Some Physical and Biological Determinants of Coral Community Structure in the Eastern Pacific

Various physical and biological factors affecting coral community structure were investigated by direct observation and periodic censusing (supplemented with laboratory observations and experiments) on three coral reefs off the Pacific coast of Panama from 1970 to 1975. The physical environment has a strong control over coral growth at shallow depth; physical factors are also important subtidally (light, sediment transport). However, paralleling the pattern on temperate shores, biological processes (competition, predation, bioturbation, mutualism) assume an increasing influence on community structure in deeper and more diverse reef assemblages. Coral zonation is marked on these biologically simple and small reefs; the following assemblages are recognized: drying reef flat–live coral cover moderate, species diversity low; reef crest and upper reef slope–highest cover, lowest diversity; lower reef slope and reef base–cover moderate to low, diversity highest. Coral populations in the different zones, though spatially close, are affected by unique sets of conditions. Recurrent extreme tidal exposures devastate reef flat corals (Pocillopora mortality = 40%—60%). The mortality rate of pocilloporid corals is higher than for other corals; this has a diversifying effect on the reef flat assemblage. Acanthaster normally feeds in deep reef zones and numerically its major prey are the predominant pocilloporid corals (most often small colonies and broken branches). Electivity indices and prey choice experiments indicate that less abundant, nonbranching corals are preferred over Pocillopora. Large, branching pocilloporid colonies harbor crustacean symbionts (Trapezia and Alpheus) which can repulse Acanthaster and therefore protect this group of corals. Experimental removal of the symbionts results in a shift of prey preference from nonbranching corals towards the branching pocilloporids. Crustacean symbionts were present in all large Pocillopora colonies sampled, but the density of Trapezia in colonies on the reef flat was about twice that in colonies from deep zones where Acanthaster forages. Further, small pocilloporid colonies and fragments contained relatively few (and a high proportion of juvenile) symbionts. The variety of preferred coral prey present along the seaward reef flanks and the relatively low abundance of Pocillopora in this habitat are considered important factors affecting the distribution of Acanthaster. In addition, a continuous live cover of pocilloporid corals, which Acanthaster avoids, can protect reef zones (e.g., the reef flat) or preferred prey species from attack. The selective destruction of nonpocilloporid corals by Acanthaster tends to lower both live coral cover and species diversity (H'). This trend is evident on the Uva Island study reef where a significant decline in coral cover (47%—18%) and H' (1.06—0.58) occurred over a 4—mo period.