Award: OCE-2235138

Project Goal The 2022 die-off of Diadema antillarum (long-spined sea urchins) in the eastern Caribbean provided a rare opportunity to study the effects of a keystone herbivore on coral reef ecosystems. This project aimed to assess how the loss of D. antillarum impacts reef community structure, algae growth, coral populations, and reef food webs. Using cutting-edge spatial tools and ecological analysis, the study focused on reefs around Culebra, Puerto Rico, with three key objectives: tracking urchin and algae abundance over time, understanding D. antillarums role in shaping reef communities, and examining changes in food web dynamics. Research Activities To address the study goal, we conducted multiple field trips between 2022 and 2024, collecting data through advanced imaging techniques like large-area imaging (LAI) and photogrammetry. Over 384 reef plots were analyzed, generating detailed 3D models to measure urchin populations, algae cover, and coral health. Laboratory work included delineation of coral colonies and algae patches using LAIs, and stable isotope analysis of 490 consumer and 87 producer samples to investigate trophic dynamics and energy flow within the ecosystem. Key Findings 1. Decline in D. antillarum The study confirmed a severe reduction in D. antillarum populations following the 2022 die-off, with no significant recovery observed even a year later. Urchin counts dropped by over 95% at both study sites, indicating a drastic loss of top-down control on algae growth. 2. Changes in Reef Community Structure The loss of urchins led to significant increases in macroalgae cover, which negatively impacted other reef organisms. For example, results suggest that sponges declined due to competition with algae. Based on percent cover estimates, coral cover showed minimal change, but when using population models, we learned that algae could have long-term impacts on coral populations via harmful effects of algae on coral growth and recruitment. For instance, coral population models indicated declining growth rates and survival, with projections showing potential extinction risks for stress-tolerant species like Diploria labyrinthiformis within 20 years. 3. Shifts in Trophic Dynamics Stable isotope analysis revealed that pelagic energy resources were a primary energy source for many species, but overall resource use changed over time. Specifically, omnivorous fish increased their reliance on native seagrass, while predator fish showed an increased reliance on algae energy sources over time. The analysis also highlighted niche partitioning, where different species reduced overlap in their resource use, reflecting adjustments to changing environmental conditions. Broader Impacts The findings highlight D. antillarums crucial role in maintaining coral reef ecosystems through its grazing on algae. Without this keystone herbivore, reefs face cascading effects, including algae overgrowth, reduced coral growth, and altered food webs. This research adds to the understanding of how top-down ecological processes and feedback mechanisms shape reef communities, offering valuable insights for conservation and management efforts. Also, the study emphasized the value of advanced imaging and AI-assisted tools in marine research. The high-resolution LAIs generated during the project can be revisited for future ecological studies, enabling long-term monitoring of coral reef dynamics. Training and Professional Development The project provided hands-on training for one postdoc, three graduate students, four undergraduates, an intern, and two hired employees. Participants gained skills in photogrammetry, ecological data analysis, and AI tools like CoralNet for benthic cover analysis. Graduate students incorporated project findings into their research, while the intern applied their experience to produce a report on urchin population trends. The training and professional opportunities contributed to marine ecology and conservation workforce development. Conclusion This research underscores the critical role of D. antillarum in maintaining healthy coral reefs and the wide-reaching effects of its decline. By combining innovative tools with rigorous ecological analysis, the study advances scientific understanding and provides practical insights for reef restoration and management in the face of ongoing environmental challenges. Last Modified: 12/27/2024 Submitted by: RolandoOSantos