Coral Reefs

Coral reefs are among the most diverse and productive biological communities in the world, though their future status appears increasingly threatened from widespread changes in tropical marine systems. Dating back approximately 30 million years, "modern" coral reefs may contain as much species diversity as tropical rain forests—or even more— though reefs are much richer in terms of higher-level diversity (for example, orders, classes, and phyla). Estimates of coral reef species richness range from 0.618 to 9.47 million species, with intermediate estimates of around 1 to 2 million species. Phyletic richness includes thirty-two of thirty-four phyla (compared with nine in rain forests and sixteen in tropical freshwaters). Such biological diversity is especially notable given that coral reefs, covering an estimated 284,300 square kilometers, take up just 0.089 percent of the world's ocean surface, less than 1.2 percent of its continental shelf area, and 2.9 percent of its tropical coastal areas. Moreover, coral reefs thrive in some of the least fertile (that is, oligotrophic) waters, where low levels of dissolved nutrients prevent substantial planktonic productivity. Rather than being plankton dependent, reef production relies on the existence of particular benthic animal-plant symbioses, especially those between many corals and single-celled, dinoflagellate algae known as zooxanthellae. Photosynthesis by zooxanthellae living inside coral tissue allows these coral colonies to act, at least in part, like plants: algal photosynthetic metabolites supplement and often surpass the nutrients that corals capture directly from prey. Because of this extra, plantlike growth ability, zooxanthellate corals are typically the most productive and important reef-building (also called hermatypic) coral species, with the trophic efficiency of the symbioses contributing to the high productivity of these ecosystems. In turn, this productivity, coupled with the rich, three-dimensional habitat complexity produced by corals and other sessile organisms and the accelerated rates of evolution that seem to be typical of tropical environments, has led to the massive proliferation and radiation of life in coral reef ecosystems.

Unfortunately, the health of coral reefs appears to have declined substantially over the last several decades. Coral reefs are now threatened globally from a variety of factors, and as these ecosystems continue to degrade, so too will the various benefits provided by reefs, upon which many people depend. These benefits include food from artisanal and commercial fisheries (for which, on many small islands, there are no alternative sources); recreational opportunities; employment in fisheries and tourism; coastline protection against waves; calcium carbonate (limestone) for natural beach replenishment and for building materials (as sand and rock); genetic resources for traditional and new medicines; collectibles such as pearls, shells, and corals used for jewelry and curios; as well as live animals for the aquarium trade. Since the maintenance of these potentially renewable resources depends on retaining healthy, intact coral reef ecosystems, these resource uses must be balanced and sustainable, a goal that is still far from being realized.

Despite the consensus on their importance to biodiversity and to humans, there are a wide range of definitions regarding what constitutes a coral reef. This situation arises from the various ways in which coral reefs have been valued and studied over the last few centuries. European explorers of tropical seas initially defined coral reefs simply as those navigation hazards (that is, reefs) associated with corals. Most current scientific definitions, however, center on certain geological and ecological features. Geologists have focused their interest and definitions on the biological processes and patterns of carbonate production, regardless of whether the resulting carbonate is shallow enough to obstruct boating. Coral reefs are therefore those biogenic (that is, biologically created) carbonate frameworks that have built up and continue to grow through the actions of scleractinian corals and other sessile organisms over decadal and longer time periods. Reefs that have previously developed, but no longer do, are considered to be fossil reefs. In contrast, some ecologists extend the concept of coral reefs beyond such structural definitions to non-structural coral communities, where coral species play important ecological roles as primary producers, filter-feeding consumers, and contributors to habitat complexity even though the communities have no net carbonate production. In these systems, despite the possible presence of reef-building corals, ecological and physical processes such as bioerosion, dissolution, disturbance, and export of rubble prevent the gradual geomorphological buildup of limestone beyond a thin veneer of living and dead corals.

Where carbonate frameworks do develop, scientists generally recognize several broad categories based on their proximity to land. Reefs running close to (that is, less than 1 km) and parallel to the shoreline are called fringing reefs; those developing farther offshore (more than 5 km) and parallel to the coast are barrier reefs; and circular reefs forming around subsided oceanic volcanoes are atolls. In some places, there are also reefs that appear to fit intermediate categories, such as the so-called bank-barrier reefs, which lack the deep lagoons of true barrier reefs, and bank or platform reefs, which may be similar to atolls but which develop on submerged hills or pinnacles rather than around subsided volcanoes.

