Disease and the Structure of Coral Reef Communities

The impact of diseases on coral reefs has been realized over only the past two decades. Two of the most extensive disease outbreaks have been on reefs in the Caribbean and have fundamentally changed the ecology of Caribbean reefs. In 1983-84 an unknown pathogen swept through the Caribbean and killed approximately 99% of the then abundant sea urchin Diadema antillarum. In many areas of the Caribbean, D. antillarum had been the dominant herbivore keeping reefs free of most fleshy seaweeds and facilitating recruitment and growth by

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Figure 9 Alternate states in coral reef ecosystems. (A) A conceptual model showing human-induced transitions between alternate ecosystem states based on empirical evidence of the effects from fishing and excess nutrients. The 'stressed' state illustrates loss of resilience and increased vulnerability to phase-shifts. (B) A graphic model depicting transitions between ecosystem states. 'Healthy' resilient coral-dominated reefs become progressively more vulnerable owing to fishing pressure, pollution, disease, and coral bleaching. The dotted lines illustrate the loss of resilience that becomes evident when reefs fail to recover from disturbance and slide into less-desirable states. (C) Pictoral representation of the different reef states shown in (A). From Bellwood DR, Hughes TP, Folke C, and Nystrom M (2004) Confronting the coral reef crisis. Nature 429: 827-833.

corals. After the mass mortality, levels of herbivory plummeted and standing crop of seaweeds dramatically increased on many reefs. D. antillarum populations are recovering in some areas of the Caribbean, and in these 'urchin zones', seaweeds cover 0-20% of the reef as opposed to 30-79% of the reef. Juveniles corals are ten times more abundant in some urchin zones. The potential recovery of this critical herbivore gives hope to Caribbean reefs many of which are still enveloped in a blanket of seaweed.

The other outbreak that altered the structure of Caribbean reefs was the epidemic of white band disease among acroporid corals in the mid to late 1980s. This disease attacked two of the major reef-building corals in the Caribbean Acorpora palmata and A. cervicornis. These two corals were once so abundant on Caribbean reefs that early coral reef ecologists named characteristic zones on reefs for these dominant corals (i.e., the 'palmata zone' and the 'cervicornis zone'). These corals that had dominated Caribbean reefs for at least a half million years are now rare to absent on most reefs and have declined so dramatically that they are both being listed as threatened species under the Endangered Species Act in the United States. If the prevalence and severity of coral diseases is linked to pollution and climate change as has been demonstrated for some studies, then a continued increase in the effects of diseases on the ecology of reefs can be expected.

Shifting Baselines, Overfishing, and the Altered Food Webs of Coral Reefs

In many regions of the world, coral reefs are mere remnants of what they were only a few decades ago. These changes to reefs are not adequately appreciated due to the problem of the 'shifting baseline syndrome' - reefs that are deemed 'normal' today are not what was 'normal' only a few decades ago, much less a century or more ago. Each new generation of divers or marine ecologists suffers from reduced expectations of what a healthy coral reef should be. For example, as a graduate student and post-doc, one of us (M.E. Hay) dove on Caribbean reefs dominated by luxuriant stands of elkhorn and staghorn coral (Acropora palmata and A. cerviconis) the size of football fields and saw reefs abundant with grouper, large herbivorous fishes, and Diadema urchins that formed 'fields' of gigantic black pincushions on regions of some reefs. In contrast, the younger author here (D.E. Burkepile) has never seen a live stand of elkhorn coral more than a few m2 and is lucky to see one Diadema on most dives. However, both of us dive on reefs that are vastly different from those that the first European colonists would have experienced. Because of this problem of shifting baselines, it is informative for ecologists to explore the history and paleoecology of reefs in order to deduce how reef communities have changed over hundreds, or thousands, of years.

Caribbean reefs were once dominated by sea turtles, crocodiles, manatees, large predatory fishes such as sharks and large groupers, and the now-extinct monk seal. Reefs with such a diversity of charismatic megafauna scarcely exist today anywhere in the world. Centuries of overfishing have made many of these species ecologically extinct and altered the strong trophic interactions that once dominated Caribbean food webs (Figure 10). Including humans into the ecological equation began a process of 'fishing down the food web' where large consumers such as sharks and manatees were the primary targets of human harvesting. After the larger animals were depleted, fisheries switched to smaller predators such as groupers and then to herbivores such as parrotfishes. The changes in the connections of these food webs have fundamentally altered the dynamics of these ecosystems and have resulted in cascading effects such as the decline of corals and increase of seaweeds and sponges.

