Threats to Coral Reefs

Brian Joseph English

Copyright 2000

 Table of Contents

Introduction. 1

Natural Threats to Corals. 1

Predators and Diseases. 1

Natural Environmental Threats. 2

Anthropogenic Threats to Coral Communities. 3

Sedimentation. 3

Anthropogenic Causal links to Predator Population Explosions. 3

Conclusion. 4

References. 4



Although corals evolved more than 450 million years ago and most coral reefs now are between 5,000 and 10,000 years old, there is still a growing international concern about the widespread global degradation of coral reefs and their related ecosystems (Hinrichsen, 1997; International Coral Reef Initiative, 1995). Overuse and abuse of coral reefs are increasing exponentially because the human population is increasing exponentially (Jackson, 1995). Although some natural phenomena such as earthquakes, typhoons, climate changes, coral eating predators and plagues may cause threats to marine ecosystems, human activity accounts for the majority of degradation to coral reefs. Siltation, pollution, poor coastal planning and inappropriate fishing techniques are some of the ways that humans threaten marine estuaries and ecosystems. This paper will first explain the severity of various natural and human factors that threaten coral communities, and then suggest plausible action to countervail those threats.

The first section of the paper will discuss natural threats to corals beginning with immediate predators and diseases living in marine environments and ending with larger scale natural threats that may result from changes in the environment. The second section of the paper will address the human threat to coral reefs beginning with direct degradation of reefs and ending with causal analysis of possible indirect threats to marine coastal environments.

Natural Threats to Corals

Predators and Diseases

Corals are living animals so it logically follows that they are part of the food chain. In a healthy environment corals can be expected to grow and multiply in spite of the presence of predators. Equilibrium relationships between prey and predator may continue for a number of years and suddenly be interrupted by a population explosion. Increases in the population of a coral predator can have a devastating impact on the coral colony. One such predator that periodically reaches plague numbers is the crown-of-thorns starfish. Numbers of these coral digesting starfish can reach into the hundreds of thousands, yet there is only speculation as to what may cause the outbreaks. The normal-event hypothesis suggests that environmental factors such as temperature, salinity and availability of food are causal variables in outbreaks. There is speculation that in years when conditions are favorable due to natural fluctuation in atmospheric and bio-spheric conditions, more planktotrophic larvae will survive to settle on reefs setting the stage for a population explosion (Endean and Cameron, 1990). The second section of this paper will discuss two anthropogenic feedback scenarios that may account for the outbreaks.

Although out breaks of coral eating predators can kill off large portions of a colony, it is possible for the colony will recover in less than 15 years. Colgan (1987) did a longitudinal study at Tanguisson Reef in Guam, which suffered an outbreak of coral eating starfish in the late 1960s. Colgan found that the reef had almost fully recovered within 12 years. However, Colgan reasons that since, in this case, the starfish did not destroy the structural integrity of the reef, the recovery was quicker than had been predicted. Endean and Cameron (1990) give several accounts of, and provide a detailed discussion on outbreaks of coral eating starfish

Corals are prey to other corallivores like parrotfish and urchins, but perhaps the deadliest enemies are the smallest. Bacterial damage to corals is often associated with "coral bleaching" which is a condition that causes corals to lose their coloring. Since corals get their color from the zooxanthellae algae that symbiotically live inside the coral, disruption to the symbiotic relationship can result in a bleaching effect. Richardson, (1998) has documented evidence of the spread of a deadly bacterium from the coastal Florida waters through parts of the Caribbean. Despite identifying the disease, the researcher and her team were unable to determine the origin of the disease or the reasons for its rapid spreading. Similar bacterial attacks have been documented from the Arabian Gulf to the Indian Ocean (Coles, 1994). The various strands of this disease are referred to as Black Band, White Band and Yellow Band.

Red tide is often documented as a threat to coral communities (Grigg and Dollar, 1990; Thia-Eng, 1998). Red tide are just one type of algae that can negatively impact coral growth and reproduction. Algae blooms can block sunlight which both corals and their symbiotic partners, zooxanthellae (the "good algae") need to survive. Algae blooms also reduce oxygen levels in the water and some species release toxins that can also harm corals and zooxanthellae. There is still speculation on what causes algae blooms, but one explanation is similar to that for starfish population explosions. When conditions are optimal there are algae blooms. Both natural and anthropogenic eutrophication are probable variables in producing optimal conditions for algae blooms.

