Study: Great Barrier Reef Has Lost Half Its Coral

Over the past 27 years, the world's largest coral reef has been ravaged by starfish and climate change.

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Australia's Great Barrier Reef, the world's largest coral reef system and one of the world's most popular tourist attractions, has lost more than half of its coral since 1985, according to researchers at the Australian Institute of Marine Science.

The reef can be seen from outer space and supports life such as sea turtles, whales, dolphins, and more—but one of its inhabitants, the crown-of-thorns starfish, preys on coral and has been one of the major causes for the reef's decline. According to the analysis, 48 percent of the decline in coral has been from storm damage, 42 percent from the crown of thorns starfish, and 10 percent from coral bleaching, which can be caused by increased water temperatures, bacterial infection, or changes in water chemistry.

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"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," John Gunn, of the institute, said in a statement. "However, we can act to reduce the impact of crown of thorns."

Crown of thorns starfish have undergone a population boom in the Great Barrier Reef over the past decade because they have few natural predators and are able to reproduce extremely quickly. Gunn says that if the starfish were eliminated from the reef, the coral would recover at a rate of about 1 percent per year. The institute is currently conducting research to take "direct intervention" reducing the starfish population.

So far, intervention efforts have been unsuccessful. Cutting up the starfish is ineffective as they possess the ability to regenerate their missing arms and can survive intense physical harm. Scientists have tried injecting them with acid, but the process is expensive. Scientists at AIMS are now considering injecting some of the starfish with transmissible diseases that will spread throughout the starfish population.

Although the starfish are a native, not an invasive species, AIMS suggests that climate change and coral bleaching have damaged the reef enough to prevent it from being able to hold off the numerous "outbreaks" of starfish that have occurred over the past several decades.

"What is clear is that this disturbance to the [reef] has never historically been so severe as to undermine its basic integrity," the institute says. "The disturbance regime has now shifted. The evidence suggests that climate change not only is exacerbating existing impacts like cyclones and coral bleaching, but also may be introducing new threats to coral reefs including increased susceptibility to disease and reduced calcification rates. The combined effect of these disturbances may result in such regular coral mortality and reduced growth that communities cannot fully recover."