Soft corals exert no direct effects on coral reef fish assemblages

Correlations between abundance of organisms and their habitat have often be en used as a measure of the importance of particular habitat features. Howe ver, experimental manipulation of the habitat provides a more unequivocal e stimate of its importance. In this study we quantified how fish communities on small patch reefs covaried with changes in benthic cover habitat featur es. A random sample of small patch reefs was selected and both fish abundan ce and habitat measures recorded. Naturally occurring patch reefs could be classed into three habitat types based on their benthic cover. Reefs domina ted by massive soft corals were the most abundant (50%), followed by those dominated by rock and soft corals in equal proportions (36%), then reefs do minated by branching corals (14%). Fish assemblages differed between the re ef types. Communities on soft-coral-dominated and rock/soft-coral-dominated patch reefs formed a continuum of species responses correlated with degree of soft coral cover. In contrast, branching-coral-dominated reefs were occ upied by a more discrete set of species. We tested the role of soft corals in contributing to this pattern by experimentally reducing soft coral cover on patch reefs from a baseline level of similar to 67% to similar to 33% a nd similar to6%, and monitoring the experiment over 2 years. Contrary to ex pectations derived from the correlative data, and in contrast with previous manipulations of hard corals, soft-coral disturbance did not generate any corresponding changes in the fish assemblage. This "negative" result indica ted that the quality and heterogeneity of habitat generated by soft corals on patch reefs was indistinguishable from equivalent-sized habitat patches formed by bare rock alone. Nevertheless, because soft corals are living org anisms they have the potential to generate indirect effects by interacting with other organisms such as hard corals. In the long-term, we hypothesize that biotic interactions between habitat forming organisms might affect com position of fish assemblages on patch reefs.