Coral reefs around the world are threatened by an onslaught of human activities. These include the coastal development, marine-based pollution, overexploitation and inland pollution and erosion.
Effects of coastal development
The growth of coastal cities and towns generates a range of threats to nearby coral reefs. Where space is limited, airports and other construction projects are built upon reef communities. Dredging of harbors and shipping channels and the dumping of spoils result in the outright destruction of these habitats. In many areas, coral ecosystems are mined for construction materials – sand and limestone, which is made into cement – for new buildings.
The indirect effects of development are the most damaging. Reef-building corals – specifically the algae (zooxanthellae) within their coral polyps, which generate energy through photosynthesis – require sunlit waters to survive. Algal blooms resulting from excess nutrients that come from sewage releases and other sources block sunlight, reducing coral growth. Shoreline construction and modification disturbs sediments, which smother corals. Nutrient-rich runoff promotes the growth of bottom-dwelling algal competitors and interferes with coral reproduction. Other threats include hot-water discharges from power plants, and mine runoff and industrial toxic waste effluents, which poison reef communities.
Even tourism, where it is unregulated, can pose a threat. Swimmers and divers in the Gulf of Aqaba (bounded by Saudi Arabia, Jordan, Israel, and Egypt), for example, have destroyed corals through trampling, while boat anchors create further damage to some areas. In many other places, hotels and resorts discharge sewage directly into the ocean, polluting reef waters and promoting algal growth. Demand for food fish and tourism curios results in overfishing of key reef species.
Overexploitation and destructive fishing practices:
Although measured together in the Reefs at Risk indicator, overexploitation and destructive fishing can be separated into two types of threats.
Overexploitation affects the vast majority of the world’s reefs. At a minimum, overfishing results in shifts in fish size, abundance, and species composition within reef communities. Evidence suggests that removal of key herbivore and predator species may ultimately affect large-scale ecosystem changes. For example, removal of triggerfish has been linked with explosions in burrowing urchin populations, their prey, who subsequently accelerate reef erosion through feeding activities.
In the Caribbean, decades of overfishing has led, in many places, to very low levels of grazing fish species. Because of this, herbivorous sea urchins (a nonburrowing species) have played an increasingly important role in keeping down algal growth. In the early 1980s, huge numbers of these urchins succumbed to disease. Without grazing fish or urchin populations, and spurred on in many areas by organic pollution, algae quickly dominated the reefs, inhibiting coral settlement and sometimes overgrowing living corals. In areas such as Jamaica, hurricanes further compounded the damage, reducing coral to rubble. Formerly thriving reefs were replaced by low-diversity and low-productivity algal systems. Some scientists claim this is a harbinger of events to come, as reefs around the world continue to be overfished. Others argue that these major ecosystem effects may be reversible in the short term, if degradation has not gone too far.
Destructive fishing practices include blast fishing; fishing with cyanide and other poisonous chemicals; muro-ami netting (pounding reefs with weighted bags to scare fish out of crevices); and in deeper waters, trawling directly damage corals. Because these methods are generally nonselective, large numbers of other species, along with undersized target species, may be swept up in nets or killed by poisons or explosives in the process. As not all fishing methods are destructive, this is less of a widespread threat than overexploitation.
Impact from inland pollution and erosion
Sediment, pesticides, and pollution from human activities in land can damage coral reefs when transported by rivers into coastal waters. These result in the smothering of corals, reduced light levels (affecting growth), and overnutrification of reef communities. Pollution is a particular threat to coral reefs near the mouths of small and medium-sized watersheds, as the high volume of freshwater flow and sediments carried by major rivers naturally inhibits coral growth. Land clearing can expand the extent of this no-growth zone. Watersheds cleared of their forests and other vegetation cover are vulnerable to erosion and flooding. During high water periods, silt and pollutants within these basins are carried far beyond the normal “plume,” or the area where coral reef growth would normally be limited by river discharges, had they been intact.
In comparison to the other stresses, oil spills and the deliberate discharge of oily ballast water by passing ships pose an unknown, but probably less significant, threat to coral reefs. Studies on the impact of some 8-9 million barrels of oil discharged into the Arabian Gulf during the Iran-Iraq and Gulf Wars found that spills appeared to be related to short-term declines in many fish and other species. In 1986, a major spill off the mouth of the Panama Canal was linked by scientists to significant losses of coral diversity and cover in heavily affected areas. In the longer term, oil spills may leave reef communities more vulnerable to other types of disturbances.
Other causes of coral reef declines
In many cases it is difficult to pinpoint the exact causes of coral reef declines now occurring around the world. Scientists believe that degradation frequently occurs through the interaction of a combination of human-caused factors, which then leaves reef communities less resistant to periodic natural disturbances. Disease, temperature extremes, pest outbreaks, tropical cyclones, and other natural events periodically devastate corals, with resulting ecosystem-wide repercussions. However, healthy reefs are resilient, and will recover with time. The impact of multiple stressors, both natural and human caused, can have a multiplicative effect on reef ecosystems. Evidence, much of it anecdotal, suggests that human-damaged reefs may be more vulnerable to some types of natural disturbances and take longer to recover. For example, some experts believe pollution contributed to the recent die-offs of Florida Key reefs in the United States from white pox disease.
Even where they are not directly affected by human activity, coral reefs may be threatened by the degradation of nearby mangroves, seagrass beds, and other associated habitats, which serve as nurseries for many reef species. In addition, mangroves play an important role in filtering out sediments washed into coastal areas from upstream runoff.In many parts of the world, mangroves are being hacked away for fuel wood, creation of aquaculture ponds, and to make room for coastal development.
Potential impacts of global climate change
One other long-term threat is global climate change. Current models predict that climate change will elevate sea surface temperatures in many places, cause sea levels to rise, and result in greater frequency and intensity of storms. Although regional and local patterns in these changes are harder to model, the effects on coral reefs are likely to include greater physical damage by storms and more frequent instances of coral bleaching. This increase in “natural” stress levels will leave coral reefs in many parts of the world more vulnerable to human disturbances.
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13. Brown, “Disturbances to Reefs,” 373-4.
14. Brown, “Disturbances to Reefs,” p. 376.
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18. Wilkinson and Buddemeier, Global Climate Change, 40, 103.