Opportunistic responses of Diadema antillarum (Echinodermata: Echinoidea) populations following the 1997–98 El Niño event in Bahia, Brazil

We present results from a seven year (1995e2001) study on the coral reefs in Bahia (Brazil), the reef invertebrate community being severely impacted by the 1997e98 El Ni~no Southern Oscillation event. Despite local extinction of some taxa, the urchin Diadema antillarum was the only species out of a total of 678 enumerated demonstrating a significant increase in population size following El Ni~no, indicating a unique response of this species to the stresses associated with these events. Other species of echinoid did not show such a response, most disappearing from the fauna during the two post-El Ni~no years. The increasing Diadema numbers were most likely due to movement of individuals onto the reef systems from deeper water, exploiting the conditions on the reefs caused by a reduction in competitors and, we speculate, an increase in available space on the reef allowing potential algal settlement to sustain this elevated urchin population. This increased grazing pressure may have prevented algae covering the reef, as seen in other Atlantic reef systems, allowing new coral settlement in 2001.