Many of the factors that make corals unique and valuable also contribute to their decline (Weber, 1993). Anthropogenic activities such as overfishing, pollution, excessive or uncontrolled tourism, coral mining and the collection of reef organisms for the curio trade are detrimental to coral reef ecosystems. Corals can usually recover over time from natural disturbances, however over exposure to human induced disturbances may cause corals to become less resilient and take longer to recover from natural disturbances such as bleaching, for example (Brown, 1997). Loss of resilience can cause system flips, when an ecosystem changes so extremely it enters a new stability domain, (Holling et al., 1995) for example, the decline of coral reefs can lead to the invasion of non reef-building organisms such as algae, which changes the ecosystem from being coral dominated to being macroalgae dominated. This usually causes a decrease in species and functional diversity (Done, 1992). However, these changes are usually reversible.
Overfishing is one of the most significant threats to coral reef ecosystems and trawling has been the cause of widespread damage to coral reef communities on tropical shelves (McManus, 1997). With human populations increasing, great demands have been placed upon coral reefs as a food source. Overfishing can cause major direct and indirect effects on the community structure of fishes and other organisms and it can reduce species diversity and sometimes lead to local extinctions of target species and also other species (Roberts, 1995).
The mining of coral rock from certain coral reefs can have negative effects not just on the coral reef ecosystem itself but on the surrounding area, for example, a study by Brown & Dunne (1988) estimated that, at the current rate of consumption of coral rock in the Maldives, the supply of living coral rock from inner atoll ‘faros’ in North Male will be exhausted within a few decades. This living coral rock protects the islands against erosion from storms and wave damage and without it, huge land loss may occur, not to mention the direct affects to the human populations living in these surrounding areas.
The use of fertilisers and their runoff into coastal waters can also have negative effects on coral reef ecosystems. As a result of this runoff, corals are becoming exposed to increasing amounts of nutrients and pollutants. Studies have shown that sedimentation and nutrient enrichment can cause serious degradation of coral reefs at local scales (Fabricius, 2005). Coral reefs are extremely sensitive to changes in their environmental conditions; they survive in nutrient poor waters so increases in nutrient richness could be detrimental to their growth and survival (Pastorok & Bilyard, 1985). Eutrophication as a result of this runoff can promote algal growth and lead to the replacement of coral communities with macroalgae (Bell, 1992).
Tourism and the collection of reef organisms
Excessive diving, snorkelling or boat anchoring near corals can physically damage them and cause them to become less resilient to other disturbances; a damaged coral is less likely to recover from a bleaching event, for example. Also, some reef organisms are often collected either for marine aquariums or for the curio trade (to be sold as souvenirs or jewellery). This if uncontrolled, can result in major declines in biodiversity and species richness, causing changes in the delicate relationships between reef organisms.