Spatial patterns of agrazing intensity on a caribbean barrier reef: Herbivory and algal distribution

Abstract

Strips of Thalassia testudinum Banks ex König were used in a field biioassay to assess the impact of herbivores in different habitats and at different depths on the barrier reef at Carrie Bow Cay, Belize. Common algal species from several habitats were then transplanted into areas with high grazing rates to evaluate: (1) the effect of herbivores on algal distribution; and (2), the extent of herbivore resistance characteristics of algal species from different habitats. Grazing rates were highest on shallow, structurally complex portions of the reef and lowest in areas that were deep, structurally simple or where turbulence interfered with feeding. Diurnal grazing by herbivorous fishes was responsible for virtually all of the plant material removed. Grazing by urchins was so low that it was unmeasurable during both day and night trials. When algal species from shallow habitats subject to low rates of grazing were moved to areas of greater herbivore activity, they were rapidly consumed by fishes. Algae from deep portions of the reef, where grazing was low, were not eaten when moved to areas with increased herbivory. These differences in the evolution of herbivore resistance are explainable as a result of growth rate differences and the relative amount, as opposed to absolute amount, of net growth that is consumed by herbivores in shallow versus deep habitats.