An important part of the marine ecosystem, coral reefs provide food and protection for fish and other aquatic species, yet around the world these beautiful areas of biodiversity are under threat from a variety of factors. The Great Barrier Reef, the world’s most iconic reef and a UNESCO World Heritage Site, has not been spared, having lost half of its coral cover in the last 27 years.
Recent results from a study conducted by the Australian Institute of Marine Science (AIMS) and published in the Proceedings of the National Academy of Sciences journal provide compelling evidence that the devastating declines in the Great Barrier Reef are a result of the cumulative impacts of tropical cyclones, coral bleaching and population explosions of crown of thorns starfish.
The observations of the decline in the Great Barrier Reef are based on the results of more than 2,000 surveys of 214 reefs conducted between 1985 and 2012, which show that coral cover on the reef has declined from 28% to 13.8%, amounting to a total loss of 50.7% in just 27 years. The study has also revealed that the rate of decline has increased in recent years, with two-thirds of the coral loss occurring since 1998.
“If the trend continues, coral cover could halve again by 2022,” said Peter Doherty, a research fellow at AIMS. This is a worrying statistic for a reef system which is classified as the world’s least threatened due to the strong legal protection it is afforded, and does not bode well for other more threatened coral reefs worldwide.
The study found that while 48% of coral loss on the Great Barrier Reef could be attributed to tropical cyclones, a worrying 42% could be linked to the crown of thorns outbreaks, with just 10% of coral loss resulting from coral bleaching.
The world’s second largest sea star, the crown of thorns starfish (Acanthaster planci) is a renowned predator of coral, and sadly there have been several outbreaks of this species in the Great Barrier Reef in the last few decades. Such outbreaks used to be a relatively rare occurrence, only happening once every 50 to 80 years. However, population explosions of these coral-eating invertebrates have increased dramatically to about once every 15 years, which may be linked to increased fertiliser and chemical run-off from the mainland after major floods.
Controlling the crown
“We can’t stop the storms, and ocean warming (the primary cause of coral bleaching) is one of the critical impacts of the global climate change. However, we can act to reduce the impact of crown of thorns,” says John Gunn, the chief executive of AIMS.
Scientists predict that the reef could recover if the crown of thorns starfish, which was first noted as a problem in 1962, can be brought under control. “When we say outbreaks, we mean explosions of populations to a level where the numbers are so large that they end up eating upwards of 90% of a reef’s coral,” said Mr Gunn.
Buying some time
The study has shown that coral cover could increase at a rate of 0.89% per year in the absence of the crown of thorns, a statistic which increases to 2.85% with the additional absence of cyclones and bleaching. As a result of this, direct action to remove the crown of thorns starfish from the area has been recommended in order to buy the Great Barrier Reef some time.
Control of crown of thorns starfish populations would involve improving the water quality of rivers running into the reef, to reduce the levels of nutrients flowing in from agricultural run-off. High levels of such nutrients in the reef feed crown of thorns starfish larvae, enabling them to thrive. Biological control of the crown of thorns starfish populations has also been suggested, but despite promising results from research on such a method, it is not yet ready to be applied in the field.
More to be done
However, controlling populations of the crown of thorns starfish will not be enough to halt coral loss entirely. Ultimately, climate change will need to be dealt with to ensure that the Great Barrier Reef, along with other coral reefs across the globe, has a decent chance of survival.
David Curnick, Marine and Freshwater Programme Co-ordinator at the Zoological Society of London (ZSL), highlighted the part we all must play in saving the Great Barrier Reef, “We can achieve better water quality, we can tackle the challenge of crown-of-thorns, and we can continue to work to ensure the resilience of the reef to climate change is enhanced. However, its future also lies with the global response to reducing carbon dioxide emissions. The coral decline revealed by this study – shocking as it is – has happened before the most severe impacts of ocean warming and acidification associated with climate change have kicked in, so we undoubtedly have more challenges ahead.”
Learn more about the crown of thorns starfish on ARKive.
Kathryn Pintus, ARKive Text Author