Relationships 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.

中文翻译：

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经常性的大规模海胆死亡和建立一个持久的替代大型藻类为主的社区状态

海胆和非结壳大型藻类之间的关系通常是非线性的，当海胆丰度超过临界阈值时，会发生从贫瘠状态到大型藻类主导状态的突然相移，反之亦然。在这里，我们使用两个大规模死亡事件作为自然实验来研究这些稳定状态之间的渐进变化和动态。在 2009-2010 年和 2018 年，两次大规模死亡使海胆Diadema Africanum的种群数量锐减在加那利群岛（东亚热带大西洋）。在这些岛屿上，这种海胆是控制大型藻类生长的关键草食动物，海胆荒芜已持续了 60 多年。大规模死亡事件在空间和时间上的强度不同，当海胆密度降至 4 indv 的临界阈值以下时，两者都检测到从海胆贫瘠之地到大型藻类床的两个相移。·m ^-2。拉帕尔马岛的第一次大规模死亡率更高，因此在该岛建立了一个由非结壳大型藻类主导的长期替代状态。随后，反复出现的大规模死亡也有利于从海胆贫瘠之地向特内里费岛的非结壳大型海藻床过渡。在大规模死亡之后，D. Africanum两个岛屿的人口减少了 94-98%。相移后没有发现这些种群恢复的迹象；生态系统的反馈可能是造成非洲斑蝽种群密度低的原因。