Using an isolated population boom to explore barriers to recovery in the keystone Caribbean coral reef herbivore Diadema antillarum
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Recovery of the keystone herbivore Diadema antillarum after the 1983–1984 mass mortality event poses one of the greatest challenges to Caribbean coral reef conservation, yet our understanding of the problem remains severely limited. Whilst some recovery has been observed, this has been restricted to the shallows (≤5 m). We report a newly discovered, isolated population recovery on Banco Capiro, Honduras, representing the largest recorded post-mortality densities beyond the shallowest environments (0.74–2.27 individuals m−2 at depths ≥10 m) alongside an unusually high mean percentage scleractinian coral cover of 49–62 %, likely no coincidence. On the nearby island of Utila, we report D. antillarum densities of 0.003–0.012 individuals m−2 and scleractinian coral cover of 12 % at depths ≥10 m, “typical” for a contemporary Caribbean coral reef. The three order of magnitude disparity in population density between sites separated by <60 km presents a unique opportunity to investigate barriers preventing their region-wide recovery by simultaneously addressing a range of previously proposed hypotheses. Despite concerns over the impacts of asynchronous spawning in low-density populations, we find that recruitment is occurring on Utila. This suggests that, whilst Allee effects are likely to be a contributing factor, the major barriers suppressing recovery are instead impacting juvenile survival into adulthood. Similarly, variations in heterospecific echinoids, interspecific competitors, and nutrient availability fail to account for population differences. Instead, we highlight a lack of structural complexity on contemporary Caribbean reefs as the most likely explanation for the limited recovery through a lack of provision of juvenile predation refugia, representing a further consequence of the recent ubiquitous phase shifts throughout the region. Using these findings, we propose future management strategies to stimulate recovery and, consequently, reef health throughout the Caribbean.
KeywordsDiadema antillarum Caribbean Coral reef Herbivory Urchin Phase shift
This work was funded by Operation Wallacea and conducted in collaboration with Tela Marine Research Centre and Coral View Research Centre, Honduras. We would like to thank Operation Wallacea staff and volunteers for contributions to data collection in Honduras and Mexico, as well as Centro Ecológico Akumal (CEA) for logistical support in Mexico. We are particularly grateful to Antal Borcsok, Maria Arroyo-Gerez, Caitlin Marsteller, Rich Astley, and Sarah Laverty for facilitating this research through logistical support. Special thanks goes to the Atlantic and Gulf Rapid Reef Assessment (AGRRA) team for providing regional data on D. antillarum population densities and to Ian Drysdale (Healthy Reefs Initiative) and Jenny Myton (Coral Reef Alliance) for their constant support of this work and their tireless efforts to protect Banco Capiro. We also thank the two anonymous reviewers whose suggestions have enabled us to make significant improvements to the manuscript.
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