Tropical Territorial Damselfishses: Is Density Limited by Aggression or Recruitment?

Abstract

The spatial distributions of two common damselfishes, Pomacentrus wardi and P. flavicauda, suggest that they compete for common resources within lagoons of the southern Great Barrier Reef. Both fishes are herbivores that defend territories on rocky substrata covered with shallow water, but are segregated by microhabitat. When young P. flavicauda recruit into the habitat where P. wardi are most numerous, they have lower persistence than counterparts on reefs where conspecifics predominate. Several experiments were done, using as replicates that large patch reefs populated by P. wardi, to determine what effects territory—holding individuals have on the distribution and abundance of conspecifics, P. flavicauda, and other grazing fishes. Total and selective removal of the numerically dominant P. wardi revealed that these reefs are mostly repopulated by larval fishes (recruitment), rather than by already recruited fishes (immigration). P. wardi are the most frequent recruits. The removals also showed that resident P. wardi do not determine the numbers of conspecific larvae invading their territories, and do not reduce their survival after recruitment (at least for 300—400 d). They do determine the growth of such juveniles. Despite this interaction, there was no evidence that either density or biomass is usually regulated in these populations. Both measures vary greatly among replicate reefs. The lack of numerical equilibrium, within the study period, was caused by a patchy and limiting supply of larval recruits. Manipulations also showed that resident P. wardi do not limit the recruitment or reduce the postrecruitment survival of P. flavicauda on large patch reefs. An unknown density—independent cause is implicated in the observed habitat—segregation and in the failure of P. flavicauda to reach breeding size on such reefs. The removal of P. wardi produced different results for the fish grazers from other families, depending upon their feeding strategy. For example, solitary acanthurids concentrated their grazing activities on reefs with fewer damselfishes, but a schooling parrotfish did not. The outcomes of these perturbations support a nonequilibrial view of reef fish communities, rather than hypotheses that emphasize the importance of competition among populations usually at carrying capacity.