Habitat Effects on Reproductive Success in the Coral Reef Fish, Acanthochromis Polyacanthus (Pomacentridae)

Current hypotheses advanced to account for the maintenance of the diversity of coral reef fishes differ fundamentally in the assumed role of competitive processes. The equilibrial hypothesis is based primarily upon an assumption of extensive competitive interactions and subsequent niche partitioning; the lottery hypothesis, instead, assumes populations limited to levels well below carrying capacity due to the high larval mortality, a large effect of chance patterns in larval recruitment, and few, if any, significant competitive interactions. To date, no direct evidence for or against the presence of such interactions has been produced, with the role of competition, if any, inferred from community characteristics. To obtain such data, reproductive success of pairs of the planktivorous reef fish Acanthochromis polyacanthus located on isolated habitat patches was recorded over a 2—yr period at One Tree Island, Great Barrier Reef. Correlations were sought between each of six measures of individual fitness and 20 physical and social characteristics of the inhabited reefs. Significant correlations were found with only four characteristics: the number of individuals of all species inhabiting the reefs, the presence of the dominant lagoonal piscivore, current strength, and, most often, the number of heterospecific planktivores inhabiting the reef. Increased numbers of these apparent food competitors correlated with delayed spawning, reduced initial brood size, and reduced adult (and possibly juvenile) rates of growth. Results of a brief series of experimental manipulations of community structure, involving selective of apparent food competitors, appear to support these observations. The results suggest that food abundance is a limiting resource for A. polyacanthus and that significant interspecific competition for food occurs. The potential therefore exists, despite possible external constraints on population size, for selection to favor niche partitioning and for resources in short supply to affect year—class success and, subsequently, community structure.