By George Stoyle

The concept of ecological disturbance is one which is possibly more applicable to coral reefs now than any other ecosystem. Coral reefs have been exposed to various disturbances for millions of years - various environmental phenomena have occurred throughout history. Over time, coral reefs have adapted to such disturances becoming necessary to maintain biodiversity through processes of reorganisation and regeneration. The maintainence of high levels of biodiversity promotes what is known as ecosystem resilience and resistance. Resilience describes an ecosystem's ability to recover, or bounce back from a disturbance, while resistance describes the ability of an ecosystem to withstand change. Problems begin to occur when there is either too little or too much disturbance. Too little allows competitively superior species to dominat, whereas to much allows only tolerant species to dominate. An intermediate level of disturbance is therefore required. In the past intermediate disturbances have come from occassional natural processes, such as hurricanes, tsunamis, El Nino and sea-surface temperature rises. However, now we have various persistent human disturbances including overfishing, pollution, excessive sedimentation and eutrophication. This combination of natural and human disturbances has reduced the resilience of many coral reefs to the extent where there is limited ability to recover. Resistance is also lowered opening up potential for mass mortalities of coral organisms as a result of disease and bleaching. Once corals are dead the available free space is then rapidly colonised by numerous faster growing benthic species, such as macroalgae, sponges and tunicates.