A number of different reef zones or geo-morphological features are also frequently recognized within a given reef system. Depending on the underlying topography, the age of the reef, or the exposure to waves, however, individual reefs may lack one or several of these zones or features. The reef crest is classically the shallowest part of the reef, where incoming swells may break into waves. A reef flat may occur immediately behind the reef crest, followed by so-called back reef areas bordering or extending into a lagoon if there is one. Fore reefs and reef slopes occur in front (relative to the dominant direction of wave exposure). Patch reefs are generally considered to be smaller structures that lie within lagoons behind other reefs (for example, barrier or atoll reefs).

Coral reefs, as structurally defined above, are largely restricted to shallow tropical seas, where a combination of warm water temperatures, high light levels, high water clarity, and low nutrient concentrations provide the conditions under which reef-building corals thrive. Found primarily between 30 degrees north and south latitudes, and along the western sides of ocean basins (for example, the Indo-Pacific and the Caribbean), these conditions have lasted continuously over a much longer evolutionary period and across a wider area in the Indo-Pacific. The result has been the evolution of dramatic species richness there.

As one moves to higher latitudes, coral reefs generally become rarer as water temperatures drop. Most reef corals cannot survive temperatures of 16 to 18 degrees centigrade for prolonged periods, either because of direct mortality or because their growth rates decline to the extent that they are susceptible to being overgrown by macroalgae. Although coral reefs, as conventionally defined, may disappear in these cooler waters, it is important to note that there are temperate and even subtem-perate species of non-reef building corals that contribute to nontropical coral communities. In some cases, such corals (for example, Lophe-lia) even form extensive, deep sea carbonate frameworks that qualify in many ways as reefs, though these structures remain less cemented and looser than in shallow tropical reefs.

At the regional and local scales, coral reefs also tend to develop more extensively around islands or off the shore of continents. This is due to the fact that substantial freshwater, sediment, and nutrient runoff from large rivers either kills or impedes the growth of corals.

A number of general threats to coral reefs are known, including overexploitation of marine populations, pollution, habitat destruction and degradation, climate change, and invasive species. In practice, since reefs are found predominantly in developing countries, these ecosystems tend to face both the common environmental impacts relating to poverty (rapid population growth, poor nutrition, low literacy rates, lack of infrastructures for environmental mitigation and enforcement, and so forth), as well as the larger-scale, global impacts that are being driven by developed nations.

Given the paucity of alternative sources of protein and income on small islands, many poor human communities within reef ecosystems are currently forced to contribute to the overexploitation of reef biota. Unfortunately,

Underwater coral formations of the Great Barrier Reef (E.O. Hoppé/Corbis)

many reef species are especially susceptible to this overfishing. For example, large, territorial, slow-growing, and late-maturing predatory species with high economic value as table fish are easy to overfish because of the slow recovery times of such populations. Similarly, slow-growing red and black corals can be easily overexploited by those selling to the jewelry trade. Although the aquarium trade tends to favor smaller, faster growing species, the large international demand for certain species can also easily lead to the strip mining of these resources from local reefs. Beyond simply removing individual species from reefs, overexploitation tends to remove whole ecological groups of species, such as large fish preda tors and herbivores, thereby causing fundamental (and difficult to reverse) changes in the way these reef communities function.

The major form of pollution on reefs is nutrient enrichment, the result mostly of the runoff of sewage and agricultural wastes. Enrichment can alter reef community dynamics is various ways, with the major direct effect being to enhance the growth of benthic and planktonic algae. Since benthic macroalgae often compete with corals for space and light on reefs, increased algal growth can lead to the overgrowth and death of corals. Planktonic blooms (that is, rapid increases in populations) of algae may also favor other coral competitors, such as filter-feeding sponges and zoanthid cnidarians, which collect small, drifting organisms or bits of food from the plankton. Sedimentation from coastal and inland development, deforestation, and agricultural activities is also problematic for reef-building corals, smothering them if intense enough, and otherwise diminishing their competitive abilities.

