A fresh look at macroalgal-coral interactions: are macroalgae a threat to corals?
Vieira, Christophe; Payri, Claude; De Clerck, Olivier
Corals and seaweeds fulfill important ecological functions in tropical reef ecosystems. In an environment where space is a limiting factor, competition between both players is critical in defining the structure of coral reef communities. Dramatic shifts from coral- to macroalgal-dominated reefs have highlighted competitive interactions between macroalgae and corals. Defining the nature of the interaction between corals and reef algae, however, has been challenging, and it was questioned whether algae are the cause or the consequence of coral reef shifts. Although it is commonly accepted that macroalgae may outcompete corals under conditions of reduced herbivory or enhanced nutrient levels, there is also evidence that algae may have a negligible or even a positive effect on corals in undisturbed reefs. Interactions between macroalgae and corals date back to the Paleocene, when ‘modern’ coralgal reefs became established. Macroalgae and corals share a long evolutionary history. A combination of abiotic and biotic interactions shaped coral reef ecosystems as we presently know them, reaching stable ecological dynamics. However, natural and anthropogenic disturbances may rupture this equilibrium resulting in unbalanced population dynamics. Intensified competition between macroalgae and corals is therefore symptomatic of damaged reefs, and usually results from decrease in herbivory as well as coral morbidity and mortality.