Species interactions affecting corals and recruitment on a protected, high-latitude reef: Hervibory, predation, and competition by fishes, urchins, macroalgae and cyanobacteria

Interactions between species can affect major processes that shape community structure in many systems. Both fishes and urchins are important herbivores on corals reefs, maintaining resilience through their grazing activities and preventing coral-macroalgal phase shifts. Despite their importance, little is known about the relative importance of these different herbivore guilds and of the scale and magnitude of their positive and negative effects on corals. First, I investigated coral recruitment at six backreef sites on Midway Atoll, including two anthropogenically-impacted sites with metal debris that have periodic blooms of the benthic cyanobacterium Hormothamnion enteromorphoides . Contrary to expectations, coral recruitment was significantly higher at the two cyanobacterial bloom sites than at the four control sites. The proportion of recruits on exposed surfaces was higher at bloom sites than at controls, indicating that cyanobacteria indirectly enhanced recruitment by inhibiting fish grazers that usually remove small corals from exposed surfaces. Next, I conducted a factorial field experiment to quantify the relative effects of herbivorous fishes and urchins (Echinometra mathaei) on macroalgal growth and coral recruitment. Fish grazing effectively limited algal biomass, which was>50 times higher in treatments without fishes. Coral recruitment was >2X higher in treatments exposed to fish grazing, indicating that algae inhibit coral recruitment more than do fishes. Algal biomass was negatively correlated with coral recruitment, suggesting that management to increase herbivorous fishes and grazing levels is likely to benefit coral recruitment. Finally, I investigated direct, species-specific effects of urchins on corals. Most fragments of all three coral species exposed to E. mathaei were damaged within days and the damage increased over several weeks to months, often ending with complete removal of all coral tissue and skeleton. Fragments in plots without E. mathaei, and all fragments exposed to H. mammillatus plots were unaffected. These studies demonstrate a number of positive and negative, direct and indirect effects of herbivores on corals. Negative effects of urchins are lessened by their spatial restrictions, and negative effects of fishes are outweighed by the positive effects of algae removal, clearing space for coral recruits and enhancing the resilience of coral reef communities.