more complex situation. At present, Lessios reports that the entire Caribbean, 

 except for much of Venezuela, Tobago, and the area between Guadaloupe and the 

 British Virgin Islands, has been affected, although these areas could be affected 

 by the time this report is published. 



Mark Hay (University of North Carolina at Chapel Hill) noted that, despite 

 the die-off of Diadema in the Florida Keys, algal abundance on reefs there was 

 not unusually high 2 months after the die-off. He suggested that either herbivorous] 

 fishes were sufficiently abundant to check algal growth or 2 months may not 

 have been enough time to detect a noticeable change. 



Douglas Morrison (University of Georgia) summarized his quantitative data 

 on the effects of the die-off on a Jamaican reef. Mean densities of urchins 1 

 year before the die-off were about 15 per m^ at 5 in depth and about 2 per m^ at 

 16 m. Several months after the die-off, mean densities had declined to about 1 

 per m^ at 5m depth, and no urchins were apparent at 16 m. After the urchin die- 

 off at the shallow site, the coverage of crustose coralline algae (an indicator 

 of high grazing intensity) declined from 45% to about 18%. Replacing the coral- 

 lines, coverage by fleshy macroalgae quadrupled (from 7% to about 28%) and coverage 

 by filamentous algae nearly tripled. These patterns were corroborated by earlier 

 urchin-removal experiments conducted before the die-off; in the experimental 

 absence of grazing, crustose corallines were replaced by upright macroalgae. 

 In contrast, the die-off had little apparent effect upon algal growth at the 

 deep site, where urchin densities were normally low, and macroalgal abundance 

 remained relatively high (73% cover). The macroalgae that increased in abundance 

 at the shallow site are known to be resistant to fish grazing but not to urchin 

 grazing. These included members of the genera Lobophora , Caulerpa , and Dictyota . 



Whatever its cause, the mass mortality of Diadema throughout the Caribbean 

 provides an excellent (although uncontrolled) "natural experiment" for determining 

 the impact of urchins on enti re reef systems. Where adequate baseline data are 

 available, observations following the die-off will test the widespread applicability 

 of previous experimental analyses of small patch-reef systems (e.g., Ogden, et al . , 

 1973) and help to resolve the controversy discussed in the next section. 



ARE URCHINS IMPORTANT GRAZERS ON UNFISHED REEFS? 



Hay summarized a study that was in press at the time of the workshop 

 (Hay, 1984). He noted that most of the studies of the impact of urchin grazing 

 on reef benthos have occurred at two sites: Teague Bay, St. Croix (e.g., Ogden, 

 et al . , 1973) and Discovery Bay, Jamaica (e.g., Sammarco, 1980). Such studies 

 have led some to believe that urchins are the most important grazers on "typical" 

 coral reefs. However, Hay suggested that these particular sites are atypical 

 in that they have been heavily fished, indirectly resulting in unusually high 

 densities of urchins which have been freed from predation by (and perhaps 

 competition with) fishes. Using pieces of the seagrass Thalassia testudinum as 

 a field bioassay for the intensity of herbivory on macrophytes, he found that 

 eight lightly fished sites scattered throughout the Caribbean (including Salt 

 River on St. Croix) showed decreasing grazing intensity with increasing depth 

 and that almost all herbivory was due to fishes. Moreover, urchin abundances 

 were low at these sites. In contrast, Haiti and Teague Bay on St. Croix, both 



28 



