Long-spined sea urchin (Diadema antillarum)

node · earth · Caribbean and tropical western Atlantic

The dominant algal grazer of Caribbean reefs until 1983, when an unidentified pathogen — the outbreak began near the Caribbean entrance to the Panama Canal in January of that year and spread with the currents through 1984 — killed more than nine in ten individuals across the region in roughly a year. It is the cleanest natural experiment in reef ecology, because it removed one functional group from an entire ocean basin while leaving everything else in place. What followed was a large-scale shift from coral-dominated to macroalgae-dominated reef in much of the Caribbean, compounding losses already underway from white-band disease in the acroporids and from overfishing of the other herbivores. Diadema populations have recovered only partially and patchily in the decades since, and a second mass mortality event affected the region in 2022. The lesson generalises beyond the Caribbean: a reef can be pushed over by the removal of a grazer rather than by anything that touches the coral directly.