Coral Reefs: A Reef Resilience Toolkit Module

Herbivory

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.

Marine Reserves Do More Than Protect Fish

A series of papers published in 20066, 20077, and 20108 on effects of marine reserves on grazing, predator communities, and coral recruitment have illuminated some positive and welcome effects of well-managed reserves. Peter Mumby and colleagues have been studying the Exuma Cays Land and Sea Park (ECLSP) of the Bahamas, a 456 km2 park established in 1956. The group of scientists has been able to answer key questions about the effects of marine reserves on coral populations.

The authors found that increased grazing inside of reserves positively influenced (increased) the density and community structure of coral recruits, demonstrating that marine reserves can do more than just protect fish communities. The study described a relationship in which increased grazing by fish, specifically parrotfish, increased the amount of substrate available for recruiting coral. Thus, the ECLSP indirectly increased coral recruits by facilitating substrate-clearing grazers. In fact, they found 2-fold increases in coral recruitment associated with high levels of grazing inside the ECLSP. This study supports one of the key aspects of enhancing reef resilience, by advocating for ecosystem management that enhances densities of important grazers, such as parrotfish.

In a previous study, the group also found that reserve protection did in fact increase top-level predators, as many scientists predicted would happen. However, the expectation that increases in top-level predators, such as groupers, would have negative effects on algal grazing was not met. Indeed, herbivory or algal grazing in protected areas increased two-fold and decreased macroalgal cover four-fold. The authors argued that the mechanism underlying this finding was that particular species of large-bodied parrotfish, the most effective macroalgal grazers on the reefs, experienced a size-escape from predation and were also protected from fishing in the reserve. Thus these larger bodied individuals were allowed to continue grazing, and ultimately made space for corals to recruit.

Herbivorous fish and invertebrates, such as parrotfish and Diadema, play a critical role in regulating the competitive relationship between macroalgae and corals. Photo © U.S. Department of Interior

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

Reef Herbivores

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.

Loss of herbivores, through overfishing, can cause shifts from coral dominated reefs to reefs with abundant macroalgae populations. To maintain reef resilience, management activities should focus on protecting herbivore populations. Photo © S. Wear/TNC

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.

Feedback loops that produce the “biotic death spiral” versus the resilience of a healthy coral reef
The biotic interactions producing positive or negative feedbacks that drive a reef toward either an unhealthy state of seaweed dominance, with declining corals, fishes, and structural complexity (left side of image), or toward a resilient healthy state of coral dominance, with few seaweeds, many fishes, and a high structural complexity formed by coral growth (right side of image). Rate of herbivory is the critical interaction determining whether the feedback is positive or negative (see Footnote 10).

Resources

Bahamas Biocomplexity Project

How to kill a coral reef: Lessons from the Caribbean by Bob Steneck

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