Data from: Competitors as accomplices: seaweed competitors hide corals from predatory starfish

Indirect biotic effects arising from multispecies interactions can alter the structure and function of ecological communities—often in surprising ways that can vary in direction and magnitude. On Pacific coral reefs, predation by the crown-of-thorns sea star, Acanthaster planci, is associated with broad-scale losses of coral cover and increases of macroalgal cover. Macroalgal blooms increase coral–macroalgal competition and can generate further coral decline. However, using a combination of manipulative field experiments and observations, we demonstrate that macroalgae, such as Sargassum polycystum, produce associational refuges for corals and dramatically reduce their consumption by Acanthaster. Thus, as Acanthaster densities increase, macroalgae can become coral mutualists, despite being competitors that significantly suppress coral growth. Field feeding experiments revealed that the protective effects of macroalgae were strong enough to cause Acanthaster to consume low-preference corals instead of high-preference corals surrounded by macroalgae. This highlights the context-dependent nature of coral–algal interactions when consumers are common. Macroalgal creation of associational refuges from Acanthaster predation may have important implications for the structure, function and resilience of reef communities subject to an increasing number of biotic disturbances.

多物种相互作用所产生的间接生物效应，能够改变生态群落的结构与功能——其影响常以出人意料的方式呈现，且作用方向与强度均存在差异。在太平洋珊瑚礁生态系统中，长棘海星（Acanthaster planci）的捕食作用与珊瑚覆盖率的大范围下降以及大型藻类覆盖率的上升密切相关。大型藻类水华会加剧珊瑚与大型藻类之间的竞争，进而进一步导致珊瑚群落衰退。然而，本研究结合控制性野外实验与野外观测手段，证实了诸如多胞马尾藻（Sargassum polycystum）在内的大型藻类能够为珊瑚提供关联庇护所，并显著降低长棘海星对珊瑚的捕食量。因此，随着长棘海星种群密度升高，大型藻类可转变为珊瑚的互利共生伙伴，即便它们本是会显著抑制珊瑚生长的竞争者。野外摄食实验结果显示，大型藻类的保护作用极强，足以促使长棘海星放弃取食被大型藻类包裹的高偏好性珊瑚，转而捕食低偏好性珊瑚。这一发现凸显了当捕食者种群较为常见时，珊瑚与藻类之间的相互作用具有环境依赖性特征。在日益频发的生物干扰事件影响下，大型藻类通过构建关联庇护所以抵御长棘海星捕食的现象，可能对珊瑚礁群落的结构、功能与恢复力具有重要意义。