Sea urchins, macroalgae and coral reef decline: a functional evaluation of an intact reef system, Ningaloo, Western Australia

The number of relatively untouched coral reefs worldwide is rapidly decreasing. Nevertheless, one coral reef ecosystem remains relatively intact: the largest west-continental reef ecosystem in the world, Ningaloo Reef in Western Australia. This study investigated the status of 2 potential bio-indicators for coral reef decline, macroalgae and sea urchin densities, on this reef. Surprisingly, both were abundant, with the presence of extensive macroalgal beds in the sandy lagoon and a sea urchin-dominated reef slope. The algal distribution on Ningaloo reflected marked cross-shelf variation in the composition of fish functional groups, with only the back reef and the reef slope exhibiting high grazing rates (completely scraped every 43 and 59 d, respectively). Estimated bioerosion rates by fishes ranged between 1 and 2.3 kg m(-2) yr(-1). Echinoids only played a significant role in bioerosion on the reef slopes owing to their high abundance in that habitat (> 12 individuals m-2). Here, estimated echinoid erosion equalled that of the most abundant excavating parrotfish, Chlorurus sordidus. High echinoid and macroalgal abundances on this relatively intact reef system highlight the need for caution when using these metrics for evaluating reef ecosystem condition.