planifrons. Population increases by S. partitus occurred within 1 yr of the removal of S. planifrons, were persistent, but only reached their maximum approximate to 4 yr (and about four generations) after the initial removal. Adult populations of S. variabilis, which is less aggressive and 20% smaller than S. planifrons, also doubled following the removal of S. planifrons. Combined populations of S. partitus plus S. variabilis increased most on reefs where S. planifrons previously had been most dense. However, increases by those two species reached only approximate to 70% of the population density and approximate to 40% of the biomass density that S. planifrons had before the removals. Removal of S. planifrons had no effect on the abundances of S. diencaeus and S. leucostictus. Removal of S. partitus from eight reefs for 3 yr did not affect the abundances of S. planifrons or S. variabilis. Eleven years monitoring of adult populations and juvenile recruitment by S. partitus before and during the experiment show that its population increase after the removal of S. planifrons occurred while its overall abundance was gradually increasing, but was not associated with a period of unusually high juvenile recruitment. Thus asymmetric competition among closely related, ecologically similar, territorial coral reef fishes does sometimes control both abundances and microhabitat use in shared habitat. Interspecific territoriality and differences in body size and aggressiveness that lead to asymmetric competitive ability occur commonly among reef fishes in various trophic groups. Interspecific interactions within such groups of species often affect their patterns of habitat use. Hence the role that interspecific competition plays in limiting abundances of coral reef fishes requires reevaluation.
