Factors constraining natural recovery of <i>Diadema antillarum</i> following a mass die-off: a case study near the island of Saba, Caribbean Netherlands

After the mass mortality of 1983-1984, recovery of the keystone herbivore <i>Diadema antillarum</i> has been limited. Persistently low population densities reduced grazing pressure, leading to algal dominance on many Caribbean reefs. To understand recovery dynamics and support restoration, greater insight into factors constraining sea urchin population recovery is essential. We assessed <i>D. antillarum</i> settlement, predator density, shelter availability, and post-die-off recovery at four locations near Saba, Caribbean Netherlands, following the 2022 <i>D. antillarum</i> die-off. One location, an artificial rock reef, had previously supported high <i>D. antillarum</i> densities, while the natural reefs showed only sporadic recruitment. One-year post-die-off, the <i>D. antillarum</i> density at the artificial reef recovered to 1.4 ± 0.5 <i>D. antillarum</i> per m², whereas no populations established on the three natural reefs. Despite high overall predator biomass, the artificial reef had lower densities of Spanish hogfish and small wrasses. Previous studies indicate that these species, together with the queen triggerfish, are key determinants of <i>D. antillarum</i> recovery. Invertebrate predator densities were broadly similar across locations, although spotted spiny lobster, an important crab predator and potentially reducing crab predation on <i>D. antillarum,</i> were more abundant on the artificial reef. Shelter availability, depth, and reef structure appeared more favorable at the artificial reef location. We conclude that <i>D. antillarum</i> population establishment is primarily shaped by predation pressure and shelter availability, while larval settlement and the presence of adults appear less influential. A regional-scale study could further detangle the factors affecting natural recovery and identify reefs with a high chance of successful <i>D. antillarum</i> restoration.