Seminar Series 2003

Facilitation by the urchin, Diadema antillarum: An essential component of scleractinian coral recovery on Jamaican reefs?

Joshua A. Idjadi, University of Delaware, Newark, DE
Nick Haring, California State University, Northridge, CA

In the last several decades, many Caribbean reefs once characterized by scleractinian coral cover are now dominated by macroalgae. This change probably was caused by multiple natural and anthropogenic disturbances including, perhaps most importantly, the region-wide mortality of Diadema antillarum. However, after nearly two decades there are signs of Diadema recovery in Jamaica. Diadema can now be found on the forereef in dense “zones” (~ 5 urchins/m2) characterized by low macroalgal cover and increased numbers of juvenile corals (< 4 cm in diameter): the presence of Diadema is associated with an increased density of scleractinians. In this study, we addressed three hypotheses to explain this relationship: (1) can juvenile coral success in urchin zones be attributed higher growth rates, or (2) is increased success of juvenile corals driven by reduced mortality in these zones, and (3) do urchins modify the substratum in such a way as to facilitate coral settlement? To test these hypotheses, juvenile corals were tagged in urchin and algal zones at three locations at Discovery Bay, Jamaica. Coral growth rates were determined by measuring the change in major diameter between January of 2001 and January of 2002, and coral mortality was determined by counting the number of corals that died from one year to the next. Standardized growth rates (using z transformations) for 7 scleractinian genera were significantly higher in urchin zones when compared to algal zones. In addition, juvenile corals were less likely to experience mortality in urchin rather than algal zones. Diadema were also found to modify the substratum, promoting crustose coralline algal cover and deterring macroalgae from growing on substratum plates. The results indicate that corals are more successful in terms of growth and survivorship in the presence of Diadema. Furthermore, substratum modification (increasing crustose coralline algae) may attract increased numbers of coral larvae and reduce competition with macroalgae. This study underscores the potential importance of Diadema in the recovery of Caribbean coral reefs and provides some “good news” for these coral communities.