The Ecological Role of Keystone Herbivore Diadema antillarum in Promoting Indices of Caribbean Coral Reef Health

Abstract

The detrimental effects on coral reef health, following the devastation of keystone herbivore Diadema antillarum populations in 1983-1984 due to a waterborne pathogen, are still evident in the Caribbean today. D. antillarum play a significant role in removing macroalgae which promotes scleractinian coral colony growth and facilitates juvenile coral recruit settlement. Recently, an isolated population that boasts the largest recorded post-pathogen population of the long-spined sea urchin past shallow waters was discovered in Banco Capiro, Honduras. This study compares Banco Capiro and Utila, a nearby island representative of a typical recovering contemporary Caribbean reef, through five defined indices of coral reef ecosystem health: organism abundance, biodiversity, stability and potential for growth, habitat complexity, and coral cover. This study attempted to determine the role of D. antillarum in facilitating these measures of coral reef health. The results show that Banco Capiro harbors a greater abundance of adult and juvenile D. antillarum population densities, heterospecifics, coral recruits, and coral cover. In contrast, Utila has greater habitat complexity and species diversity for adult coral and juvenile coral recruits. The comparison of these two reefs provides insights into the major barriers to D. antillarum recovery: Allee effects of low- density populations and poor juvenile survival into adulthood. The results show that Banco Capiro is in a successional stage further along in recovery than Utila, largely due to the greater abundance of D. antillarum. This study emphasizes the ecological significance of this sea urchin and makes population restoration a conservation priority.