Herbivore abundance and grazing intensity on a Caribbean coral reef

Abstract

Herbivory is a primary factor in determining the structure of coral reef communities. Spatial variation among reef habitats in the intensity of herbivory has been documented, but underlying variation in species composition and abundance within the herbivore guild has received little attention. The distribution and relative abundances of herbivorous fishes and sea urchins across several habitats were studied on the Belizean barrier reef off the Caribbean coast of Central America. Marked variation in total herbivore density as well as major changes in the composition of the herbivore guild were found across reef habitats. Acanthurids (surgeonfishes) predominated in shallow areas (< 5 m) while scarids (parrotfishes) were dominant in deeper habitats. Significant differences among habitats in an experimental assay of grazing intensity were strongly correlated with herbivore abundance. The spatial distribution of herbivorous fishes across reef habitats does not appear to be simply explained by differences in reef topography, but may depend on complex interactions among proximity to nearby shelter, predator abundance, density of territorial competitors, and local availability of food resources.