Competitors as accomplices: seaweed competitors hide corals from predatory sea stars.

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.