Wednesday, October 31, 2018

Losing the rainforest of the sea: Coral reef decline and loss of future ecosystem benefits and services

*This is a guest post by Karuna Sehgal - student in my 'Causes & Consequences of Biodiversity' course. 

The past three decades of human activity has altered the earth in more ways than one. The Earth is losing species, ecosystems and biodiversity because of warming climates, among other factors. Coral reefs, in particular, are greatly impacted by the rise of global surface temperatures.

Coral Reefs throughout tropical and sub-tropical oceans are under tremendous heat stress resulting in coral bleaching and mortality. Corals are animals that live in a symbiotic relationship with microscopic dinoflagellate algae that inhabit the coral tissues (Baker et al., 2008). Increased water temperatures result in corals expelling dinoflagellates living in their tissues, causing the coral to turn white, ending its symbiotic relationship (Heron et al., 2017). This does not necessarily mean death for the coral; however bleaching still adversely impacts corals by inhibiting growth and reproduction (Heron et al., 2017). This symbiotic relationship provides the coral with about 90% of the energy it needs to thrive, it also enables corals to construct limestone skeletons that form the three-dimensional structure of reefs, which provides habitat for over a million species (Heron et al., 2017. They are referred to as the Rainforests of the Sea because they are the most bio-diverse ecosystem in the ocean, comparable to rainforests on land. Species richness and the diversity found in these systems are phenomenal and breathtaking, and yet they are dying at an alarming rate.

Fig. 1: Examples of a healthy and a bleached coral reef (images modified from Wikipedia pages on coral reefs and reef degradation, respectively)

Coral Reefs provide a lot of ecological and economically important services; they gross an estimated value of over $1 trillion (USD) globally, because of their social, economic and cultural services (Heron et al., 2017). With that being said, reefs only account for less than 0.1% of the ocean floor, but host more than one-quarter of all marine fish species (Heron et al., 2017). Climate change alters the pristine attractiveness of coral reefs to tourists, which directly affects low-income coastal countries and small developing islands within coral reef regions (Hoegh-Guldberg et al., 2007). Developing countries are not equipped to respond to climate change, and many rely on tourism for the majority of their economies (Hoegh-Guldberg et al., 2007). But tourist visits are one form of valuation, coral reefs are also critical for supporting fisheries and protecting shorelines from erosion,  For the loss of reef ecosystem services it is going to cost the US about $500 billion per year by 2100 (Hoegh-Guldberg et al., 2015).

This loss of economic value through bleaching is ultimately caused by our activities. Anthropogenic activity has resulted in rising temperatures and increases in the atmospheric concentration of carbon dioxide; this has been the largest increase in global temperature since the pre-industrial times (Stocker et al., 2013). Widespread mass coral bleaching was first documented in 1983 at the time of an extremely strong El Nino (Cofroth et al., 1989). It is important to note that coral reefs have been around a long time and residing in oceans since at least the Triassic period over 200 million years ago, and are well adapted to specific environmental conditions and human activity has damaged them in a matter of 30 years. Therefore water temperatures of even 1-2oC above the normal temperature would result in severe coral bleaching (Heron et al., 2017). It was estimated that coral reefs would take approximately 15- 25 years to recover from mass mortality, but if the frequency of mass mortality events increases to a point where the return time of mortality event is less than the time it takes to recover, the abundance of corals on reefs will decline (Heron et al., 2017).

Ocean acidification is another factor affecting coral reefs because it hinders the coral's ability to build their limestone skeletons and increases bio-erosion of reefs (Heron et al., 2017). With approximately 25% of the emitted CO2 from anthropogenic sources entering the ocean and producing carbonic acid, which then dissociates to form bicarbonate ions and protons, reducing the availability of carbonate to biological systems (Hoegh-Guldberg et al., 2007). These high CO2 levels and ocean acidification are expected to cause coral reefs to erode. A number of studies have determined that the doubling of pre-industrial atmospheric CO2 to 560 ppm decreases coral calcification and growth by up to 40% through the inhibition of aragonite formation as carbonate-ion concentrations decrease (Hoegh-Guldberg et al., 2007). Studies have concluded that the corals will not thrive again until the atmospheric CO2 has been reduced to 320-350 ppm (Heron et al., 2017).

Building the resilience of these reefs by reducing human impacts is now the main focus of organisations like the World Heritage Committee of UNESCO and the Reef Resilience Network. A World Heritage Committee analysis showed that nearly all of the 29 World Heritage coral reef sites were exposed to levels of heat stress that cause coral bleaching, more than twice per decade during the 1985-2013 period (Heron et al., 2017). Roughly 21 of the World Heritage reef properties have been exposed to repeated heat stress during the past three years (Heron et al., 2017), threatening the long-term persists of these unique and valuable places.

Fig. 2: Satellite image of coral bleaching alerts from  2014–2017 (image from NOAA Coral Reef Watch)
Bleaching and heat stress spread across tropical oceans and intensified during El Niño, and continued from La Niña and beyond (Heron et al., 2017). This period has included the three warmest years on record: 2014, 2015, and 2016 (Heron et al., 2017). Figure 2 shows that more than 70% of the global coral reef locations have experienced bleaching and most of these have experienced it twice or more, since June 2014 (Heron et al., 2017).

What is the future of these reefs? Will the next generation be able to see and explore them as we have or will they have to watch documentaries of what used to be? Coral Reefs are the most biologically diverse and economically important ecosystem on the planet, providing ecosystem services, essential to human societies and they are at danger (Hoegh-Guldberg et al., 2007).


Baker AC, Glynn PW, Riegl B (2008) Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook. Estuarine, Coastal and Shelf Science 80:435-471.
Cofroth MA, Lasker HR, Oliver JK (1989) Coral mortality outside of the eastern Pacific during 1982-83: Relationship to El Niño. In: Global Ecological Consequences of the 1982-83 El Niño-Southern Oscillation. Glynn, PW. (ed.). Elsevier.
Heron et al. 2017. Impacts of Climate Change on World Heritage Coral Reefs : A First Global Scientific Assessment. Paris, UNESCO World Heritage Centre.
Hoegh-Guldberg O, et al. (2015) Reviving the Ocean Economy: the case for action - 2015. WWF International, Gland, Switzerland.Geneva, 60p.
O. Hoegh-Guldberg, P. J. Mumby, A. J. Hooten, R. S. Steneck. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidificaition. Science, 318, 1-7. Doi: 10.1126/science.1152509
Stocker TF, et al. (2013) Climate Change 2013: The Physical Science Basis. Working Group 1 (WG1) Contribution to the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5), Cambridge University Press. 


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