Habitat complexity and consumer-mediated positive feedbacks on a Caribbean coral reef

Theoretical and empirical evidence suggest that positive feedbacks can increase resilience in ecological communities. On Caribbean coral reefs, there have been striking shifts from physically complex communities with high coral cover to relatively homogenous communities dominated by macroalgae, which have persisted for decades. However, little is known about positive feedbacks that may maintain coral reef community states. Here, I explore a potential consumer-mediated feedback on a Jamaican reef by examining how grazing by a keystone herbivore (Diadema antillarum) is enhanced by physical structure, which offer refugia from predation. Surveys revealed that habitat complexity and Diadema density were positively related. Increasing habitat complexity by adding physical structure significantly decreased macroalgal cover and increased the proportion of urchins in algal habitats in field manipulations. Experimental increases in urchin density also decreased macroalgal cover, but did not affect the proportion of urchins in algal habitats. These results suggest that the low habitat complexity of macroalgal-dominated reefs may inhibit an urchin-mediated shift to coral dominance and that positive feedbacks must be considered in reef restoration efforts.