Spatial and temporal patterns in herbivory on a Caribbean fringing reef: the effects on plant distribution.

Blades of Thalassia testudinum Banks ex König were used in a field bioassay to assess the impact of herbivorous fishes and urchins in different habitats, at different depths, and at different times of day on a fringing reef at Galeta Point, Panama. Algal transplants were then performed to determine the effect of herbivory on algal distribution and on potential plant-plant interactions in the absence of grazers. Thalassia loss on the reef flat (0%/h) and sand plain (0%/h) was very low relative to that on the shallow reef slope (14%/h). Within the reef slope habitat, herbivore activity decreased linearly with depth. On the shallow reef slope, herbivory was highest in habitats that were most topographically complex. Herbivory was reduced in microhabitats where algal turfs had decreased structural heterogeneity by overgrowing small branching corals and filling in the cavities that these corals normally provide. On the shallow reef slope, grazing of Thalassia was lowest at night (0.3%-1%/h), rose rapidly in the morning to peak between 0900-1300 h (27%-45%/h), then dropped gradually throughout the afternoon and returned to nocturnal rates at 1900-2000 h.Herbivorous fishes did 97% of their fedding during the daytime and were responsible for more than 90% of the Thalassia removed from the reef. Urchins accounted for only 9% of the Thalassia removal, and day and night grazing rates were not markedly different.Algal species from the reef flat and sand plain are excluded from the reef slope by herbivores, not physical parameters, and should outcompete reef-slope species in the absence of herbivory. On this tropical reef, plants in the more physically benign environment appear to be selected primarily for herbivore resistance. Selection for competitive ability among seaweeds becomes increasingly important in habitats where herbivores are more exposed to their own predators or where herbivore populations are low due to periodic physical disturbances.