The density and size distribution of Diadema antillarum in Dominica (Lesser Antilles): 2001–2004

Diadema antillarum populations at many Caribbean locations have failed to recover from a wide-spread mortality event during the early 1980s. Quantitative assessments of Diadema in Dominica have only recently begun and can now be put into a regional context. Diadema and benthic algae were monitored in 4-month intervals at six 100 m² sites, spread over a distance of 38 km, along the west coast of Dominica. Diadema density surveys began in July 2001, while algal cover was monitored as of November 2001. Mean densities of individual sites ranged from 0.81 m⁻² (SD=1.02) to 2.98 m⁻² (SD=1.33), and overall means increased from 1.50 m⁻² (SD=0.12) in 2001, to 2.00 m⁻² (SD=0.05) in 2004. The weak negative relationship observed between the Diadema density and macroalgal cover (R ²=0.25, P=0.0030), indicates that Diadema play a limited role in controlling macroalgal species. In contrast, the abundance of turf algae was positively related to low Diadema densities, yet this relationship becomes negative beyond a critical point between 2.0 and 3.0 Diadema per m² (R ²=0.25, P=0.0075). As of November 2001 size measurements of 100 randomly sampled urchins at each site were included in the surveys, resulting in overall mean test size diameters ranging between 5.54 cm (SD=1.55) and 6.00 cm (SD=0.96). The mean test size diameter per site ranged from 4.54 cm (SD=1.91) at Tarou Point in November 2003 to 6.77 cm (SD=0.83) at Salisbury November 2001. Across all sites except Tarou Point, the most commonly occurring Diadema sizes throughout this study were within 1 cm, in the range between 4.5 and 7.5 cm. It is unclear how this Dominica scenario fits into the context of the pathogen-induced mass mortality events of the 1980s because neither qualitative nor quantitative records exist for that time in Dominica. The recent increase in Diadema density may be a symptom of recovery from Hurricane Lenny. Given the indiscriminant local pressure (affecting herbivorous fishes as well as fish predators of Diadema) Diadema’s role as grazer of algal turfs is highlighted. On a regional scale, its stable population of reproductive size classes may serve as source of larvae for nearby downstream islands.