It Pays to Be Pushy: Intracohort Interference Competition between Two Reef Fishes

Competition is often most intense between similar sized organisms that have similar ecological requirements. Many coral reef fish species settle preferentially to live coral at the end of their larval phase where they interact with other species that recruited to the same habitat patch at a similar time. Mortality is high and usually selective and individuals must compete for low risk space. This study examined the competitive interactions between two species of juvenile damselfish and the extent to which interactions that occurred within a recruitment cohort established the disjunct distribution patterns that were displayed in later life stages. Censuses and field experiments with juveniles found that one species, the ambon damsel, was dominant immediately after settlement and pushed the subordinate species higher up the reef and further from shelter. Presence of a competitor resulted in reduced growth for both species. Juvenile size was the best predictor of competitive success and outweighed the effects of short term prior residency. Size at settlement also dramatically influenced survival, with slightly larger individuals displaying higher aggression, pushing the subordinate species into higher risk habitats. While subordinates had higher feeding rates, they also sustained higher mortality. The study highlights the importance of interaction dynamics between species within a recruitment cohort to patterns of growth and distribution of species within communities.