The process of herbivory is particularly intense in coral-reef ecosystems, where herbivorous fishes play a key role in promoting the resilience of coral reefs by mediating competition between corals and benthic macroalgae. Understanding and quantifying the species-specific roles of herbivores in ecosystem processes is critical for the proactive management of coral-reef systems, particularly in the context of their ability to overcome disturbance.
This study investigated variation in herbivory on macroalgae by coral-reef fishes at small (within-reef; <1 km) and broad (regional; 50-300 km) spatial scales at Ningaloo Reef in north-west Western Australia.
The levels of herbivory by fishes significantly differed between regions of Ningaloo Reef and were shown to be similar to those at lower latitudes of the Great Barrier Reef. No differences in herbivory were evident at the smaller within-reef spatial scale, with herbivory instead being highly variable between successive sampling days. Using stationary high-definition video cameras to film the removal of transplanted macroalgal assays by fishes, the species responsible for the patterns of herbivory were also identified and their bite rate quantified. Seven fish species, namely Naso unicornis (f: Acanthuridae), Kyphosus sp., K. vaigiensis (f: Kyphosidae), Siganus doliatus (f: Siganidae), Scarus ghobban, S. schlegeli and initial-phase Scarus sp. (f: Scaridae), accounted for 95% of the 32,395 bites on the macroalgal assays across regions. However, analyses demonstrated that only the bites of N. unicornis, Kyphosus sp., and K. vaigiensis removed significant macroalgal biomass, indicating the functional inability of many other species to rapidly remove fleshy macroalgae from the reef. Together, the feeding of these three ‘macroalgal browsers’ accounted for more than 75% of the quantified variability in herbivory, essentially making these species the ‘drivers’ of macroalgal herbivory in Ningaloo Reef. Regions exhibiting similar levels of herbivory were characterised by different feeding species, indicating some degree of functional redundancy within Ningaloo Reef. However, the potential for this redundancy appears to be limited, given the process of macroalgal herbivory was effectively confined to only three out of the 31 roving ‘herbivorous’ fish species observed throughout this study. The disproportionate effects of these three herbivorous fish species could not be inferred by visual presence alone, as these three dominant species only comprised 1-7% of the herbivorous fish biomass across regions, thus highlighting that the species critical to the healthy functioning and resilience of coral-reef systems may go unnoticed using standard visual census techniques alone. The differential roles of species in the removal of macroalgae from the reef stress the necessity to evaluate roles in ecosystem processes at the species level. Given the exclusivity of the role in macroalgal herbivory to a select few species, this study also emphasises that a high diversity of herbivores may not necessarily offer a high level of resilience against a change in macroalgae cover in coral-reef systems.