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

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