Partitioning Herbivory and Its Effects on Coral Reef Algal Communities

Abstract

Coral reef herbivores were classified into three functional groups according to criteria describing the frequency and intensity of disturbance to the algal community created by their herbivory. These herbivore groups included: microherbivores, which have small foraging ranges (1—100 cm2) and a high frequency of grazing any point within their range; the echinoid Diadema antillarum, which has intermediate—size foraging ranges (0.5—1 m2) and intermediate frequencies of grazing any one area within an individual's range; and herbivorous fishes, with large foraging ranges (up to 0.5 ha) and the lowest grazing frequencies at any one site. Algal communities on experimental coral—settling plates were subjected to treatments designed to detect the effects of grazing by representatives of each functional herbivore group on algal biomass, community structure, and primary productivity in a backreef/reefcrest habitat in St. Croix, United States Virgin Islands. Abundances of microherbivores and herbivorous fishes (mainly juvenile scarids) fluctuated over time, while Diadema antillarum abundance remained relatively constant. Grazing intensity (as measured by the rate of algal biomass removal and algal standing crop under different grazing regimes) was greatest for D. antillarum; grazing by D. antillarum resulted in low—biomass, highly productive algal turfs (O2 release per unit chlorophyll a per unit time: 11.3—22.4 mg · mg—1 · h—1). Algal turfs grazed by D. antillarum were dominated by epilithic and endolithic filamentous and crustose species. Grazing by herbivorous fishes led to algal communities with algal turfs of higher biomass than in D. antillarum—grazed treatments; these communities became dominated by a single algal species (Sphacelaria tribuloides). Fish—grazed algal communities had lower biomass—specific productivity rates than algal turfs grazed by D. antillarum (2.4—8.4 mg · mg—1 · h—1). Microherbivore—grazed and ungrazed treatments had the lowest biomass—specific productivity rates and eventually became dominated by a few macroalgal species. Although algal biomass in sea urchin—grazed treatments was only 25—50% of the biomass values in treatments not grazed by sea urchins, productivity per unit area was not significantly lower in the former treatments, suggesting a positive effect of sea urchin grazing on algal productivity. This positive effect is a result of the reduction of algal self—shading when grazing is intense and/or the input of regenerated nutrients from D. antillarum excretions. Coral reef plant—herbivore interactions are similar to those in terrestrial grassland—herbivore systems. As in many terrestrial systems, plant—herbivore interactions result in higher overall ecosystem primary productivity and apparently facilitate the flow of energy and materials in this case from the highly productive algal turf component to higher levels in the reef trophic web.