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.
Influence of macroalgal contact on coral growth
Excel spreadsheet (.xls) of data from field-based manipulative experiment measuring monthly changes in coral (Montipora hispida) mass from December 2013 to March 2014. Corals were exposed to one of five algal treatments: 0, 2, 4, 6, or 8 fronds of the brown alga Sargassum polycystum (frond length = 15-20 cm). Numbers in column A indicate the parent colony from which a coral originated, and numbers in column B indicate the number of Sargassum fronds in contact with the coral. The percent change in coral mass was determined using the total change in mass (column G) and initial coral mass (column C).
Clements and Hay_Influence of macroalgal contact on coral growth.xlsx
Influence of coral-macroalgal contact on Acanthaster planci feeding behaviour
Excel (.xls) spreadsheets detailing results of field surveys and manipulative experiments investigating the influence of coral-macroalgal contact on Acanthaster planci feeding behaviour. Sheet 1 includes data from the field survey conducted during July-August 2013 on Votua Reef, Viti Levu, Fiji. Column A indicates individual Acanthaster feeding incidents on the coral Montopira hispida. These include the coral that was attacked (Primary) and 5 nearest-neighboring Montipora colonies (Secondary), which are indicated in column B. The percent of each colony covered in macroalgae or attacked by Acanthaster are included in columns D and E, respectively. Sheet 2 includes data from feeding choice experiments conducted during July-August 2012. Individual starfish were given a choice between Montipora surrounded by Sargassum polycystum or plastic Sargassum mimics of varying frond densities and lengths vs. Montipora that lacked Sargassum or Sargassum mimics. Column F indicates the 1st coral attacked (0 = lacked Sargassum/mimics, 1 = surrounded by Sargassum/mimics). Sheet 3 includes data from feeding choice experiments conducted during June 2013. Individual starfish were offered a choice between either Montipora hispida or Porites cylindrica that both lacked surrounding Sargassum (0, column B), or where the choice was between Montipora surrounded by Sargassum or Porites that lacked surrounding Sargassum (1, column B).
Clements and Hay_Influence of coral-macroalgal contact on Acanthaster planci feeding behaviour .xlsx