The destruction and degradation of coral reef habitats is sometimes intentional, though more often it occurs as a by-product of other activities. For example, when ports decide to dredge and destroy relatively small areas of reef during the construction of channels, the consequences of the altered circulation and wave exposure patterns often lead to unintended but more widespread habitat degradation from sedimentation and storm disturbances. Similarly, the deforestation and conversion of mangrove habitats for shrimp aquaculture frequently have many indirect effects on nearby coral reefs, including increased sedimentation and the loss of nursery grounds for reef species. Even fishers who depend on reefs for their livelihoods, and who do not intend to harm reef habitats, use certain techniques that contribute to extensive reef damage and degradation: anchoring on fragile corals; using nets that become lost and entangled on reefs; using toxicants such as sodium cyanide and chemical bleach to stun animals; and especially "blast fishing" with explosives to nonselectively kill whole assemblages of fish (as well as their living habitats).

Although environmental tolerances of reef-building coral species can be relatively broad across their whole range, coral populations in particular places seem to have evolved relatively narrow tolerances (for example, 4 degrees centigrade is the normal range of annual temperatures experienced by corals on a reef slope). These environmental requirements make corals and other reef species espe cially vulnerable to climate change, particularly the periodic warmings of surface waters in the ocean that cause mass "coral bleaching" events. Coral bleaching (not to be confused with the use of chemical bleach mentioned above) refers to the dramatic whitening in a coral's appearance that results from the loss of its symbiotic zooxanthellae, or the zooxan-thellae's loss of its photosynthetic pigments. Bleaching can be caused by various stresses, such as exposure to pollution, air, high levels of UV radiation, and extreme temperatures. Of these, thermal factors seem to have the widest impact, and increases in water temperature of 1 to 2 degrees centigrade above the normal maximum for a period of several weeks are sufficient to cause mass bleaching events, affecting a significant fraction of coral colonies and other zooxanthellate taxa. Corals can often recover from mild or even moderate bleaching, but severe bleaching leads to death. Some coral populations seem to have evolved greater resistance to bleaching, but it is uncertain whether other populations can similarly adapt to changing sea temperatures.

In addition to predictions of increased bleaching with climate change, there are other predictions that subtle changes in the chemistry of sea surface water, resulting from the absorption of higher levels of atmospheric carbon dioxide, will reduce the ability of corals and other calcareous species to grow their skeletons. As this occurs, it will likely shift the balance between carbonate production and erosion toward the latter, causing some reefs to gradually lose rather than gain material. Given that this shift away from reef buildup will occur during the same time that sea levels are expected to rise (via thermal expansion of water and melting of the polar ice), the consequences for reef ecosystems, including those human communities relying on reefs, is expected to be substantial.

Although these threats to reefs are widespread, they differ in importance and differentially interact, depending both on the region of the world and on the local socioeconomic conditions. For example, blast fishing is much more common in the Indo-Pacific than elsewhere, and diseases of sea urchins, corals, and sea fans, presumably caused by invasive pathogens, are especially widespread and damaging in the Caribbean. Since the incidence of these diseases has increased in the last few decades, it is likely that stresses from other factors (for example, nutrient and sediment pollution, competition with algae, and thermal stress) are causing marine organisms in certain regions to become increasingly susceptible to disease. At local scales, overexploitation of reef species and pollution both have dramatic effects on reefs, though the latter impacts (including both sedimentation and sewage releases) may be especially exacerbated near human settlements; the effects from overfish-ing may be more widespread, given a well-developed commercial fleet and expanding economic demand. Certain industries, such as tourism, may also affect reefs in multiple ways—for example, by causing more pollution from coastal development, by driving up the seafood demand in restaurants, and through physical impact on reefs by poorly trained snorkelers and boaters.

Given this onslaught of diverse threats, what conservation efforts can help to protect these valuable but fragile ecosystems? Apart from critical, long-term efforts to stabilize human populations and achieve reductions and greater equity in our consumption of resources, coral reef conservation efforts have focused on several core areas, including better education of the public and decision makers about the importance of coral reef ecosystems, and ways to increase ownership of reef resources and responsibility for their management.

Although many traditional societies formerly had extensive rules governing the sustainable use of coral reef resources, modern societies have generally treated their marine resources as relatively open and readily exploitable (or degradable) by anyone. Since such open access frequently leads to a race to exploit and take advantage of resources before others can, the establishment (or reestablishment) of various legal controls for ownership, stewardship, or other modes of responsibility are essential for coral reef conservation. Such responsibilities need to be allocated not just among coastal communities but also between coastal and inland jurisdictions, in order to prevent upstream activities (for example, careless deforestation practices) from having severe downstream effects on coral reefs.

