Project: RAPID: Here we go again - The fate of Diadema antillarum and Caribbean reefs in the 21st century

NSF Award Abstract:
Coral reef benthic community assemblage and functioning are controlled in part by herbivores that graze on algae, which compete for space with various taxa, including coral and sponge species. In the Caribbean, it is hypothesized that the black sea urchin, Diadema antillarum, is a keystone grazer that controls macroalgae species composition and biomass at low levels and facilitates the recruitment and survival of reef-building corals, leading to a coral-dominated state of reefs. The recent D. antillarum die-off provides the conditions of a natural removal experiment useful to understand the top-down control exertion of this herbivore, especially by quantifying the spatial arrangement progression of benthic functional groups across urchin abundances. This study is assessing the impact of the loss of this keystone species on reef community structure and function using innovative imaging tools to quantify the fine-scale changes in the community structure, the size distribution, and spatial dispersion of D. antillarum, the dominant coral species (Porites), and turf-algae and macroalgae on the reefs of Culebra, Puerto Rico, during the initial phase of the Diadema die-off in the Caribbean. The Caribbean-wide D. antillarum die-off highlights the importance of understanding how and when herbivores play a role in shaping reef communities. This RAPID award is creating a rich digital dataset that will be leveraged to assess multiple ecological questions and provide numerous opportunities for undergraduate and graduate research studies.

The loss of this keystone species is expected to impact overall reef community structure and function and the abundance and distribution of coral turf-algae and macroalgae. The project uses innovative spatially explicit tools to quantify the fine-scale spatiotemporal changes in the community structure, size distribution, and spatial dispersion of D. antillarum, Porites astreoides (dominant coral species), and turf-algae and macroalgae (Dictyota spp.) on the reefs of Culebra, Puerto Rico, during the initial phase of the Diadema die-off in the Caribbean. Based on top-down control, competition, and percolation theory, the investigators hypothesize (H1) a strong relationship between D. antillarum and the community structure of reefs, (H2) an increase in macroalgae/turf algae cover starting with a non-uniform dispersion of algae clumps, followed by the coalescing of algae clumps percolated across the reefs and around P. asteroides colonies, which will result in (H3) a decrease of P. asteroides demographic parameters (e.g., growth, size-structure, recruitment) due to smothering, shading, and interactive effects with algae. They hypothesize (H4) that the trophic dynamics of reefs will shift, especially by the diversification of basal resources and expansion of trophic niches of consumers. The investigators are using Structure-from-Motion software to create large-area images that reproduce a digital reconstruction of reef plots. They leverage data from an existing project in Culebra (Puerto Rico) that has collected over 90 high resolution (cm-scale), large-area images (LAIs, 100 m2) since 2020. They are generating bi-monthly LAIs (8 time points) to assess the fine-scale and fast responses as a function of the die-off. Samples across different basal resources and trophic guilds/levels are being collected to perform stable isotope analysis and Bayesian mixing models to quantify shifts in trophic dynamics due to the response of the reef community to the urchin die-off. These research activities follow an analytical framework based on a before-and-after experimental approach and high-resolution temporal trends. Different statistical modeling techniques are being incorporated in this framework to address the projects' hypotheses and synergistic activities.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.