There has been a vitriolic debate about whether or not Queensland's farms are damaging the reef. Conservationists argue that the increased run-off of agricultural sediments, nutrients and chemicals has reduced coral cover and biodiversity in recent years. On the other hand, farmers claim that shoddy science is being used to sway public opinion and bulldoze them into adopting uneconomic practices. Scientists fall into both of these camps (New Scientist, 4 January, p 8).
The trouble is a direct link is very hard to prove. The water quality naturally fluctuates, and there are threats to the reef's health from global warming, cyclones and occasional plagues of crown-of-thorn starfish. The historic record of what the reef should look like is sparse. And to run controlled experiments on the reef would be too risky.
But now Katharina Fabricius and Glenn De'ath of the Australian Institute of Marine Science in Townsville, Queensland, have tried to inject some objectivity into the debate by applying an epidemiological technique first used to link smoking to lung cancer in the 1960s. They have compared the health of the reef at several locations close to agricultural areas with that at several others around 400 kilometres away.
The scientists adapted six criteria that were used to pick up causal relationships between smoking and cancer. In their modified form, these included whether known biological facts support the hypothesis that poor water quality stresses coral; whether run-off is associated with poor coral health in different circumstances; and whether or not there is a clear relationship between the pollutant dose and the effect on the coral.
Together, the results suggest a causal link between agricultural pollution, low coral biodiversity and poor recolonisation of the reef. They found, for instance, that hard coral biodiversity was almost twice as high on the reefs far from agricultural areas than on the reefs close by. In both areas, coral cover and biodiversity decreased as the dose of pollutants increased, says Fabricius. She plans to submit the work to the journal Ecological Applications.
Many scientists are pleased to see a new approach. "You are never going to be able to run controlled trials on the Great Barrier Reef, or know what it looked liked in its pristine state. But this provides insights that you could never get with traditional methods," says David Williams of the Cooperative Research Centre for the Great Barrier Reef in Townsville.
But some reef scientists have yet to be convinced. Peter Ridd of James Cook University, also in Townsville, says that while it is true that in other parts of the world high nutrient concentrations have reduced coral biodiversity, the new study does not prove that pollution is high enough on the Great Barrier Reef to cause damage.
Written by Rachel Nowak, Melbourne
New Scientist issue: 7th June 2003
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