Here we examine the role of herbivore richness and identity in affecting the primary succession of the algal community and coral growth on a coral reef. In two experiments, we enclosed equivalent total densities of either single-herbivore or mixed-herbivore groups of herbivorous fishes in replicate, 4 m2 cages at a depth of 16-18 m on a reef in the Florida Keys, USA. In Year 1, we used the redband parrotfish (Sparisoma aurofrenatum) and the ocean surgeonfish (Acanthurus bahianus); in Year 2, we used the redband parrotfish and the princess parrotfish (Scarus taeniopterus). Within each treatment, we placed cinderblocks to monitor primary succession of the macroalgal community and attached pieces of live coral to the cinderblocks to assess coral growth in Year 1.
Results/Conclusions
Across both years, herbivore identity rather than species richness appeared to control the development of the macroalgal community. In Year 1, ocean surgeonfish had strong effects on the algal community by suppressing upright macroalgae and thick algal turfs (>0.5cm) while facilitating cover of closely cropped algal turf (<0.5cm) which led to significant positive effects on coral growth. Similarly, in Year 2, princess parrotfish suppressed thick algal turfs (>0.5cm) and facilitated communities dominated by short algal turf (<0.5cm) and crustose coralline algae. In contrast, redband parrotfish had minimal effects on algal community development in both years with redband-only treatments having similar community structure to herbivore exclosures. The patterns for primary succession are in contrast to those we reported previously for the natural benthic community where the redband parrotfish was the dominant herbivore and herbivore richness significantly affected macroalgal abundance and coral growth. Comparing patterns for algal communities on cinderblocks vs. that for the natural benthic community suggests that different species have different effects on the algal community depending on the stage of succession. Further, our data suggest that different species of herbivorous fishes may play different roles in preventing vs. reversing shifts from coral-dominated to macroalgal-dominated states on reefs.
