Image caption The name "largemouth bass" appears to suit the fish

An experiment in a US lake suggests that ecosystem collapses could be predicted, given the right monitoring.

Researchers changed the structure of the food web in Peter Lake, in Wisconsin, by adding predatory fish.

Within three years, the fish had taken over, producing a decline in tiny water plants and an explosion in water fleas.

Writing in the journal Science, the researchers say the change was preceded by signals that could be used to predict similar collapses elsewhere.

In particular, rapid swings in the density of plants and fleas indicated the food web was unstable and about to change.

The idea that such early warning signals ought to exist is not new - but the researchers say this is the first time it has been demonstrated experimentally.

"For a long time, ecologists thought these changes couldn't be predicted," said research leader Stephen Carpenter from the University of Wisconsin in Madison, US.

"But we've now shown that they can be foreseen. The early warning is clear; it is a strong signal."

Peter and Paul

The Peter Lake food web contained four key components. Insects such as fleas ate tiny water-borne plants, small fish such as golden shiners ate the fleas, and much bigger largemouth bass ate the little fish.

We are surrounded by problems caused by ecological regime shifts Jonathan Cole, Cary Institute of Ecosystem Studies

Beginning in 2008, the researchers began to add more bass, and more than a thousand hatched the following year.

Sensing the threat from these predators, the golden shiners began to spend more time in the shallows or sheltering under floating logs.

Larger fleas moved in, eating the floating plants (phytoplankton).

But the changes were anything but smooth, with wildly varying numbers of fleas and phytoplankton seen at different times.

Eventually, by late 2010, the ecosystem appeared to have finalised its transition from one stable state to another.

This second state, dominated by fleas and largemouth bass, is similar to the situation that had existed for years in neighbouring Lake Paul.

This lake showed no major changes during the three years, indicating that the changes seen in Peter really were caused by the addition of bass.

Banks collapse

Many natural systems appear capable of existing in more than one stable state.

Image caption Lakes Peter and Paul in Wisconsin have a 50-year history of use for ecological research

Until 20 years ago, the Grand Banks off Canada's east coast were dominated by cod - so many as to prevent the growth of other species.

Overfishing caused the cod population to collapse.

Other species have since taken their prime position, some of which prey on juvenile cod - perhaps meaning that the prized fish will never return to their former dominance.

The new research suggests it might be possible to detect signals of such a coming crash before it happened.

"Early warning signs help you prepare for, and hopefully prevent, the worst case scenario," said Jonathan Cole from the Cary Institute of Ecosystem Studies near New York, another of the scientists involved.

"We are surrounded by problems caused by ecological regime shifts - water supply shortages, fishery declines, unproductive rangeland - and our study shows that there is promise in identifying these changes before they reach their tipping point."

The principle may have been proved, but the application would still appear to be some way away.

Monitoring any ecosystem with the intensity used at Peter Lake will be expensive, although the ever growing fleet of Earth observation satellites could help in some cases.

Even more problematic is knowing which early warning signs apply in which ecosystem.