Top-down Trophic Shifts in the Subtidal Habitats of Cuba and the Role of Herbivorous Fish and Urchins

A healthy coral reef is expected to be governed by top-down population control. This study tests this hypothesis in the subtidal zone of a Cuban coral reef system and also aims to reveal any differences between the grazing effects of herbivorous fish and sea urchins. Population data on grazers were collected for three habitats – beach, cliffs, and rocks – this information was then correlated with palatable algae cover for the corresponding quadrat. The comparison of regression lines for each habitat suggested that there was no difference between the grazing effects of herbivorous fish and sea urchins. Also, the trendlines indicate a significant positive correlation in the rocks habitat for urchins with amount of algae (P < 0.05, R2 = 0.177). This indicates a trophic shift from a top-down regulated system and can be attributed to hurricane damage, and mechanisms outside of the study’s original design, such as the strong positive dependency of crustose coralline algae on urchins (P < 0.01, R2 = 0.264). This study demonstrates that species-level interactions are so diverse in such a highly complex ecosystem and may not be detectable at a community level, which obscures the overall strength of the studied response.