Recurrent large‐scale sea urchin mass mortality and the establishment of a long‐lasting alternative macroalgae‐dominated community state

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AbstractRelationships between sea urchins and noncrustose macroalgae are typically nonlinear, and an abrupt phase shift from a barren to a macroalgae‐dominated state and vice versa happens when the sea urchin abundance exceeds a critical threshold. Here, we have used two mass mortality events as a natural experiment to study progressive changes and dynamics between these stable states. In 2009–2010 and 2018, two mass mortalities decimated the populations of the sea urchin Diadema africanum in the Canary Islands (Eastern Subtropical Atlantic). In these islands, this sea urchin is a key‐herbivore controlling the growth of macroalgae, and sea urchin barrens have persisted for more 60 yr. Mass mortality events differed in intensity both in space and time, and two phase shifts from sea urchin barrens to macroalgal beds were detected in both when sea urchin density declined below a critical threshold of 4 indv. · m−2. The first mass mortality was greater in La Palma Island, consequently in this island a long‐lasting alternative state dominated by noncrustose macroalgae has been established. Subsequently, a recurrent mass mortality also favored the transition from the sea urchin barrens to noncrustose macroalgal beds in Tenerife Island. After mass mortalities, D. africanum populations were reduced by 94–98% in both islands. No signs of recovery of these populations have been identified after phase shifts; ecosystem feedbacks may be the cause of the low population density of D. africanum.