J. Exp. niiar. B&l. Ecol., 1987, Vol. 113, pp. 39-59 Elsevier 39 JEM 00960 Herbivory on coral reefs: community structure following mass mortalities of sea urchins Terence P. Hughes, Daniel C. Reed and Mary-Jo Boyle Department ~~3i~l~~~al Sciences, university of Ca~~~~~. Santa Barbara, California, U.S. (Received 20 March 1987; revision received 12 May 1987; accepted 15 July 1987) Abstract: The community structure of Jamaican coral reefs has undergone drastic change since mass mortalities ofthe long-spined black sea urchin Diadema anrillarum Philippi occurred in 1983. In the absence of Diadema, algal abundance has increased enormously, up to a mean of 95% cover or 4.6 kg wet weight m-a. Coral cover, which was already low on some reefs following Hurricane Allen in 1980, has been further reduced by as much as 60% since 1983 by competition with algae. Densities of D. antillurum at 10 sites in 1986 ranged from 0 to 12% of pre-1983 levels. Other echinoids, which might potentially compensate for the lack of herbivory from D. anfillarum, have not increased significantly in density. Numbers of herbivorous scarids and acanthurids also remain at relatively low levels, because of overfishing. In the absence of high densities of fish and sea urchins, it is likely that recent changes in community structure will continue, resulting in further replacement of corals by algae in shallow water. The impact of the urchin mass mortalities is qualitatively similar to previous experimental removats of this species. In both cases, removal of echinoids resulted in substantial increases in macroalgae. However, quantitatively, the responses of algaI and coral communities to the natural die-off were signiticantiy greater, probably due to wide differences in spatial and temporal scales of the respective perturbations. Key words: Herbivory; Coral reef; Alga; Diadema antillarum INTRODUCTION A major question in community ecology is how interactions between species influence their distribution and abundance. On tropical reefs, the abundance of algae is often maintained at low levels because of herbivory (e.g., Sammarco, 1977, 1982a,b; Ogden & Lobel, 1978; Hay etai., 1983; Hay, 1985; Carpenter, 1986; Lewis, 1986). Con- sequently, substrates that are protected experimentally from grazing rapidly become colonized by macroalgae (Ogden et al., 1973a,b; Sammarco et al., 1974; Vine, 1974; Wanders, 1977; Borowitzka, 1981; Carpenter, 1981; Sammarco, 1983; Hay&Taylor, 1985) which in turn inhibit recruitment and growth of corals (Birkeland, 1977; Potts, 1977; Bak & Engel, 1979; Sammarco, 1980; Lewis, 1986). Furthermore, algal biomass is often high where herbivores are naturally scarce, e.g., on many highly turbulent reefs or reef flats (Wanders, 1976; Adey et al., 1977; Connor dc Adey, 1977; Hay, 1984; Adey Publication 394 of the Discovery Bay Marine Laboratory. Correspondence address: T. P. Hughes, Department of Biological Sciences, University of California, Santa Barbara, CA 93106, U.S. OO2~-0981~87~SO3.50 % 1987 Elsevier Science Publishers B.V. (Biomedical Division)