Among-habitat algal selectivity by browsing herbivores on an inshore coral reef

Understanding how the impact of different herbivores varies spatially on coral reefs is important in qualifying the resistance of coral reefs to disturbance events and identifying the processes that structure algal communities. We used assays of six common macroalgae (Acanthophora spicifera, Caulerpa taxifolia, Galaxaura rugosa, Laurencia sp. Sargassum sp., and Turbinaria ornata) and remote underwater video cameras to quantify herbivory in two habitats (reef crest and slope) across multiple sites on Orpheus Island, Great Barrier Reef. Rates of herbivory varied among macroalgal taxa, habitats, and sites. Reductions in algal biomass were greatest for Sargassum sp. (36 % 4 h(-1)), intermediate for A. spicifera, Laurencia sp., C. taxifolia, and T. ornata (17-33 % 4 h(-1)) and lowest for G. rugosa (6 % 4 h(-1)). Overall, rates of herbivory were generally greater on the reef crest (30 % 4 h(-1)) than the reef slope (21 % 4 h(-1)). This difference in rates of herbivory coincided with a marked shift in the dominant herbivores between habitats. Kyphosus vaigiensis, despite only feeding on three species of macroalgae (Sargassum sp., T. ornata, and A. spicifera), was responsible for 34 % of all bites recorded on the reef crest yet did not take a single bite from algae on the reef slope. In contrast, Siganus doliatus took bites on every species of algae in both habitats, accounting for 40 % of bites on the reef crest and 74 % of all bites recorded on the reef slope. This difference in the number of macroalgal species targeted by herbivores and the habitat/s in which they feed adds another dimension of complexity to our understanding of coral reef herbivore dynamics.