Cross-shelf variation in browsing intensity on the Great Barrier Reef

Herbivory is widely accepted as a key process determining the structure and resilience of coral reefs, with regional reductions in herbivores often being related to shifts from dominance by coral to leathery macroalgae. The removal of leathery macroalgae may therefore be viewed as a critical process on coral reefs. However, few studies have examined this process beyond a within-reef scale. Here, browsing activity was examined across the entire Great Barrier Reef shelf using bioassays of the leathery macroalga Sargassum to directly quantify algal removal. The assays revealed marked cross-shelf variation in browsing intensity, with the highest rates recorded on mid-shelf reefs (55.2-79.9% day(-1)) and decreasing significantly on inner- (10.8-17.0% day(-1)) and outer-shelf (10.1-10.4% day(-1)) reefs. Surprisingly, the variation in browsing intensity was not directly related to estimates of macroalgal browser biomass; rather, it appears to be shaped primarily by the local environment and behaviour of the component species. Removal rates across the inner- and mid-shelf reefs appear to be related to the attractiveness of the assays relative to the resident algal communities. Controlling for the influence of the resident algal communities revealed a positive relationship between removal rates and the biomass of a single macroalgal browsing species, Naso unicornis. In contrast, the low removal rates on the outer-shelf reefs displayed no relationship to algal or herbivore communities and appeared to reflect a negative behavioural response by the resident fishes to a novel, or unfamiliar, alga. These findings not only highlight the complexities of the relationship between fish presence and ecological function, but also the value of examining ecological processes across broader spatial scales.