Multiple defender effects: synergistic coral defense by mutualist crustaceans.

The majority of our understanding of mutualisms comes from studies of pairwise interactions. However, many hosts support mutualist guilds, and interactions among mutualists make the prediction of aggregate effects difficult. Here, we apply a factorial experiment to interactions of 'guard' crustaceans that defend their coral host from seastar predators. Predation was reduced by the presence of mutualists (15% reduction in predation frequency and 45% in volume of coral consumed). The frequency of attacks with both mutualists was lower than with a single species, but it did not differ significantly from the expected frequency of independent effects. In contrast, the combined defensive efficacy of both mutualist species reduced the volume of coral tissue lost by 73%, significantly more than the 38% reduction expected from independent defensive efforts, suggesting the existence of a cooperative synergy in defensive behaviors of 'guard' crustaceans. These emergent 'multiple defender effects' are statistically and ecologically analogous to the emergent concept of 'multiple predator effects' known from the predation literature.