Arrested recovery of Diadema antillarum population: Survival or recruitment limitation?

Densities of the long-spined sea urchin Diadema antillarum remain significantly below pre mass-mortality levels at most Caribbean localities. The arrested recovery of this formerly abundant herbivore has been attributed to low supply of recruits and high post settlement mortality. There is, however, some debate as to which of these factors is determinant of the local dynamics of this echinoid. In this study, we use demographic modeling to analyze the contribution of recruitment and post settlement survival on the dynamics of D. antillarum in four localities of Puerto Rico Archipelago. Our results indicate relatively high adult survival, and low stasis but high growth transition in the small individuals. Recruitment rates were low and exhibited high spatial and temporal variability. The four populations exhibited asymptotic growth rates (λ) below 1.0, with λ varying from 0.918 to 0.964. The elasticity analysis showed that the survival of large-sized Diadema can potentially contribute most to the changes in λ for all sites. Numerical projections of the populations indicate that no site would exhibit an increase in density under current recruitment rates, but doubling recruitment would produce an increase in sea urchin density in three of the four sites. Recovery of D. antillarum populations would require the spatial and temporal co-occurrence of high recruitment and survival rates.