Prohibiting or limiting the take of herbivorous species should be a high priority for reef managers, and is critical for maintaining reef resilience. Recent research has demonstrated the importance of herbivores in facilitating coral recovery following major disturbances such as a bleaching event.1 Herbivores are known in many ecological systems as key actors regulating both community structure and function.
In the case of coral reefs, herbivores play a critical role in regulating the competitive relationship between macroalgae and corals. Macroalgae and corals compete for space and when herbivores are not present, the faster growing macroalgae often overgrow corals, depriving them of essential sunlight and causing their decline. For example, in the early 1980’s Caribbean reefs experienced a sudden shift from coral dominated reefs to reefs with substantial macroalgae populations, following chronic fishing of herbivores and then subsequent die-off of a key herbivore, Diadema antillarum.2
A variety of types of reef herbivores, including fish and invertebrates, promote coral resilience. Research indicates that herbivorous fish diversity, identity and size can all be critical for controlling seaweeds and facilitating corals. A mix of herbivores with complementary diets facilitates both survival and growth of corals. Some commonly known reef herbivores are parrotfish (Family Scaridae) and long-spined urchins (Diadema spp.). There are three functional groups of coral reef herbivores—grazers, bioeroders, and scrapers—and each has a particular role in maintaining healthy reef systems.
Despite this critical role, in the last few decades there have been major declines in grazer densities on many reefs around the world. Much of this decline has been regionally specific and dependent on fishing patterns.
Many Caribbean reefs continue to be algal dominated, as herbivorous fish and urchin populations have not recovered. It is not yet certain if these phase shifts to algal dominated areas can be reversed, as it takes time for herbivore populations to rebuild after major declines. Even if herbivore populations recover, many prefer epilithic turf algae over macroalgae3, making it difficult to mow down mature stands of macroalgae, and thus making it even more important to prevent decline of herbivorous fish populations.
Managing Herbivory Regimes
A coral reef that has transformed or is in the process of transforming into an algal reef is usually fairly obvious because healthy coral reefs do not typically have substantial stands of macroalgae. Most healthy coral dominated reefs have pockets of turf algae and occasional macroalgae.
Reef managers should work to maintain a balanced assemblage of coral and algal communities. Once algae have taken over, it is difficult to reverse the trend. When this occurs, management activities should focus on rebuilding and protecting herbivore populations. Following a major disturbance event, herbivores play an important role in inhibiting algal growth, providing coral larvae opportunity to recolonize dead substrate.4 Species-specific effects of herbivorous fishes suggest that a species-rich herbivore community can be critical in providing the resilience that reefs need to recover from common disturbance.10 In addition to species identity, fish size within a species can be critical. Large fish are disproportionately better grazers than small fish. For example, for some parrotfish, it takes 75 fish of 15 cm length to graze as much as one fish of 35cm length.9
Recent studies have identified specific types of herbivores (large-bodied parrotfish) that seem to be more important, at least at the regional scale (see sidebar Marine Reserves Do More Than Protect Fish). Recent studies make another point relevant for management: It appears that the fish who prevent seaweed from taking over reefs in the first place may not be the same fish that reverse the shift once it occurs. Any management strategy that reduces algal cover may enhance the recovery of coral and the resilience of the community.5 Refer to Monitoring Herbivory for methods of assessing and monitoring functional groups of herbivorous reef fishes as indicators of coral reef resilience.
How to kill a coral reef: Lessons from the Caribbean by Bob Steneck
1 Nyström and Folke 2001, Bellwood et al. 2004
2 Lessios 1988, Knowlton 1992, Hughes 1994
3 Hay et al. 1987
4 Hughes et al. 2007
5 Bellwood et al. 2004
6 Mumby et al. 2006
7 Mumby et al. 2007
8 Mumby et al. 2010
9 Lokrantz et al. 2008
10 Hay and Rasher 2010