Drivers of herbivory on coral reefs: species, habitat and management effects

Ecosystems are under increasing pressure from external disturbances. Understanding how species that drive important functional processes respond to benthic and community change will have implications for predicting ecosystem recovery. Herbivorous fishes support reefs in coral-dominated states by mediating competition between coral and macroalgae. Spatiotemporal variability in herbivore populations and behaviour have direct effects on the removal of algae, but knowledge of how different drivers impact on herbivore populations and their foraging is currently lacking. Such knowledge is important to understand whether herbivory is likely to compensate for changing resource availability, and thus, the potential for reefs to recover from disturbance. The relative importance of these drivers has implications for the suitability of specific management actions put in place to support herbivory. Variability in density, body size, foraging movements and grazing rate of 2 parrotfish species was investigated across reefs exhibiting a range of benthic and fish community compositions. Foraging movements were influenced by the benthos, with foraging distances greatest on degraded reefs. In contrast, parrotfish densities were driven by the management status of the reef; parrotfish size was primarily linked to species identity, whereas grazing rate was influenced by both management status and species. These findings suggest that the distribution of foraging effort will vary over time in response to reef condition, such that feeding becomes more dispersed as reefs degrade. Gear restrictions that protect large, high-grazing-rate species, or designation of no-take areas, are likely to maximise algal removal, regardless of reef condition.