A potential positive feedback loop for algal dominance on coral reefs: Why do fish eat less algae where it’s abundant?

Background/Question/Methods

The causes of Caribbean coral reef decline are hotly debated but both local and global stressors have been implicated. In Jamaica, it appears that coral loss due to disease, bleaching, and storm damage has allowed algae to gain a foothold. The presence or absence of herbivorous fish and urchins may mediate the abundance of these algae. This is reinforced by evidence from the Great Barrier Reef that shows that reefs with high coral mortality can remain virtually algae free if the right herbivores are present. In a striking result, other investigators working on the GBR found that herbivorous fish eat less algae when algae is dense. They hypothesized that this might be due to the fear of potential predators hiding in the structure of the algal beds. We tested an alternative hypothesis; that perhaps due to their evolutionary history of foraging on coral dominated reefs; fish forage poorly when algae occurs against a homogenous background of other algae. To test this hypothesis in Jamaica, we presented fish with palatable algae against both contrasting (beige) and algae-mimicking backgrounds. In addition we conducted benthic surveys and herbivore counts over several years to quantify trends in coral/algal cover on “healthy” and degraded reefs. 

Results/Conclusions

Our reef surveys revealed that algal cover was lower in the presence of herbivorous urchins. Herbivorous fish were relatively abundant on algal-dominated reefs versus coral dominated reefs. Our herbivory experiment provided preliminary evidence that fish eat less algae when it appears against an algae-like background. These patterns and processes may reveal a mechanism underlying a positive feedback loop that allows algae-dominated reefs to remain so.