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，可改变生态群落的结构与功能——其作用模式往往出人意料，且作用方向与强度存在显著差异。在太平洋珊瑚礁生态系统中，长棘海星（Acanthaster planci）的捕食行为与珊瑚覆盖率的大范围缩减、大型藻类覆盖率的大幅上升密切相关。大型藻类暴发会加剧珊瑚与大型藻类之间的种间竞争，进而进一步诱发珊瑚种群衰退。然而，本研究结合控制性野外实验与野外观测数据，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（associational refuges），并显著降低长棘海星对珊瑚的捕食强度。因此，随着长棘海星种群密度升高，大型藻类尽管会作为竞争者显著抑制珊瑚生长，却可能转而成为珊瑚的互惠共生伙伴。野外摄食实验结果显示，大型藻类的保护效应足够显著，可促使长棘海星转而取食低偏好性珊瑚，而非被大型藻类环绕的高偏好性珊瑚。这一发现凸显了当捕食者种群较为普遍时，珊瑚与藻类之间的相互作用具有环境依赖性特征。在面临日益增多的生物干扰的背景下，大型藻类通过构建关联庇护所以抵御长棘海星捕食的生态效应，可能对珊瑚礁群落的结构、功能与恢复力产生重要影响。