Declines in sea urchin populations across the Caribbean threaten the region’s coral reefs, a concern highlighted by Florida State University researchers.
The sea urchin Diadema antillarum has long been regarded as a key herbivore in Caribbean reefs, grazing on algae that would otherwise overgrow and suffocate coral growth. Over the past four decades, two major events have sharply reduced urchin numbers. In 1983, an unidentified aquatic pathogen triggered a catastrophic wipeout, erasing about 99% of Diadema across its Caribbean, Gulf of Mexico, and western Atlantic range. Recovery has occurred in pockets, largely in shallower waters where conditions favor survival.
Recent work from John’s Island in the Virgin Islands has provided a clearer view of how hedgehog urchin populations move and respond to changing conditions. The second mass die-off occurred in 2022, and scientists now report a population decrease of roughly 98% compared with 2021 and about 99.96% relative to the 1983 event. Concurrently, coral cover in 2021 reached a nadir during the full observation period, signaling a coral ecosystem that was already precarious and not showing a rebound in mortality.
From 2021 onward, only small urchin populations remained in shallow zones where they grazed and maintained algae-free microhabitats that allowed coral larvae to settle, attach to substrates, and begin reef-building. A recent surge in urchin mortality has effectively erased these survivors, heightening the risk that critical algae-control remains unavailable in key reef zones.
Researchers warn that if these trends persist, the Caribbean coral reefs could face ongoing, possibly irreversible, stress due to the loss of a primary natural regulator of algal growth. The implications extend beyond biodiversity, affecting fisheries, tourism, and coastal protection that rely on healthy reef systems. The broader picture shows how the decline of a single functional group—reef herbivores—can ripple through the entire ecosystem, undermining resilience in the face of other stressors such as warming seas and disease. In related observations, separate wildlife reports have noted unexpected declines elsewhere, underscoring a pattern of sudden, wide-ranging losses in fragile habitats across regions.