Seaweeds protect corals from predatory starfish: Competitors become accomplices as reefs degrade
Positive indirect effects arising from multispecies interactions play an integral role in the structure and function of ecological communities and can lead to negative interactions between two species becoming positive when these species are embedded within a diverse community. A better understanding of context-dependency is needed for modeling these interactions, as well as for building robust management strategies for ecosystems subject to increasing disturbance. On Pacific coral reefs, predation by the crown of thorns starfish, Acanthaster planci, has been associated with broad-scale coral decline and increased cover of macroalgae that often replaces living corals. Increased macroalgal cover enhances macroalgal-coral competition and is generally considered an additional driver of coral decline. Using a combination of field observations and manipulative experiments, we investigated the effects of an abundant brown macroalga, Sargassum polycystum, on the coral Montipora hispida and how the cost and benefits for the coral may vary due to Acanthaster predation. We hypothesized that at sufficient densities, Sargassum would reduce coral growth, but also might provide Montipora with an associational refuge from Acanthaster. For the latter, this included testing whether the probability of Acanthaster attacking Montipora declined with macroalgal density, as well as whether the preference of Acanthaster for Montipora over Porites cylindrica (a low preference prey) would reverse if Montipora was competing with high densities of Sargassum.
Sargassum depressed coral growth in direct proportion to its abundance. However, choice experiments and field surveys of Acanthaster feeding behavior revealed that corals in contact with macroalgae were also less likely to be attacked and consumed by Acanthaster than those that lacked surrounding macroalgae; protection increased with increased density of macroalgae. Additional feeding experiments revealed that the deterrent effects of macroalgae were strong enough to cause prey switching, with Acanthaster consuming low preference (Porites) rather than high preference corals (Montipora) if the latter were surrounded by macroalgae. This study highlights the context-dependent nature of interactions between scleractinian corals and macroalgae, which are considered fundamental to reef ecosystem dynamics. Coral-seaweed interactions are context-dependent and can be diverse in direction and magnitude. The ability of some macroalgae to create associational refuges by altering Acanthaster predation may have important implications for the structure, function, and resilience of reef communities subject to an increasing number of biotic disturbances.