Macroalgae can inhibit resilience by overgrowing and killing coral colonies, and by producing toxic chemicals which can kill corals or limit coral recruitment by lowering coral fecundity, settlement rates, and post-settlement survival. Herbivores help keep the substrate free from algae so that coral recruits can settle. When herbivores are not present, faster growing macroalgae can overgrow corals. This deprives corals of essential sunlight which can cause declines in coral condition and cover, and reduces the space available for coral settlement. This can result in a phase shift.
Herbivores are diverse and do not constitute an ecologically uniform group. They include several groups that differ in terms of how they feed, what they eat, and their impact on the substrate. There are four functional groups of coral reef herbivores—scrapers, grazers, browsers and excavators—and each has a role in maintaining healthy reef systems. ref
Herbivores help to regulate community structure and function in many ecological systems. In addition to regulating and influencing the competitive interactions between corals and macroalgae, herbivorous fishes and urchins are also agents of bioerosion. For example, parrotfishes excavate the surface of the reef matrix or living coral as they feed, and the material is processed by their jaws, reduced to sediment and expelled back into the system. Such processes on coral reefs play an important role in maintaining the balance of reef growth and decay. Bioerosion contributes to reef recovery by removing dead coral and cleaning areas of substratum for colonization by benthic organisms, facilitating the settlement, growth, and survival of coralline algae and corals.
Ecological feedback mechanisms on coral reefs may have either positive or negative trajectories, and these are controlled primarily by the amount of grazing intensity (i.e., if the grazing intensity is high enough to control overgrowth of macroalgae). Disturbances such as overfishing of herbivores, coral bleaching, and coral disease may contribute to the decline of corals or overgrowth of macroalgae. Once a reef is dominated by macroalgae, negative feedbacks reinforce the dominance of macroalgae making it hard for corals to recover. An example of a negative feedback is insufficient grazing intensity. This could be caused by reduced herbivorous fish biomass (e.g., due to overfishing), high algal productivity (e.g., due to elevated nutrient concentration), and/or low coral cover (e.g., recent bleaching event). Once macroalgal growth outpaces the ability of reef herbivores to control macroalgal biomass, macroalgae blooms, and reef degradation can be quick and difficult to reverse because macroalgae can damage corals and reduce coral settlement. A decline in corals reduces the structural complexity of the reef; lower habitat complexity can reduce coral recruitment (reduced availability of refugia from algae), and herbivorous fish recruitment (due to increased predator efficiency). ref Researchers have documented coral decline in response to removing herbivorous fish from a reef at both local and regional scales. ref
In an experimental manipulation of large herbivorous fishes, Hughes et al. 2017 tested the influence of herbivores on the resilience of coral assemblages. ref The experiment was undertaken on the Great Barrier Reef after the regional-scale bleaching in 1998 within a no-fishing reserve where coral abundances and diversity had been sharply reduced by bleaching.
In control areas, where fishes were abundant, algal abundance remained low, whereas coral cover almost doubled (to 20%) over a three year period, primarily because of recruitment of species that had been locally extirpated by bleaching. In contrast, exclusion of large herbivorous fishes caused a dramatic explosion of macroalgae, which suppressed the fecundity, recruitment, and survival of corals. Consequently, management of fish stocks is a key component in preventing phase shifts and managing reef resilience.