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

Abstract

The number of relatively untouched coral reefsworldwide 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 seaurchin densities, on this reef. Surprisingly, both were abundant, with the presence of extensive macroalgal beds in the sandy lagoonand a sea urchin-dominated reef slope. The algal distribution on Ningaloo reflected marked crossshelf variation in the composition of fishfunctional groups, with onlythe 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 abundancein that habitat (>12 individuals m-2). Here, estimated echinoid erosion equalled that ofthe most abundant excavating parrotfish, Chlorurus sordidus.High echinoid and macroalgal abundances on this relatively intact reef system highlight theneed for caution when using these metrics for evaluating reef ecosystem condition. © Inter-Research 2010.