Functional structure of herbivorous fish communities in the Wakatobi National Park, Southeast Sulawesi

Marine fish play important functional roles on coral reef systems in regulating ecosystem resilience. Herbivores are key indicators of a reefs resilience to disturbances in particular to regime shifts to algae dominated states. The importance of understanding the functional role of key taxa in maintaining coral reef biodiversity and resilience is now more important than ever as acute and long-term environmental change threatens these systems further. We aimed to understand the spatio-temporal variation among herbivore functional roles and how these were partitioned across species. We quantified the functional role of herbivores by excluding grazing activity on the reef. Algal communities responded with a significant increase in benthic cover (+47.47%) over the 12 month exclusion period. We further identified herbivore abundance and biomass and how this was partitioned across reefs zones and how benthic composition drove variation in the Wakatobi National Park, Indonesia. Herbivore abundance and biomass decrease within the reef from the flat to the slope, this was consistent from 2013 to 2017; however overall abundance and biomass increased over time. Herbivore functional roles were found to be distributed across 64 species. Abundance and biomass of herbivores and secondary functional groups (SFG), decrease with increased algal cover across all grazing guilds, this was best predicted by the fish abundance of guilds and particularly the SFG’s nested within herbivores. SFG’s revealed that Acanthuridae spp. were the most abundant and contributed to the fish most biomass within herbivore communities and Signanidae spp. with greater individual mean biomass. Ctenochaetus striatus was responsible for driving community differences. These taxa are key in maintaining reef resilience through high functional grazing, and vital for increased conservation and monitoring efforts against direct pressures to mitigate effects of further environmental change.