Because seaweeds can overgrow and kill corals, herbivores are critical for coral reef function because they keep reefs free of seaweeds, thus facilitating the recruitment, growth, and resilience of corals. Fishes and urchins are typically the dominant herbivores on coral reefs with fishes in some reef areas biting the bottom at rates of >100 000 times perm2 every day. When in sufficient numbers, either fishes alone or sea urchins alone can remove greater than 90% of the daily primary production on reefs. By feeding on seaweeds that are competitively superior to corals, herbivorous fishes both clear the substrate for settling coral larvae and prevent seaweed overgrowth of established corals. In return, the biogenic structure and topographic complexity of reef corals benefit herbivorous reef fishes and urchins by providing food, habitat, and refuges from predation. When herbivores are removed by experimentation, overfishing, or disease, seaweeds replace corals and the biogenic structure of the reef degrades. Both reductions in coral structure and increases in seaweeds are associated with losses of herbivorous reef fishes. Interestingly, large-scale manual removals of seaweeds from reefs have resulted in only temporary increases in herbivorous fish abundance with seaweeds becoming the dominant benthic organism once again after several months. Thus, reductions in seaweeds without recovery of corals may inhibit the recovery of many reef fishes, leading to the continued degradation of coral reefs.

The main herbivorous fishes on coral reefs are generally surgeonfishes (Acanthurdiae) and parrotfishes (Scaridae) with rabbitfishes (Siganidae), chubs (Sparidae), and dam-selfishes (Pomacentridae) also responsible for considerable herbivory in some locations (Figure 3). Surgeonfishes typically feed on turf algae and foliose seaweeds with some species feeding primarily on detrital material. Parrotfishes have robust jaws with teeth fused into a beak-like formation (hence the name parrotfish), which allows them to feed on tough, calcified seaweeds in addition to algal turfs and foliose seaweeds. Although the important role ofherbivores in influencing reefcommunity is well established, less is known about the importance of individual herbivore species or the role ofherbivore diversity in affecting coral reef health. Herbivore diversity should benefit reefs because a more diverse herbivore assemblage should include herbivores with varied attack strategies, which in turn should increase the efficiency of seaweed removal because particular seaweeds are unlikely to be well defended against all types of herbivores. Experimental manipulations of herbivorous fish diversity demonstrate that species-richness is important for reef function because complementary feeding by different herbivorous fishes suppresses upright seaweeds, facilitates crustose corallines and turf algae, reduces coral mortality, and promotes coral growth. Hence not only are herbivores critical for coral reefs, but herbivore species-richness is also essential as a range of feeding strategies and physiologies allows efficient removal of seaweeds and promotes coral health.

Figure 3 Herbivores, like this mixed-species school of parrotfishes in the Caribbean, are important to coral reef health because they remove seaweeds that would otherwise overgrow and kill corals. Photo credit M.E. Hay.

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