Negative trophic relationship between parrotfish biomass and algal cover on Philippine coral reefs

Abstract

The importance of herbivory in preventing phase shifts to algal dominance on coral reefs is widely acknowledged. However, there is a paucity of information on which and how much herbivores are needed to effectively prevent an overgrowth of different algal types in natural environments. Our study showed the overall negative relationship between herbivorous fish biomass and benthic algal cover on Philippine coral reefs. The strongest negative relationships were between parrotfish biomass and both total algal and algal turf covers, and between total herbivore biomass and macroalgal cover. We further identified 4.0 mt/km2 and 2.0 mt/km2 for parrotfish, and 1.0 mt/km2 for all herbivores, as the potentially critical biomass levels needed to keep total algal, algal turf, and macroalgal covers, respectively, at low levels. The mean (SE) cover of all algae was about 37.0 ± 0.7% in areas with 4.0 mt/km2 parrotfish biomass compared with 46.2 ± 0.8% and 44.0 ± 1.1% in areas with 2.0–3.9​ mt/km2 and <2.0 mt/km2 parrotfish biomass, respectively. Algal turf cover of 10.1 ± 0.7% in areas with <2.0 mt/km2 parrotfish biomass was almost twice as high as those with 2.0 mt/km2 parrotfish biomass. Macroalgal covers were only 4.7 ± 0.5% and 10.6 ± 1.1% in areas with 1.0–2.0 and >2.0 mt/km2 total herbivore biomass, respectively, compared with 39.8 ± 1.6% in those with <1.0 mt/km2 total herbivore biomass. Our findings support the hypothesis that phase shifts to algal dominance on coral reefs would not happen as long as there are no major losses of important herbivore species and herbivore biomass levels do not fall below critical levels.