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Science & Technology

Indicator of doom

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Dec 31, 2012 | From the print edition

Ecosystems undergo dramatic fluctuations just before collapse

Enlou Zhang, one of the researchers, collects sediment core samples from Lake Erhai in ChinaTHE WORLD, as we know it, changes often. Established ecosystems can become incapable of supporting life, a phenomenon known as ecosystem collapse. This is worrisome. But to be able to take remedial action, researchers first need to identify ecosystems that are at a risk.

A team of geographers and ecologists from the UK, China, The Netherlands and Spain have prepared a model to predict the collapse of an ecosystem as early as 30 years before it occurs. They have found that shortly before collapse, ecosystems flicker dramatically between healthy and unhealthy states. To reach the conclusion, the researchers studied Lake Erhai in Yunnan province of China. It is one of the seven biggest freshwater lakes in the country.

The team, led by Rong Wang from the Palaeoecological Laboratory at the University of Southampton in the UK, extracted three sediment cores representing a 125- year period from the bottom of the lake. They studied the levels and variation of fossilised algae in these cores and the available environment monitoring data from the area. The data thus generated was then analysed using a mathematical model. The analysis revealed the algae communities in the lake were relatively stable until about 30 years ago. Beyond that, the researchers observed a lot of fluctuation in the types and concentrations of algae in the lake.

“Dramatic swings can be seen in data, suggesting large external impacts on the lake—pollution from fertilisers, sewage and changes in water level— caused the system to rapidly switch between alternate states. Eventually, the lake’s ecosystem could no longer cope, reached a tipping point and collapsed altogether,” says Wang. The study was published in Nature on November 18. Pete Langdon, co-author of study, says, “Who’s to say we couldn’t use this method in other ways? Perhaps we should look for flickering signals in climate data to try and foretell impending crises?”

Stephen Carpenter, director of the Center for Limnology at University of Wisconsin Madison in US, has also published a similar study in 2011. His team had studied similar ecosystem fluctuations caused due to introduction of largemouth bass in a Wisconsin lake that was earlier dominated by small algae-eating fish. “Flickering is well known from models. However, evidence of flickering in real ecosystems is sparse. This paper demonstrates the phenomenon rather clearly in the time series of a degrading lake,” he says of Wang’s study.

Carpenter had found unusual oscillations in the amount of algae in the Wisconsin lake over a year before the lake’s food web shifted. He observed the smaller fish shifted to shallow waters where bass threatened them less. This freed the algae in the more open waters of their predators, making their population rise fast.

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