Natural Environmental Threats

Since coral communities are an intricate part of coastal environments they are subject to the awesome forces of nature. Grigg and Dollar (1990) site a number of studies that document major disturbances and mortality of coral communities caused by low temperatures, storms, major El Nino seasonal changes, low tides, and volcanic eruptions earthquakes.

Although coral bleaching can be caused by the presence of parasitic bacteria feeding on the zooxanthellae algae living in the coral pulp, Kushmaro et al., (1996) acknowledge that disruptions to the symbiotic relationship with zooxanthellae could also result from increased seawater temperature, ultraviolet radiation and climate change. Gates (1990) studied the effects of seawater on corals in Jamaica. Gates found much of the bleaching effects to be seasonal and not always lethal to the coral. If minor seasonal fluctuations in temperature can cause sublethal coral bleaching, does more severe seawater temperature changes cause lethal bleaching?

Coral reefs may also be damaged by storms and hurricanes. Although the effect is usually minimal, severe storms may do visible damage to reef systems. Blair, McIntosh and Mostkoff (1994) documented the effects of hurricane Andrew on soft and hard coral systems in southern Florida. They concluded that the hurricane affected the benthic cover of algal and soft coral communities were heavily impacted, whereas the benthic cover of hard corals was slightly to moderately impacted.

Research in Australia (Lough, 1994) and throughout the Pacific (Glynn, 1988) describes how the El Nino-Southern oscillation event is responsible for aperiodic influences on corals. The El Nino phenomenon causes changes in surface temperature, wind fields and rainfall. Increased rainfall may be the strongest of these three variables because it leads to increased river flow and run off. Still, Lough did not find any major damage or change in the structure or health of the corals in the Great Barrier Reef due to the El Nino phenomenon, but he acknowledges the need to consider high variability in the physical environment when examining changes in reef processes due to global change or human degradation.

Because corals need salt water to live, low tides that expose surface coral can result in bleaching and death of near surface reef corals. Similarly, in regions where there are heavy seasonal rains, fresh water runoff from mountains and rivers can reduce the salinity of the water effecting the health of corals. Certainly, this type of seasonal stress to corals is not as detrimental as more catastrophic occurrences such as earthquakes and volcanoes that destroy the structural integrity of reefs over vast areas. In the long term, the impact that naturally occurring periodic changes in climate and ecosystems have on corals is dependent on the magnitude and return period (Grigg and Dollar, 1990). Grigg and Dollar analyzed species composition and community structure of coral reefs in the Hawaiian archipelago to develop a theoretical model of succession that suggests "differences between reef communities in comparable habitats off different islands are due largely to differences in successional age" (p.441). If the recovery time is less than the return period of severe disturbances, then reefs are usually high cover, dominated by one or only a few species, and have low diversity. Conversely, reef systems that have frequent disturbances (i.e., seasonal storms, high waves, or periodic low tides) are characterized as having "low cover, high diversity, high equitability" (p. 441).

Anthropogenic Threats to Coral Communities

Corals have an natural ability to recover from natural "disturbances"; however, the human ‘disturbances to coral communities can impact marine environments more severely (Lindahl, 1998, p.645) According to the National Oceanic and Atmospheric Adminiasration (NOAA):

The most serious anthropogenic stresses include: sedimentation caused by poor land-use practices; pollution and over-nutrification from domestic, agricultural, and industrial waste; physical alteration of coral reefs during coastal construction projects; destructive fishing practices such as poison and blast fishing; ship groundings; and coastal tourism, which brings millions of eager divers and snorkellers to coral-rich areas of the world each year. (NOAA, 1997)

These variables and there causal links to coral degradation will be discussed in this section.


