Since healthy herbivore populations can keep macroalgae from overgrowing corals or inhibiting coral recruitment, they are critically important to the resilience of coral reefs. For this reason, healthy populations of herbivores, especially herbivorous fishes, can be one of the most useful indicators of reef resilience.
Herbivorous fishes can be divided into four functional groups, based on their role in controlling algal growth and maintaining the reef substrate for coral recruitment:
- Scrapers/small excavators
- Large excavators/bioeroders
Each functional group above makes an important and complementary contribution to reef resilience. Resilient reefs will often have species present that can perform all of these functions (functional diversity), and may have more than one species performing each function (functional redundancy).
Monitoring herbivorous fishes can be done with standard methods, and can be readily incorporated into existing fish monitoring programs. A comprehensive protocol ref is available to guide managers who want to incorporate herbivorous fishes into a resilience-monitoring program.
Scrapers/small excavators 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. Scrapers include the majority of parrotfishes (Hipposcarus and Scarus species). Scrapers and small excavators help control the establishment and growth of macroalgae while intensely grazing epilithic algal turf and providing areas of clean substratum for coral recruitment
Large excavators/bioeroders are major agents of bioerosion on reefs, removing dead coral and exposing hard, reef matrix for coral recruitment. Excavating species (Bolbometopon muricatum and all Chlorurus species) differ from scrapers by taking deeper excavating bites and remove greater quantities of substrata with each bite. They include all large individuals of excavating species (individuals > 35 cm standard length).
Grazers/detritivores intensely graze 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, butterflyfish, small angelfishes (all Centropyge species), and many species of surgeonfishes (all Zebrasoma and Acanthurus species except those that feed 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.
Browsers feed on macroalgae. 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.