With ownership and a shared value of stewardship for future generations in place, the tendency for short-term benefits to outweigh long-term benefits is reduced, allowing groups to balance opportunities for present and future resource use. Indeed, abundant evidence shows that, in cases where communities are relatively cohesive and heavily dependent on their natural resources, these groups can effectively and sustainably manage their resources if allowed to do so. Other communities may require alternative methods of cooperative resource management, comanagement between communities and national authorities, or even exclusive management by the latter.

Because of the ease of overexploiting many marine species, better stewardship requires some limitation of fishing practices. Such restrictions may affect the number and identity of people allowed to enter the fishery, the types of fishing gear or technologies that can be used, the species that can be caught, the numbers that can be caught, the time periods when they can be caught, or the places that they can be caught. In many locations, these options are tailored in species-specific ways that require substantial knowledge about the natural history and local ecology of each exploited species, as well as intensive monitoring and enforcement. Alternatively, some of these rules can be implemented in broader forms that simultaneously address the needs of multiple interacting species and their habitats.

The use of marine protected areas (MPAs), especially marine reserves that restrict the exploitation of all species, provides one example of an "ecosystem management" approach. MPAs have proliferated in recent decades, and many have suggested that marine reserves are particularly appropriate for coral reef ecosystems. By protecting habitats and providing refuges to populations that are particularly vulnerable to overexploitation, marine reserves can rebuild overexploited populations and enhance surrounding fisheries in relatively short periods of time (for example, two to five years). Coral reef MPAs also provide sites for economic development of nonconsumptive uses of protected marine resources, such as dive tourism. Revenues from tourism in areas with large fish populations and healthy reefs frequently surpass those from nearby fisheries. Of course, for tourism to be part of an effective conservation strategy, part of the revenues from tourism must be directed to the management of the resources, and potential negative impacts of the tourism need to be monitored and strategically managed.

The suite of conservation approaches for coral reefs also includes mariculture—the farming of marine life for food, the aquarium trade, and for reef restocking—as well as other forms of reef restoration. These areas, despite various scientific controversies and uncertainties, warrant further research. Because of ongoing losses to coral reef ecosystems, successful conservation will eventually require not only approaches for preventing further degradation (for example, marine reserves) but also techniques for active and widespread recovery of degraded ecosystems.

Finally, more effective communication is needed between reef scientists, conservationists, the public, and decision-makers. Public awareness of the ongoing crisis in coral reef ecosystems remains low, hampering the ability of well-intentioned decision-makers to make positive changes to conservation policies. More effective cross-sectoral attention to reef conservation will likely only occur after enough people become aware of the ecological, cultural, and economic importance of coral reefs, their current downward trajectories, and the cost-effectiveness of active management of these resources. As awareness of coral reef issues increases, other methods of conservation, such as consumer choice in seafood, aquarium fish, and ecotourist destinations, will likely become more viable and effective. Although environmental interests are too often latent within the larger public, market-based conservation tools—assisted by "ecolabeling" certification programs—offer immense potential in harnessing and directing these interests once they manage to cross the critical threshold of public attention. Such private sector approaches to conservation will become increasingly important complements to local and national regulation as economic globalization continues to expand this century.

—Daniel R. Brumbaugh

See also: Adaptation; Benthos; Carbon Cycle;

Cnidarians (Sea Anemones, Corals, and Jellyfish);

Coloniality; Communities; Ecosystems; Extinctions,

Direct Causes of; Food Webs and Food Pyramids;

Lagoons; Protoctists


Birkeland, Charles, ed. 1996. Life and Death of Coral

Reefs. New York: Chapman and Hall; Kleypas, Joan

A., Robert W. Buddemeier, and Jean-Pierre Gattuso.

2001. "The Future of Coral Reefs in an Age of Global

Change." International Journal of Earth Science 90:

426-437; Porter, James W., and Jennifer I. Tougas. 2001. "Reef Ecosystems: Threats to Their Biodiversity." In Encyclopedia of Biodiversity, vol. 5, edited by Simon Levin. pp.73-75. New York: Academic; Reaka-Kudla, Marjorie L. 1997. "The Global Biodiversity of Coral Reefs: A Comparison with Rainforests." In Biodiversity II: Understanding and Protecting Our Natural Resources, edited by Marjorie L. Reaka-Kudla, Don L. Wilson, and Edward O. Wilson, pp. 83-108. Washington, DC: Joseph Henry/National Academy Press; Spaulding, Mark D., Corinna Ravilious, and Edmund P. Green. 2001. World Atlas of Coral Reefs. Berkeley: University of California Press.

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