According to Grigg and Dollar (1990) sedimentation poses the most common and the most serious anthropogenic threat to corals. Sedimentation is often linked to expanding human populations. More people translates into more buildings. Construction of buildings may involve products made from corals such as cement or limestone. Coral mining not only destroys the corals being mined, but also disturbs the greater marine eco-system by clouding the water with sediment and smothering corals. Construction near the shore makes muddy also creates muddy estuaries and lagoons. Inland, too, certain practices can add to the increase in sedimentation. Clear-cut logging, deforestation, inappropriate farming methods, mining for ore and removing vegetation from banks can greatly increase the amount of river run-off (Grigg & Dollar, 1990; McAllister & Ansula, 1993; Berg et al, 1998).

Corals are able to withstand some levels of sedimentation because in the natural environment sediment particles are suspended in seawater. Griggs and Dollar (1990) assert that most coral species can withstand only a low level of sediment supply to the living surface. Corals have a natural sediment-rejection capacity. "Many species have the ability to remove sediment from their tissues by distension of the coenosarc with water, or by ciliary action which can nullify lethal effects of sedimentation" (Yonge as cited in Grigg and Dollar, 1990; p. 443). Although corals can naturally "clean" some of the sediments from their system, corals are unable to deal with major sedimentation caused by either cataclysmic natural disaster or by massive anthropogenic disturbances.

Much needs to be done to circumvent the impact that human-origin sedimentation has on corals. One idea includes sewage abatement processes. If sewer outfalls are placed at depths below reef growth, there may be less effect on the reef. Still, the best way to avoid destroying coral reefs may be to educate people on how to reduce sedimentation in marine eco-systems.

Anthropogenic Causal links to Predator Population Explosions

As mentioned earlier, the crown-of-thorns starfish has periodic outbreaks that increase its predacious effect on corals. Although there is little current evidence to support the role of humans in these outbreaks, McAllister and Ansula (1993) suggest that elimination of crown-of-thorns’ natural enemies through over-collecting of their shells may be one factor. With fewer natural enemies, the equilibrium is upset and the starfish are able to multiple until the reduction of their own food supplies force declines in population. They also describe the plausibility that over-fertilizing is a causal factor in increasing numbers of these starfish, "Fertilizers from farms, sewage or other sources wash on to coral reefs and cause blooms of tiny plants called phytoplankton. The survival of the larvae, ... is better on this rich source of food, and more grow up to be an adult." (p.51)

Some of the diseases that attack corals might also have connections to human behavior. Richardson (1998) claims the Black Band disease that destroys coral tissue originates from a group of algae that secrete toxic sulfides into the marine environment. C.R. Wilkinson (as cited in Wilhelmsson et al., 1998) suggests anthropogenic factors for the increases of algae. Pollution from oil and petrochemical industries and other human threats to the marine environment have decreased fish populations and provided conditions conducive to algae blooms. Hierta (1994) claims, ‘Excessive nutrients from sewage and agricultural runoff spur algae blooms, such as red or brown tides, that can smoother coral. Turbidity from sediment runoff blocks the sunlight needed for growth.


Although there are many natural threats to marine ecosystems, corals have evolved into animals that are able recover from environmental and predatorily threats. Many of the established coral reefs in the world today have been thriving and surviving for thousands of years. Despite the natural ability to persist, many coral reef ecosystems are in poor health. The recent major declines in coral reef health can be traced to anthropogenic origins. Inappropriate fishing practices and development related causes are destroying reefs at alarming rates. Humans are currently a major threat to coral.

Ironically, it may be humans and human technology the rescues ailing coral reefs. Grigg and Dollar optimistically claim "the technology to mitigate impacts of many sources of anthropogenic stress are presently available" (p.448). They give examples of sewage abatement, deep-water disposal, containment booms and adsorbents (for oil spills).

Coral reef management at the local level may offer the best answer to anthropogenic stress. Community-based environmental education programs are becoming popular in many areas where coral reef have been damaged. As a form of non-formal education these programs aim to help local residents of coastal communities learn how to maintain a sustainable relationship with the marine environment.