Although the largest megafauna are now largely gone from Caribbean reefs, we have some idea of their historical populations. For example, green turtles were once so abundant that ships' naturalists from the sixteenth to seventeenth centuries remarked that they could navigate to the Cayman Islands via the sounds of turtles swimming and that congregations of turtles seemed so thick as to confound a ship's path. One estimate puts the total pre-Columbian population of green turtles in the Caribbean at greater than 30 million as opposed to the tens of thousands today. Green turtles typically eat sea-grasses and seaweeds, but the top-down force that this historical population would have exerted on seagrass production and biomass is unrivaled by any current estimates of herbivory in seagrass beds. Because the biota of coral reefs has changed so dramatically over the past few hundred years, Jeremy Jackson writes that scientists trying to understand the ecological processes that structure coral reefs are ''.. .trying to understand the ecology of the Serengeti by studying the termites and the locusts while ignoring the elephants and the wildebeest.'' Basically, the biotic forces that impact coral reefs today are mere shadows of what they once were, and humans have radically changed the ecological and evolutionary trajectories that have influenced coral reef ecosystems for millennia.

Protection and Resurrection of Coral Reefs

One of the saving graces of coral reefs over the next few decades may be the creation and enforcement of marine reserves that protect reefs from overfishing. Overfishing is one of the most devastating threats to reefs, as fishers preferentially remove the large-bodied fishes that are

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Figure 10 Simplified coastal food web for coral reefs showing changes in some important top-down interactions due to overfishing; before (left side) and after (right side) fishing. Bold font represents abundant; normal font represents rare; 'crossed-out' represents extinct. Thick arrows represent strong interactions; thin arrows represent weak interactions. Modified from Jackson JBC, Kirby MX, Berger WH, etal. (2001) Historical overfishing and the recent collapse of coastal ecosystems. Science 293: 629-638.

the strongest interactors in these ecosystems, resulting in fundamental changes to the food webs of reefs. The establishment of marine reserves limits or prevents the harvesting of fishes and invertebrates from areas of reef and theoretically allows populations of overharvested species to rebound, reestablishing viable populations of fishes and crucial ecosystem processes on reefs. Recent studies of the efficacy of marine reserves show that reducing fishing pressures on reefs allows increases in the density, biomass, individual size, and diversity of fishes and invertebrates inside the reserves and that these effects occur rapidly and are longlasting. In addition, these reserves not only allow increases in fish density and biomass within the protected areas but also result in the 'spillover' of fishes as they migrate from the reserves into unprotected areas. Thus, marine reserves may subsidize fish populations on reefs that are not directly protected from fishing, although the extent to which this spillover effect will actually affect unmanaged reefs is equivocal.

Marine reserves can also restore trophic linkages that enhance the recovery of coral reefs. For some reefs in the Bahamas, long-term protection from fishing (i.e., roughly 50 years of enforcement) has led to increases in the abundance of medium-sized predatory fishes such as the Nassau grouper (Epinephelus striatus) (Figure 11).

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Figure 11 (a) Patterns of parrotfish biomass and their predators (±SE) within the Exuma Cays (Bahamas) and for all other survey areas combined. 'Park' denotes the Exuma Cays Land and Sea Park which is 456 km2 and was established in 1959. 'South' and 'North' represent reef systems that are near the southern and northern borders of the park. (b) Mean macroalgal cover (gray bars) (±SE) and grazing intensity of parrotfishes (black bars) inside and outside the Exuma Cays Land and Sea Park. Reserve impacts are significant (P<0.01) for each variable. From Mumby PJ, Dahlgren CP, Harborne AR, et al. (2006) Fishing, trophic cascades, and the process of grazing on coral reefs. Science 311: 98-101.

Figure 11 (a) Patterns of parrotfish biomass and their predators (±SE) within the Exuma Cays (Bahamas) and for all other survey areas combined. 'Park' denotes the Exuma Cays Land and Sea Park which is 456 km2 and was established in 1959. 'South' and 'North' represent reef systems that are near the southern and northern borders of the park. (b) Mean macroalgal cover (gray bars) (±SE) and grazing intensity of parrotfishes (black bars) inside and outside the Exuma Cays Land and Sea Park. Reserve impacts are significant (P<0.01) for each variable. From Mumby PJ, Dahlgren CP, Harborne AR, et al. (2006) Fishing, trophic cascades, and the process of grazing on coral reefs. Science 311: 98-101.

Increases in grouper abundance resulted in increased predation rates on small herbivorous parrotfishes, which would seemingly decrease the rate of herbivory on reefs. However, the protection from fishing also allowed large parrotfishes to recover and actually increased the overall rate of herbivory in the reserve despite increased predation on smaller herbivores (Figure 11 ). These increased rates of herbivory decreased macroalgal abundance and may increase coral abundance and cover over time if this balance between predation and herbivory can be maintained. Although the benefits of reserves to conservation and fisheries are promising, one of the main challenges to the success of marine reserves is the enforcement of no-harvesting policies once the reserve is established. In many areas, reserves are 'paper parks' or parks in name only as there is insufficient money or political will to achieve the enforcement necessary for the reserves to succeed. However, if marine reserves can be implemented and enforced they will be one of the best tools that conservation science currently has to protect, and hopefully resurrect, many coral reefs.

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