Monitoring Herbivory

Herbivores play a critical role in coral reef resilience by limiting the establishment and growth of algal communities that impede coral recruitment. Studies have demonstrated the critical role herbivorous reef fishes play in influencing competitive interactions between corals and macroalgae. Herbivorous reef fishes are also important agents of bioerosion on reefs. Herbivorous reef fishes can also play an important role in reversing coral-algal phase shifts, although this process is not well understood.

Yellow tang (Zebrasoma flavescens), black Achilles tang (Acanthurus achilles), and bluefin trevalley (Caranx melampygus) play an important role in coral reef resilience by grazing on algal turfs. Photo © David Burdick/NOAA's Coral Kingdom Collection

Herbivorous reef fishes are diverse and do not constitute an ecologically uniform group. They comprise several functional groups that differ in terms of how they feed, what they consume, and their impact on the underlying substratum. Four functional groups of herbivorous reef fishes each play different and complimentary roles in coral reef resilience:

scrapers/small excavators
large excavators/bioeroders
grazers/detritivores
browsers

Scrapers/small excavators: The majority of parrotfishes (Hipposcarus and Scarus species) are scrapers. They take non-excavating bites and remove algae, sediment and other material by closely cropping or scraping the reef surface, leaving shallow scrape marks on the reef substratum. Excavating species (Bolbometopon muricatum, Cetoscarus bicolor and all Chlorurus species) differ from scrapers by taking deeper excavating bites and removing greater quantities of substrata with each bite. Scrapers and small excavators (individuals < 35cm standard length) play similar roles in coral reef resilience by limiting the establishment and growth of macroalgae while intensely grazing epilithic algal turf, and providing areas of clean substratum for coral recruitment.

Sleeper Functional Groups

While diverse herbivorous reef fishes play a role in reversing coral-algal phase shifts, two recent studies demonstrate phase-shift reversal by an “unsuspected” single species.1,2 Both studies used large-scale experimentally induced phase shifts on the Great Barrier Reef and the use of underwater cameras to document the species responsible for the transition from macroalgal domination to epilithic algae and coral.

One study2 revealed that the single species, Naso unicornis, was almost solely responsible for the removal of Sargassum biomass. The second study1 documented recovery of the reef (removal of Sargassum) was primarily due to the batfish, Platax pinnatus. The results of both studies reveal the critical issue to identify and protect those groups that underpin the resilience of the local ecosystem. Management and conservation strategies may need to look beyond species diversity, and also focus on the maintenance of ecological processes and protection of key species in functional groups.

Large excavators/bioeroders play a similar role in coral reef resilience to scrapers and small excavators. However, they are also major agents of bioerosion on reefs, removing dead coral and exposing hard, reef matrix for coral recruitment. They include all large individuals of excavating species (see above: individuals > 35cm standard length). Five species have also been observed grazing on live corals on Indo Pacific reefs, although coral only accounts for a substantial proportion of the diet of one species (B. muricatum). Since these species have a greater affect on the underlying substratum than scrapers and small excavators, they play a different role in coral reef resilience by opening up new sites for colonization by coralline algae and corals.

Grazers/detritivores play an important role in coral reef resilience by intensely grazing epilithic algal turfs, which can limit the establishment and growth of macroalgae. Unlike parrotfishes, grazers do not scrape or excavate the reef substratum as they feed. Grazers include most rabbitfishes, small angelfishes (all Centropyge species), and many species of surgeonfishes (all Zebrasoma and Acanthurus species except those that feed on exclusively on plankton or are grazers/detritivores). Grazers/detritivores include Acanthurus species that feed on a combination of epilithic algal turf, sediment and some animal material. Although only a small proportion of their diet is algae,
grazers/detritivores are combined with grazers because many are schooling species that can be abundant and consume significant amounts of algal turf.

Browsers consistently feed on macroalgae. They select individual algal components and remove only algae and associated epiphytic material. Browsers play an important role in reducing coral overgrowth and shading by macroalgae, and can play a critical role in reversing coral-algal phase shifts. They include some unicornfishes, rudderfishes, batfishes, a rabbitfish and parrotfishes of the genus Calotomus and Leptoscarus.

Key components of the monitoring protocol include3:

A school of humphead parrotfish, Bolbometopon muricatum, approaching the shallows from the deep blue lagoonal waters of the Great Barrier Reef. B. muricatum are large excavators/bioeroders, playing a role in coral reef resilience. Photo © David Burdick/Marine Photobank