Steep decline in Diadema antillarum populations in Honduras’ Mesoamerican barrier reef (2014-2022) and its impact on benthic communities

Many Caribbean coral reefs shifted from coral to macroalgal-dominated communities following the die-off of the herbivorous urchin Diadema antillarum in the 1980s. Whilst most reefs remained algal-dominated, some have shown signs of coral recovery. This study compared benthic cover and D. antillarum populations (2014-2022) in two differently degraded reef systems in Honduras: Banco Capiro (Tela Bay) and Utila island. In 2014, Banco Capiro had high coral cover (~59%) and D. antillarum densities (155 individuals/100 m2), whereas Utila’s reefs had low coral cover (~17%) and low D. antillarum densities (2.5 individuals/100 m2). By 2022, D. antillarum populations in Banco Capiro had drastically declined (~88% decrease in adults and ~85% decrease in juveniles) and were encompassed by a large increase in macroalgae (~193%) and decrease in hard coral (~31%). In Utila, juvenile D. antillarum populations decreased by ~52%, while adult populations increased by ~700% between 2018 and 2022, although densities remained lower than Banco Capiro (0.5 individuals/100 m2). Macroalgal and hard coral cover did not change significantly in Utila, though decreases in Montastrea spp. and Siderastrea siderea (species susceptible to Stony Coral Tissue Loss Disease) were detected between 2018 and 2022. These results highlight one of the steepest D. antillarum declines in the Caribbean since 1983, preceding the 2022 mortality event. Further research is needed to understand the causes behind this collapse and potential links to the 2022 event.