Great Barrier Reef coral loss is probably our fault

The Great Barrier Reef is the largest coral reef system in the world, stretching over 2,600 kilometres along the coast of Queensland and covering an area of 344,400 square kilometres. But over the past 27 years it’s lost half its coral, apparently thanks to human activity.

Researchers from the Australian Institute of Marine Science in Townsville have been monitoring the amount of coral since 1985. Back then, the reefs they studied had 28% coral cover, but in 2012 they only had 13.8%. That’s a reduction of 50.7% (De’ath G, Fabricius KE, Sweatman H & Puotinen M 2012, “The 27–year decline of coral cover on the Great Barrier Reef and its causes”, Proceedings of the National Academy of Sciences, vol. 109, no. 44, pp. 17995-17999, doi:10.1073/pnas.1208909109).

This reduction wasn’t uniform across the whole system, as the far northern reefs have remained fairly stable at about 24% coral cover. But there’s been a decline in the central region – which they classify as between Cooktown and Mackay – and in the southern region below Mackay, where there’s been a steep drop of over 75% in the past decade alone.

The researchers also looked at what caused these reductions, by modelling possible causes against the observed fluctuations in coral cover. What they found was that 48% of the coral reduction could be attributed to tropical cyclones, 42% to outbreaks of crown-of-thorns starfish, and 10% to mass coral bleaching (primarily two events, in 1998 and 2003).

Climate change would seem to be a factor here, as it’s been linked to the increasing intensity of tropical cyclones (see Knutson TR, et al. 2010, “Tropical cyclones and climate change”, Nature Geoscience, no. 3, pp.  157–163, doi: 10.1038/ngeo779 [PDF 641 KB]).

Temperature also seems to be a major cause of coral bleaching, which is when the coral loses its symbiotic zooxanthellae. These single-celled organisms photosynthesise and provide energy for the coral polyps – as well as the vibrant colours of the coral. In turn, they get nutrients and a home. However, the zooxanthellae seem to be sensitive to temperature, as a rise of only 1°C  can cause mass deaths of them and subsequently their host coral.

But even though climate change is the biggest culprit, the researchers admit it’s unlikely that in the near future we’re going to make a big impact or reduce temperatures. So instead, they suggest concentrating on the crown-of-thorns starfish. If we could cut out just the starfish, but cyclones and bleaching continued, the coral cover would still increase by 0.89% per year.

And we have a chance of doing this, because the crown-of-thorns starfish itself is influenced by human activity.

Crown-of-thorns starfish competing to eat the last remaining piece of coral (click to embiggen)
Crown-of-thorns starfish, Acanthaster planci, competing to eat the last remaining piece of Acropora coral (Photo by JSLUCAS75, via Wikimedia Commons)
As the name suggests, the crown-of-thorns, or Acanthaster planci, is a spiny starfish, or sea star. It’s the second-largest species of sea star in the world, with adults reaching 25-35 cm in diameter and having up to 21 arms. It feeds by latching onto coral with its multiple tube feet and then extruding its stomach out through its mouth to digest coral polyps.

It sounds like a nasty, introduced species, but actually it’s been in the Great Barrier Reef for at least 8000 years. In fact, it’s found in coral reefs across the Indian and Pacific Oceans, from the coast of Africa to the coast of America.

And at normal population numbers, it seems to be a natural part of the reef ecosystem. It prefers to eat the faster-growing coral species, giving the slower-growing species a chance to compete. But occasionally the numbers increase to plague proportions, and the starfish have to eat everything.

Now it’s not 100% certain what causes these outbreaks, but the leading theory is that it’s due to water quality. The starfish larvae feed on phytoplankton, and phytoplankton numbers increase with inorganic nutrients in the water. And these inorganic nutrients increase greatly when fertiliser is washed off farmland, particularly after floods.

So the researchers recommend more effort to improve water quality in order to reduce the numbers of crown-of-thorns starfish. This is in preference to hunting them down one-by-one – which is favoured by MP Bob Katter – because in the past that’s proven to be rather expensive and labour-intensive, but overall ineffective across the whole reef (although hunting does work for protecting a small area, so it’s good for tourist operators).

In the end, it all comes down to pollution, whether greenhouse gases or fertiliser run-off. And until we can cut the former, we need to concentrate on the latter. The crown-of-thorns starfish may in fact be a natural feature of the Great Barrier Reef, but it’s our activities that turn it into a threat.

(This story aired on 22 November 2012 – you can listen to the podcast.)

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