Threats To Coral Reefs – Natural Impacts

Coral Reefs—Part 3

Coral Reefs are suffering from natural and anthropogenic threats. Anthropogenic means human influence or human impact. Many anthropogenic influences are resulting in the degradation and destruction of coral reefs causing loss of biodiversity, essential food supplies and economic revenue. Combined with threats from nature in the form of storms, typhoons and diseases, coral reefs are struggling to survive. Natural stressors are made worse by human disturbances. For instance, the presence of disease may be higher in corals stressed by human impacts such as mechanical damage and pollution.

Natural Impacts

Storms and Earthquakes

Disasters such as storms and earthquakes occur naturally and periodically and devastate large areas of reefs. These natural events are more severe if reef communities are already weakened by other impacts and recovery is inhibited by algal overgrowth due to the lack of grazing organisms, removed by fishing.

Water temperature rise and coral bleaching

Coral Bleaching. © NameUnusual climatic patterns can result in stress to coral reefs. The dramatic effects of El Nino have raised concern over the effect of climate change on corals. Climate change and heated water released from coastal developments can lead to warm waters and change local ecological conditions. An increase in the sea temperature can cause the phenomenon known as coral bleaching where the corals, stressed by the temperature change, expel their algal symbionts and turn bright white.

Other causes of bleaching are changes in nutrient levels and salinity, extreme low tides and increased UV radiation. Corals may recover but are generally presumed to be weakened by such an incident. Death, if it does occur, may be largely attributed to starvation, although it is thought that some autolysis (tissue destruction) occurs. The physiological mechanisms involved with bleaching are not fully understood and are currently a source of investigation. It has been hypothesised that the algae are expelled to make way for the potential repopulating of the coral by more stress resistant algae.

Crown-of-Thorns (CoTs)

Recent increases in the populations of the coral eating Crown-of-Thorns sea star have posted another natural threat to reefs. When present in huge numbers, these animals are able to devastate huge areas of reef. Recovery of the coral from these outbreaks may take as long as 20-40 years, where damage is not severe. However, recovery in some parts of the world may never happen as the coral is being taken over by algal cover and other coral species. CoTs can have several million babies in a year. We have contributed to their increase through over harvesting their natural predator the Triton Trumpet (Davui) and through nutrients from sewage etc. By giving baby CoTs more food (seaweed) when they are young, they are more likely to survive to become the destructive adults we know today.


Corals under stress often suffer from bacterial infections due to excess production of protective mucus. Excessive mucus production resulting from natural and man made influences (e.g. increased sedimentation, toxic chemicals) may also enhance the number of blue green algae thought to be responsible for black band disease, which is seen as dense band of filaments across the coral colony. Coral polyps are killed as the band advances leaving only white limestone behind.

This is part 3 of 4 in the Coral Reefs series.