One impressive example of coral reef management is in the Philippines (Bagarinao, 1998; Hinrichsen, 1997). By the mid-1980s, Apo Reef was almost totally destroyed by villagers’ inappropriate fishing practices. The use of dynamite, cyanide and destructive nets to eke out a living from the failing reef nearly destroyed the island community’s livelihood. Apo reef has made a dramatic comeback in the last decade and a half due to proactive reef management by the local community and experts from Silliman University in Dumaguete. The villagers have learned sustainable practices that are essential to maintaining healthy reefs. Hinrichsen comments on the hope that the Apo reef case provides for community-based coral reef management programs:

Apo demonstrates that it is not too late to protect these wonderfully diverse underwater ecosystems and to preserve their productivity for the people who depend on them. The model that Apo sets offers encouragement to the coral-reef nations that recently launched a new international protection plan, culminating in the designation of 1997 as the International Year of the Coral Reef. (p.14)

This paper only gives a general overview of the threats to coral reefs. Many of the anthropogenic threats have roots in the political, economic and social institutions of society. To begin to seek solutions to the human causes of coral reef degradation, it is necessary that scientists and researchers engage in interdisciplinary efforts sharing their knowledge and resources.


Berg, Hakan; Ohman, Marcus C.; Troeng, Sebastian and Linden, Olf. (1998).

Environmental economics of coral reef destruction in Sri Lanka. In Ambio; Vol. 27, no. 8, pp. 627-634.

Blair, S.M.; McIntosh, T.L.; Mostkoff, B.J. (1994). Impacts of Hurricane Andrew on the

offshore reef systems of central and northern Dade County, Florida. Bull. Mar.

Sci. 54(3): 961-973.

Coles, S. L.(1994). Extensive coral disease outbreak at Fahl Island, Gulf of Oman, Indian

Ocean. In Coral Reefs 13: 242.

Endean, R. And cameron, A.M. (1990). Acanthaster Planci Population outbreaks.  In
        Dubinsky (Ed.), Ecosystems of the World Voume 25. Amsterdamn: Elsevier.
Gates,-R.D. (1990).  Seawater temperature and sublethal coral bleaching in Jamaica. In
        Coral Reefs; vol. 8, no. 4, pp. 193-197.
Glynn, Peter W.(1988).  El Nino-Southern Oscillation 1982-83: Nearshore population,
        community, and ecosystem responses. In Annual Review of Ecology Systems.

             Vol. 19 pp. 309-45.

Grigg, R.W. and Dollar, S.J. (1990). Natural and Anthropogenic Disturbance on Coral

Reefs. In Dubinsky (Ed.), Ecosystems of the World Voume 25. Amsterdamn: Elsevier.

Hierta, Ebba. (1994). Rescuing the reef. In National Parks, Vol. 68, Nov./Dec., pp.32-7.

Hinrichsen, Donald. (1997). Requiem for Reefs. In International Wildlife. May/April

1997; pp. 12-20.

International Coral Reef Initiative. (1995). The Coral Reef Initiative. In Report of the

INCR workshop. Silliman University, Dumaguete City, The Philippines. Available: On-line at

Jackson, B.C. (1995). "Messages from keynote speakers: The role of science in coral reef

conservation and management." Available: On-line at:

Kushmaro, A.; Y. Loyola; M. Fine and E. Rosenberg. 1996. Bacterial infection and coral

bleaching. Nature 380(4): 396.

Lough, J.M. 1994. Climate variation and El Nino-Southern Oscillation events on the

Great Barrier Reef: 1958 to 1987. Coral Reefs 13(3): 181-195.

McAllister, Don E. And Ansula, Alejandro. (1993). Save Our coral Reefs. Ottawa,

Canada: IGS Printing and Lamenating.

NOAA. (1997) Corals Can’t Stand the Heat: Global Climate Change and Coral Reefs.

Available: On-line at:

Richardson, L.L., et al. Florida’s mystery coral-killer identified. Nature 392(April 9):557.

Wilhelmsson, C. R., Ohman, Marcus C., Stahl, Henrik and Shlesinger, Yechiam. (1998).

"Artificial reefs and dive tourism in Eilat, Isreal." In Ambio, Vol.27, no.8

December, 1998.