Tree Reef Research Articles

Coral Reef Fish Communities: Unstable, High‐Diversity Systems?

We examined the pattern of development of assemblages of coral reef fishes on artificial reefs at One Tree Reef, Great Barrier Reef. Two sets of 8 reefs were built. To investigate the effects of habitat structure on species diversity, each set consisted of replicate pairs of 4 different reef types: plain (no holes), small—holed, medium—holed, and large—holed reefs. To examine seasonal differences in colonization, 1 set was established in summer (October 1971) and the other set in winter (July 1972). Visual censuses of the reefs' fish assemblages were made at monthly intervals over 32 mo (summer set) and 23 mo (winter set). We recorded a total of 105 resident species from the artificial reefs. Although less diverse, the fish assemblages differed little in terms of species composition from those of small natural patch reefs. Artificial reefs showed a high between—reef variability in species composition that was largely unrelated to habitat structure. Competitive interactions between species appeared unimportant in explaining the distribution of species between reefs. Likewise, possible positive associations between species were indicated for only a few species. Almost all settlement of the artificial reefs was by juvenile fishes. Recruitment was markedly seasonal and occurred mainly over summer (September—May). Juvenile recruitment patterns showed little year—to—year constancy. Most species tested showed nonrandom, clumped patterns of settlement on replicate reefs. More than half of species tested showed no significant differences on numbers of individuals which settled on different reefs, but the remainder showed significantly greater settlement on some reefs, suggesting habitat selection. For a few species, there was also greater recruitment to reefs already occupied by conspecifics, suggesting possible social facilitation of settlement. For most species, there were no significant interspecific interactions between adults and newly settling juveniles. Between—census variability in species composition was high and species turnover ranged from °17% of species/mo in winter to °39% in summer. Maximum times of persistence for most species were <12 mo, and for many individuals survivorship was of the order of only a few months. Much of the high turnover appears due to losses through predation. There was no evidence that a persistent species equilibrium was attained during colonization. Because of predation and the seasonally varying, uncertain nature of recruitment, we see the development of any long—term species equilibrium as unlikely. We suggest that nonequilibrium conditions are a characteristic of coral reef fish communities and that because of these conditions, high within—habitat diversities are maintained.

Holocene reef growth — One Tree Island, Great Barrier Reef

Data from investigations at One Tree Reef and from other sources suggest that post-glacial coral reefs in the Great Barrier Reef probably date from only about 9,000 years ago. They are likely to have grown on the elevated parts of an irregular karst surface, their general morphology having been largely determined by the shape and depth of this surface. Growth has been dominantly vertical because of the rapid sea-level rise, with lateral modification being quite recent and dominantly in a leeward direction.