Diadema antillarum (Philippi) is a common echinoid grazer in Caribbean reef areas. Field observations in st . Croix, u.s.v.r. an.d Disc. overy Bay, Jamaica and laboratory experiments demonstrate that these urchins have a strong feeding preference for algal turf. Seasonal variation in the abundance of this preferred food item results in increased incidences of live coral predation. Experimental manipulations demonstrate that over the short term food availability is more important than urchin density in determining prey item choice. Size and abundance data show an inverse relationship between urchin density and mean individual urchin size. There is also an inverse relationship between urchin density and algal turf abundance. This relationship is also shown experimentally as well as the effects of urchin grazing intensity on algal primary productivity and the numbers of algal species surviving. Tagging experiments demonstrate that homing behavior exists for these urchins. A simple model is presented to summarize factors regulating Diadema populations and their effects on the benthic algal community.

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Effects of variation in eutrophication and in Diadema antillarum densities (grazing pressure) on the abundance and life history characteristics of Favia fragum on seven reefs along the west coast of Barbados were investigated. Densities of D. antillarum were negatively correlated with eutrophication levels, and interpretations of their effects have been made simultaneously. Abundance of F. fragum was lower on more eutrophic reefs with lower D. antillarum densities. This may result from effects of eutrophication and of D. antillarum on algal cover on reefs. Macrophytic algae were more abundant, and crustose coralline algae less abundant, on eutrophic reefs with lower D. antillarum densities; and macrophytic algal cover was negatively correlated with crustose coralline algal cover across reefs. F. fragum abundance was positively correlated with crustose coralline algal cover, and F. fragum occurred less frequently on reef areas where macrophytic algae were abundant. High eutrophication and low D. antillarum density (grazing pressure) may therefore reduce F. fragum abundance by increasing the cover of macrophytic algae relative to crustose coralline algae on reefs. Neither planulation periodicity, colony fecundity, nor polyp fecundity in F. fragum differed at different eutrophication levels and D. antillarum densities. Moreover, adult growth and adult mortality did not differ with eutrophication levels and D. antillarum densities on reefs. However, growth of juveniles appeared slower and mortality higher on eutrophic reefs with low D. antillarum densities. Moreover, larvae of F. fragum preferred to settle on crustose coralline algae than on turf algae (macrophytic algae), and the former are comparatively scarce on eutrophic reefs with low D. antillarum densities. The results suggest that the negative correlation between adult abundance of F. fragum and eutrophication levels/grazing pressure (D. antillarum densities) on Barbados reefs are caused primarily by effects

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Multivariate ordination techniques were used to characterize the direction and magnitude of coral community changes on west coast reefs in Barbados between 1982 and 1992, and to relate these changes to coastal eutrophication and variation in Diadema antillarum densities (grazing pressure). D. antillarum densities were substantially lower in 1992 than in 1982, reflecting the 1983 mass mortality event. Reductions in urchin density were greater on less eutrophic reefs than eutrophic reefs. Cover by macrophytic algae increased, cover by crustose coralline algae decreased, and the number of coral species decreased between 1982 and 1992. With the notable exception of the most eutrophic reef, coral cover decreased over the 10 year period. Changes in algal cover across reefs between 1982 and 1992 were strongly correlated with decreases in D. antillarum densities, and were therefore typically greater on less eutrophic reefs, indicating that reduced grazing pressure has more strongly influenced algal cover changes than prevailing eutrophication levels. Coral species composition changed at all sites between 1982 and 1992, and except at the most eutrophic site, the direction of change was directly related to reduced D. antillarum densities and associated algal cover changes. Temporal change at the most eutrophic site was best explained by the prevailing high level of eutrophication, and resulted primarily from a substantial increase in the cover of one species, Porites porites. The decrease in coral cover that occurred on most of the study reefs in the face of the increasing cover by macrophytic algae was typically accompanied by an increase in the relative abundance of Type 1 (high recruitment, high natural juvenile mortality) to Type 2 (low recruitment, low natural juvenile mortality) corals.

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