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Coral reef stresses


By Charles Sheppard

Coral reefs are the most diverse ecosystem in the sea. In some ways they are very robust marine ecosystems, but in other ways, perhaps because of their huge numbers of species, they are very delicate and susceptible to being damaged or killed. On the one hand, healthy reefs are glorious riots of life, and marine scientists have spent several decades unravelling the complicated ways in which they work. On the other hand, at least one third of the world’s reefs have already died — gone for ever in terms of human lifetimes at least — even when the cause of their demise is lifted.

How coral reefs lived and grew right up to the sea surface remained a mystery for years for several reasons. First, where were all their plants? It was known that plants are the world’s food base, yet there were hardly any visible plants, let alone waving fields of them such as the naturalists knew about from their own (mostly cold) Northern shores. The answer is that the main plant base comes from the symbiotic algae living in the cells of reef building corals. This helped answer the second mystery: how could such vibrant reefs live in the nutrient-poor oceans of the tropics? Nutrients, it was known, were needed for plants to grow. But the waters that bathe oceanic reefs in particular were the poorest on Earth in terms of nutrients. The answer was clear once the symbiosis was discovered; there is a very tight cycling of nutrients between the symbiotic components of the coral-algal symbiosis and little ‘leakage’ from the reef into the sea.

There was a third, long running mystery also, namely, how do reefs form? In particular, why do they invariably grow to the surface of the sea from a wide range of depths and, why do they all have rather similar shapes? This was explained in several ways. Firstly it became clear that the Earth’s crust moves, both across oceanic distances over huge time periods, and they move vertically by hundreds of metres. Corals need light (because of their symbiotic algae) so they only live at the contemporary sea level, and the sea level changed hugely over the millennia that corals have lived and made their limestone skeletons. Darwin was the first to deduce this, in particular the importance of growth on subsiding substrates such as volcanoes.


The numerous shapes and kinds of corals, soft corals, and sponges (and many other forms) live together in what has been called a ‘super-symbiosis’ or a ‘super-organism’, terms which, while strictly not true, do give a sense of the intimate linkages that occur between so many of the component groups of species. This may provide one reason why they are, in so many ways, very susceptible to human impacts today. Raised nutrients (e.g. from sewage and shoreline construction) are hugely damaging. Burial of reefs for building on are also fatal to the reefs of course, and, sadly land made by landfill on a reef foundation (something easy to do because reefs are shallow) has a higher economic price when sold for building than the reef did in the first place. Shallow sea and reefs, we might say, become more valuable when they are no longer sea but are converted to expensive, sea-side building land! Eco-nomics and eco-logy have the same root word and should work hand-in-hand, but clearly they don’t, to the detriment of these complex living systems.

Reefs are valuable – but to whom? Reef and beach based tourism forms over half the foreign exchange earnings for many countries. Without reefs to attract tourists, many states would become impoverished; many already are. More importantly (again to whom?) they provide food for huge numbers of coastal dwellers throughout the poorest parts of the world. Not only do fish form the basis for human existence, but so do molluscs, sea cucumbers, octopus… the list is endless. Too many people extracting food from a reef readily collapses the elaborate ecosystem, with the result that there is nothing left for the next year, or the next generation.

Various aspects of climate change are adding to the mix of stresses for reefs. As CO2 builds up, it warms the oceans, and this has killed off countless areas of reef already or at least added an additional stress. When that gas dissolves in the ocean, the water becomes more acidic, again causing damaging effects, in this case reducing the ability to lay down their limestone skeletons. These are not predicted effects – something for the future. We measure it and know that we are already well along that path.

Coral reefs are a canary in the environmental coal mine, showing us, before any other system can perhaps, what we are doing to the earth today. We know enough of their science now to understand this and avert the problems. What we don’t have is the will to do so. It is no longer a problem of science but of sociology and politics.

Charles Sheppard is Professor in the School of Life Sciences at the University of Warwick. His research focuses mainly on community ecology, particularly on ecosystem responses to climate change. He works for a number of UN, Governmental, and aid agencies to advise on topical marine and costal developmental issues. He is the author or editor of 10 books, including The Biology of Coral Reefs (2009) and Coral Reefs: A Very Short Introduction (2014).

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Image credit: Iolanda reef in Ras Muhammad nature park (Sinai, Egypt), By Mikhail Rogov, CC-BY-SA-3.0 via Wikimedia Commons

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