Human threats to coral reefs (Introduced species (Non indigenous species…
Human threats to coral reefs
This is not always damaging but if the catch rate is excessive then over-fishing can reduce fish populations and affect other inter-dependent species. The Crown of Thorns Starfish is a predator of coral polyps in the Indian and Pacific oceans. They are eaten by Giant Triton molluscs. Over fishing of the Giant Triton on Australian coral reefs allowed Crown of Thorns Starfish populations to rise, leading to the destruction of large areas of coral reef by the starfish.
hard objects can kill polyps as they are pushed against the hard limestone beneath the living cells. Litter such as floating bottles, lost fishing gear, boat anchors and careless divers can all damage reefs. Damage is also caused by extraction of coral for land reclamation and construction.
this occurs when the symbiotic algae within the coral polyps are expelled. Bleached corals look pale, hence the term 'bleaching'. This can be caused by a variety of natural and anthropogenic factors. One natural factor is the raised light levels caused by increased solar activity, but many human activities can cause bleaching including:
increased water temperatures caused by global climate change
low oxygen levels due to high zooplanktpon activity caused by overfishing of their fish predators
pollution, including sewage pesticides, cyanide and sunscreen products
bleached coral polyps, which are not dead and may regain algae in the future grow very slowly and are less able to recover from damage
Non indigenous species have been introduced in several ways:
from ballast water discharged from ships
escaped fish and crustaceans from fish farms
accidental and intentional releases from aquaria
The Red Lionfish was accidentally introduced to the Caribbean in the 1990s. They are indigenous to the Indian and Pacific Oceans but have few predators in the Caribbean sea so they have spread rapidly. They are general predators and have reduced the populations of many indigenous fish species.
Loss of associated habitats
Mangroves and seagrass beds help to trap suspended sediments and reduce water turbidity. They also provide nursery grounds for many of the fish species that live as adults on coral reefs. If the mangroves and seagrass beds are damaged, then the coral reefs nearby become damaged too.
turbid water carried by rivers or sediments disturbed by coastal developments can cover and kill corals
Collection of ornaments and souvenirs
many species from coral reefs are collected to be sold as ornaments such as mollusc shells , hard corals and sea fans. Trade in about 2000 species of coral is restricted by CITES Appendix II.
The expansion of coastal towns, the construction of tourist resorts, ports, and marinas can destroy coral reefs directly and increase turbidity causing damage to reefs near the developments
a wide range of pollutants can harm coral reefs. Large oil spills can cover coral reefs. Even at lower concentrations oil is toxic to corals. Nutrients such as nitrates and phosphates from agricultural runoff and sewage effluents can stimulate growth of algae that covers corals. The shading caused by the algae reduces the growth of the symbiotic algae within the polyps and prevent the release of eggs when the corals reproduce.
Ocean acidification is caused by increased carbon dioxide concentrations in the atmosphere. More carbon dioxide dissolves in the oceans producing carbonic acid which makes the oceans more acidic it is therefore difficult for the coral polyps to produce their coral skeleton