Natural history of coral−algae competition across a gradient of human activity in the Line Islands

Abstract

Competition between corals and benthic algae is prevalent on coral reefs worldwide and has the potential to influence the structure of the reef ben- thos. Human activities may influence the outcome of these interactions by favoring algae to become the superior competitor, and this type of change in com- petitive dynamics is a potential mechanism driving coralalgal phase shifts. Here we surveyed the types and outcomes of coral interactions with benthic algae in the Line Islands of the Central Pacific. Islands ranged from nearly pristine to heavily fished. We observed major differences in the dominant groups of algae interacting with corals between sites, and the outcomes of coralalgal interactions varied across reefs on the different islands. Corals were generally better competitors against crustose coralline algae regardless of location, and were superior competitors against turf algae on reefs surrounding uninhabited islands. On reefs surrounding inhabited islands, how- ever, turf algae were generally the superior competi- tors. When corals were broken down by size class, we found that the smallest and the largest coral colonies were the best competitors against algae; the former successfully fought off algae while being completely surrounded, and the latter generally avoided algal overgrowth by growing up above the benthos. Our data suggest that human disruption of the reef ecosys- tem may lead to a building pattern of competitive dis- advantage for corals against encroaching algae, par- ticularly turf algae, potentially initiating a transition towards algal dominance.