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. 2001 Apr 24;98(9):5067-71.
doi: 10.1073/pnas.071524598. Epub 2001 Mar 27.

Recovery of Diadema antillarum reduces macroalgal cover and increases abundance of juvenile corals on a Caribbean reef

P J Edmunds  1 , R C Carpenter
Affiliations

Recovery of Diadema antillarum reduces macroalgal cover and increases abundance of juvenile corals on a Caribbean reef

P J Edmunds et al. Proc Natl Acad Sci U S A. .

Abstract

The transition of many Caribbean reefs from coral to macroalgal dominance has been a prominent issue in coral reef ecology for more than 20 years. Alternative stable state theory predicts that these changes are reversible but, to date, there is little indication of this having occurred. Here we present evidence of the initiation of such a reversal in Jamaica, where shallow reefs at five sites along 8 km of coastline now are characterized by a sea urchin-grazed zone with a mean width of 60 m. In comparison to the seaward algal zone, macroalgae are rare in the urchin zone, where the density of Diadema antillarum is 10 times higher and the density of juvenile corals is up to 11 times higher. These densities are close to those recorded in the late 1970s and early 1980s and are in striking contrast to the decade-long recruitment failure for both Diadema and scleractinians. If these trends continue and expand spatially, reefs throughout the Caribbean may again become dominated by corals and algal turf.

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Figures

Figure 1
Figure 1
Map of Jamaica showing the location of the study sites along the north coast. RB = Rio Bueno, LTS = Long Term Survey, M1 = Mooring 1, DB = Dairy Bull, EDB = East Dairy Bull (≈0.7 km east of DB), and * = Discovery Bay Marine Laboratory. Sites were selected to sample reefs that have been the subject of long-term studies [RB, LTS, M1 (refs. , , , and 18)] and recent surveys (DB; ref. 19), and to span the greatest scale accessible with small boats (e.g., EDB).
Figure 2
Figure 2
Population densities of Diadema antillarum in sea urchin and algal zones at five sites along the north coast of Jamaica. (Bars = mean population densities; error bars = 1 SE; see Fig. 1 for site designations.) The size–frequency distribution of Diadema pooled across all sites (sea urchin zones only) is shown in the Inset graph with the percent occurrence of nine size classes based on the maximum test diameter. Size-class designations represent the following ranges of maximum test diameters: 1 < 20 mm, 2 = 20–29 mm, 3 = 30–39 mm, 4 = 40–49 mm, 5 = 50–59 mm, 6 = 60–69 mm, 7 = 70–79 mm, 8 = 80–89 mm, and 9 = 90–99 mm.
Figure 3
Figure 3
Abundances of macroalgae and corals (Inset) in sea urchin and algal zones at five sites along the north coast of Jamaica. (Bars = mean percent cover; error bars = 1 SE.) See Fig. 1 for site designations; the order of sites in the Inset is the same as for the main graph.
Figure 4
Figure 4
Density and number of genera per quadrat (Inset) of juvenile corals in sea urchin and algal zones at five sites along the north coast of Jamaica (mean ± SE; n = 10 for each bar). See Fig. 1 for site designations; the order of sites in the Inset is the same as for the main graph.
Figure 5
Figure 5
(A) Population densities of Diadema at depths between 4 and 10 m along the north coast of Jamaica from 1977 to 2000. Reported abundances are averaged over sites and depths within a sampling period (year) for each study. (a) Discovery Bay (R.C.C., unpublished data), (b) Discovery Bay (36), (c) Discovery Bay and Rio Bueno (37), (d) 14 sites in Jamaica (1), (e) Discovery Bay (12), (f) algal zones at five sites (this study), (g) sea urchin zones at five sites (this study). (B) Densities of juvenile corals along the north coast of Jamaica between 1976 and 2000. There are no comparable data from the mid-1980s to the early 1990s (d), but it is likely that there was little or no coral recruitment over this period (1). Densities of juvenile corals from the present study (averaged across sites) for the algal and urchin zones (f and g, respectively). (h) The densities of juvenile corals (≤5 cm diameter) on Discovery Bay (11-m depth) in 1976–1978 (34) calculated from ref. (i) for foliaceous corals (≤50 cm2) at Rio Bueno (10-m depth) between 1977 and 1980, and calculated from ref. (j) by using the densities of new recruits (≤2.6 cm diameter) of Agaricia agaricites and Leptoseris cucullata at 10-m and 20-m depths, assuming that they represented 75% of the coral recruits. (k) The average densities of juvenile corals (≤4 cm diameter) at four sites (10-m depth) on, or close to, Discovery Bay (19). (l) Juvenile corals (≤4 cm diameter) at Dairy Bull (10-m depth; P.J.E., unpublished data).
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