Key herbivores reveal limited functional redundancy on inshore coral reefs

Marine ecosystems are facing increasing exposure to a range of stressors and declines in critical ecological functions. The likelihood of further loss of functions and resilience is dependent, in part, on the extent of functional redundancy (i.e. the capacity of one species to functionally compensate for the loss of another species) within critical functional groups. We used multiple metrics; species richness, generic richness, abundance and reserve capacity (i.e. the relative number of individuals available to fulfil the function if the numerically dominant species is lost), as indicators to assess the potential functional redundancy of four functional groups of herbivorous fishes (browsers, excavators, grazers and scrapers) in two of the worlds' most intact coral reef ecosystems: the Great Barrier Reef (GBR) and Ningaloo Reef in Western Australia. We found marked variations in potential redundancy among habitats within each reef system and functional groups. Despite negligible fishing of herbivorous fishes, coastal habitats in both reef systems had lower functional redundancy compared to offshore locations for all herbivorous fishes collectively and the four functional groups independently. This pattern was consistent in all four indicators of redundancy. The potential vulnerability of these coastal habitats is highlighted by recent shifts from coral to macroalgal dominance on several coastal reefs of the GBR. Our approach provides a simple yet revealing evaluation of potential functional redundancy. Moreover, it highlights the spatial variation in potential vulnerability and resilience of reef systems.