LAUSR

Competitive processes and their outcomes, such as interactions between scleractinian corals and macroalgae, are important drivers of the structure and function of coral reef ecosystems. Human communities can alter the dynamics of coral-algal interactions by changing species abundance and by affecting competitive ability. Here, we investigated how a natural human population gradient in the Maldives influences the relative abundance of benthic organisms, and if changes in benthic cover can influence the diversity, frequency and outcomes of coral-algal interactions. We observed a decline in some coral assemblages and an increase in coral mortality and filamentous algae on reefs with the highest human population pressures. At the highest level of human population, the diversity of coral-algal interactions was significantly reduced, with some genera of plating corals locally sparse. Human population pressures did not increase the frequency of coral-algal interactions or the competitive ability of macroalgal types. Regardless of human population, interactions between filamentous algae and cyanobacteria were the most damaging to competing corals. Interactions between crustose coralline algae and Halimeda were not only the most common and least harmful to coral, but were also positively correlated with coral cover, emphasizing the role that positive species interactions can play in regulating community structure and function.