Coral Bleaching - Expelling Biodiversity

Corals are persistent, they have survived for tens of millions of years, through all the climate fluctuations of the Pleistocene and the Holocene (Stoddart 2008). You wouldn’t think they were candidates for extinction. However periods of mass coral extinctions have occurred before, up to 45% of all coral species went extinct around the Cretaceous-Tertiary boundary (Carpenter et al 2008). Corals are under threat; a major one being coral bleaching which is mainly caused by increased sea temperatures but also high solar irradiance and disease (Brown 1997). Coral reefs are among the most productive and diverse ecosystems; thousands of species coexist, meaning its vital to protect for biodiversity (Parker 2012). Coral bleaching is a whitening of corals due to loss of symbiotic algae and/or their pigments (Brown 1997). The problem of coral bleaching is far more serious than its loss of colour, it means that the home of many creatures has died and no longer provides food for marine species and therefore humans.

The region is diverse in terms of natural events. Climate change is, and will continue to, increase the frequency and intensity of these natural occurrences. These include; ENSO, tropical cyclones, floods and drought and by persistent features such as the trade winds and convergence zones (Hay et al 2002). Due to global warming, the Pacific region is warming by between 0.6 and 3.5ᵒC, a rate of warming which is much larger than the observed changes during the last century (Hay et al 2002). This will and has increased the occurrences of coral bleaching in the Pacific as corals bleach when water temperatures rise 1° to 2°C and persist over three or four weeks (Parker 2012). It is thought by some scientists that coral extinction may be coming again (Carpenter et al 2008). 

When sea temperature rises corals expel their symbionts, this causes colony death if the heat persists (Carpenter et al 2008). Although corals in the Pacific can cope in warm waters better than others due to ENSO, waters are getting warmer at a faster rate. A rate, that some scientists fear, is too quick for adaptation (Hay et al 2002). It is thought that the Pacific area will become more ‘ENSO like’, so wet areas will become wetter (increase in flooding), dry areas will become drier (increase in drought) and waters will become warmer (Hay et al 2002). Typhoons increasing in frequency and intensity will affect corals, strong typhoons mean corals are damaged for around 10 years (Stoddart 2008).

Increased damage from; natural occurrences (typhoons), sea temperature rise (coral bleaching), and over-fishing, is set to damage biodiversity in coral reefs considerably. 

Species closer to the sea surface are at a higher risk, the families that are most at risk are; Euphylliidae, Dendrophylliidae, and Acroporidae (Carpenter 2008). The species found in deeper reefs are less dependent on coral reeds as a habitat and the deeper waters are less affected by human and physical changes (Carpenter et al 2008). The proportion of threatened species is greatest in corals than terrestrial animal groups excluding amphibians (Carpenter et al 2008). Fish stocks are set to decline as coral bleaching increases, this spells out bad news when population sizes of the larger islands are set to increase (Lovell et al 2004).

The Reefs at Risk Revisted report predicts that by 2050 many Pacific reefs will bleach annually (Parker 2012).

Reference

Brown, B. (1997), ‘Coral Bleaching: Causes and Consequences’, Coral Reefs, 16, 5, 129-38 

Carpenter, K. et al (2008) ‘One-Third of Reef-Building Corals Face Elevated Extinction Risk from Climate Change and Local Impacts’, Science, 321, 560-63

Hay, J. E. (2002), Climate variability and change and sea level rise in the Pacific Islands region: A resource book for policy and decision makers, educators and other stakeholders, Tokyo, Japan: South Pacific Regional Environment Programme and Japan Ministry of the Environment

Lovell, E. H. Sykes, M. Deiye, L. Wantiez, C. Garrigue, S. Virly, J. Samuelu, A. Solofa, T. Poulasi, K. Pakoa, A. Sabetian, D. Afzal, A. Hughes and R. Sulu, (2004), ‘Status of Coral Reefs in the South West Pacific: Fiji, Nauru, New Caledonia, Samoa, Solomon Islands, Tuvalu and Vanuatu’, Australian Institute of Marine Science 2, 337-62

Parker, B. (2012) Pacific coral reef management in a changing climate in Keener, V. et al (ed.) Climate Change and Pacific Islands: Indicators and Impacts, PIRCA: Island Press, 15-17

Stoddart, D. (2008), ‘Ecology and Morphology of Recent Coral Reefs’, Biological Reviews, 44, 4, 433-98