Response of herbivore functional communities to multiple disturbances in Moorea, French Polynesia

Background/Question/Methods

Herbivores are crucial to the prevention of coral reef phase shifts. Multiple disturbances, such as a crown-of-thorns seastar (COTS) outbreak followed by a large tropical cyclone, can result in loss of live coral cover, which can be followed by a phase shift to dominance by macroalgae. Identifying how disturbances affect herbivore communities can lead to an increased understanding of processes that underlie return to a coral-dominated state. This study is one of the first to assess changes in the functional composition of the herbivore community as a result of two large disturbances. The island of Moorea, French Polynesia underwent a severe COTS outbreak during 2008-2009, which resulted in the reduction of live coral cover from ~40% to < 5% on the forereef. This was immediately followed by an intense tropical cyclone in February 2010. The aims of this study are to (1) assess whether there are unique community assemblages of herbivorous fish among the three reef habitats (fringing reef, backreef, and forereef), (2) quantify responses of the fish communities to the disturbances, and (3) assess the changes in the herbivore functional assemblage among the three habitats. Annual fish survey data from 2006-2012 were used in the analyses.

Results/Conclusions

A total of 52 herbivorous fish species was observed from 2006-2012. Rare species (i.e., observed with < 35 total individuals over the 7 year period) were removed from analyses, leaving a total of 34 species. Among these 34 species, 7 functional groups were represented: browsers, detritivores, excavators, farmers, grazers, grazers/detritivores, and scrapers. Non-metric multidimensional scaling analyses revealed that there are unique functional assemblages among the fringing reef, backreef, and forereef habitats of Moorea. The backreef assemblage was differentiated from the other two habitats by greater abundances of farmer fish whereas the fore reef habitat had more detritivores. Despite having somewhat different functional assemblages, the three habitats responded similarly to the disturbances—all moved towards functional communities with increased representation of scrapers. In addition to scrapers, detritivores and excavators increased in abundance and biomass following the disturbances whereas grazers declined overall. Although coral cover has not recovered to pre-disturbance levels, the herbivorous fish community suppressed the establishment of macroalgae and there has been a massive recruitment of juvenile coral on the forereef. Results suggest that a functional community with a substantial biomass of grazers and increased representation of scrapers may help prevent coral phase shifts by preventing the establishment of macroalgae.