Herbivore species richness and the structure of Caribbean coral reefs

On coral reefs, herbivorous fishes have strong effects on community structure, macroalgal abundance, and coral resilience, but little is known about how herbivore richness affects the health and structure of these communities.  In two experiments over two years, we enclosed equivalent densities of either single-species or mixed-species of herbivorous fishes in replicate, 4 m² cages at a depth of 17 m on a reef in the Florida Keys, USA to evaluate the effects of herbivore identity and species richness on community structure.  In Year 1, we used the redband parrotfish (Sparisoma aurofrenatum) and the ocean surgeonfish (Acanthurus bahianus); in Year 2, we used the redband parrotfish and the princess parrotfish (Scarus taeniopterus).  We found strong effects of herbivore richness on seaweed abundance, coral growth and survivorship, and community structure due to feeding complementarity between herbivores.  In Year 1, ocean surgeonfish and redband parrotfish synergistically suppressed cover and biomass of upright macroalgae by 54-76% compared to single-species treatments due to complementary feeding on different seaweeds.  The mixed-species treatment also increased cover of crustose coralline algae by 23-117%, increased coral cover by 22%, and decreased coral mortality from the 8-24% in single-species treatments to 0% in the mixed-species treatment.  In Year 2, redband parrotfish and princess parrotfish fed on different algal functional groups with redbands suppressing upright macroalgae and princesses suppressing filamentous algal turfs, again increasing cover of crustose coralline algae, which are favored sites for coral recruitment.  Nonmetric multidimensional scaling showed that the morphologically and taxonomically dissimilar princess parrotfish and ocean surgeonfish had more similar effects on macroalgal community structure than did the two species of parrotfishes.  These fishes play functionally diverse roles in the herbivore guild on Caribbean reefs and their complementary effects on algal communities produce crucial positive, but indirect, effects on corals, the foundation species for the ecosystem.