The coral-seaweed-herbivore triangle is an accepted generalization embedded within a highly complex web of biotic interactions and abiotic conditions that bring exceptions. The pattern is confirmed by observations that herbivorous fishes and urchins can have very strong effects on the standing crop of reef macroalgae, thereby opening space for corals to thrive. However, other factors, such as the abundance and distribution of territorial damselfishes, shelter for schooling herbivores, water motion, and nutrient flux, as well as multiple stressors on corals, can modify this basic pattern, sometimes strongly. High levels of herbivory lead to dominance by low-lying algae, including crustose corallines that may foster coral settlement. The intensity of herbivory by schooling fishes often varies unimodally with depth: low in very shallow water due to wave stress and other factors limiting accessibility by fishes, high at moderate depths, and low in deeper water where coral growth that provides shelter for fishes declines. Dense stands of macroalgae tend to thrive where herbivores are rare due to lack of habitat complexity that provides refuges from predation. Herbivorous damselfishes can act as natural cages by inhibiting schooling grazers and maintaining a high diversity of mid-successional algae within their territories via moderate grazing (intermediate-predation effect). These algal mats not only inhibit coral growth, but also serve as localized refugia for small invertebrates and newly settled fishes. Nutrients also play a pivotal role in structuring benthic algal productivity, standing crops, and species assemblages. Besides directly consuming corals or algae, reef fishes can also affect invertebrate corallivores and herbivores, causing subsequent indirect effects on reef benthos. Examples include predation on the corallivorous crown-of-thorns seastar and herbivorous sea urchins, the latter causing halos around patch reefs where urchins remain near cover. From a management perspective, conserving herbivores is clearly important for keeping reef algae in check. Maintaining both the abundance and species diversity of herbivores of a variety of sizes, especially via marine reserves, is likely the best means of ensuring that macroalgae do not displace corals. Such rules of thumb are likely essential for fostering the ecological resilience of coral reefs, especially in the context of a warming and acidifying ocean.
Hixon, M. A. (2015). Reef fishes, seaweeds, and corals: A complex triangle. In Coral Reefs in the Anthropocene (pp. 195–215). Springer Netherlands. https://doi.org/10.1007/978-94-017-7249-5_10