Tipping the balance: Reef community shifts after a regional urchin population collapse

Coral reef ecosystems are maintained by complex interactions among organisms, with herbivory playing a critical role in regulating benthic algae populations and preserving reef health. Herbivores such as the black spiny sea urchin prevent macroalgae from outcompeting corals by exerting top-down control, sustaining ecosystem stability and diversity. There is evidence that the mass mortality of D. antillarum in the 1980s contributed to phase shifts from coral-dominated to algal-dominated states across the Caribbean, demonstrating their critical role in reef dynamics. Certain areas of the Caribbean experienced a slow recovery of D. antillarum , until 2022, when another mass mortality event occurred. This study investigates the ecological impacts of the recent 2022 die-off using monitoring data collected from Culebra, Puerto Rico, between 2021 and 2024. High-resolution orthomosaics generated through photogrammetry and Structure-from-Motion (SfM) techniques were used to assess shifts in benthic community structure, focusing on algal and coral cover changes in response to D. antillarum abundance. In addition, we modeled coral recruitment dynamics and found that both urchin cluster density and mean cluster size at baseline were related to declines in coral recruitment, with larger and more numerous urchin clusters associated with reduced post-die-off recruitment. Our results also showed increases in macroalgal cover and decreases in sponge cover following the loss of D. antillarum , highlighting the potential community assembly regulative role of this herbivore. These findings underscore the urgency of protecting and restoring key herbivore species and highlight the need for targeted management strategies to mitigate further degradation of Caribbean reef systems.