Richard Black BBC News 28 Apr 11;
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.
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.
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 predate 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.
Early Warning Signal for Ecosystem Collapse: Fluctuations Before the Fall
ScienceDaily 28 Apr 11;
Researchers eavesdropping on complex signals emanating from a remote Wisconsin lake have detected what they say is an unmistakable warning -- a death knell -- of the impending collapse of the lake's aquatic ecosystem. Researchers have found that models used to assess catastrophic changes in economic and medical systems can also predict environmental collapse. Stock market crashes, epileptic seizures, and ecological breakdowns are all preceded by a measurable increase in variance—be it fluctuations in brain waves, the Dow Jones index, or, in the case of the Wisconsin lake, chlorophyll.
The finding, reported April 29 in the journal Science by a team of researchers led by Stephen Carpenter, a limnologist at the University of Wisconsin-Madison, is the first experimental evidence that radical change in an ecosystem can be detected in advance, possibly in time to prevent ecological catastrophe.
"For a long time, ecologists thought these changes couldn't be predicted," says Carpenter, a UW-Madison professor of zoology and one of the world's foremost ecologists. "But we've now shown that they can be foreseen. The early warning is clear. It is a strong signal."
The implications of the National Science Foundation-supported study are big, says Carpenter. They suggest that, with the right kind of monitoring, it may be possible to track the vital signs of any ecosystem and intervene in time to prevent what is often irreversible damage to the environment.
"With more work, this could revolutionize ecosystem management," Carpenter avers. "The concept has now been validated in a field experiment and the fact that it worked in this lake opens the door to testing it in rangelands, forests and marine ecosystems."
Ecosystems often change in radical ways. Lakes, forests, rangelands, coral reefs and many other ecosystems are often transformed by such things as overfishing, insect pests, chemical changes in the environment, overgrazing and shifting climate.
For humans, ecosystem change can impact economies and livelihoods such as when forests succumb to an insect pest, rangelands to overgrazing, or fisheries to overexploitation.
A vivid example of a collapsed resource is the Atlantic cod fishery. Once the most abundant and sought-after fish in the North Atlantic, cod stocks collapsed in the 1990s due to overfishing, causing widespread economic hardship in New England and Canada. Now, the ability to detect when an ecosystem is approaching the tipping point could help prevent such calamities.
In the new study, the Wisconsin researchers, collaborating with groups from the Cary Institute for Ecosystem Studies in Millbrook, N.Y., the University of Virginia in Charlottesville, and St. Norbert College in De Pere, Wis., focused their attention on Peter and Paul lakes, two isolated and undeveloped lakes in northern Wisconsin. Peter is a six-acre lake whose biota were manipulated for the study and nearby Paul served as a control.
The group led by Carpenter experimentally manipulated Peter Lake during a three-year period by gradually adding predatory largemouth bass to the lake, which was previously dominated by small fish that consumed water fleas, a type of zooplankton. The purpose, Carpenter notes, was to destabilize the lake's food web to the point where it would become an ecosystem dominated by large predators. In the process, the researchers expected to see a relatively rapid cascading change in the lake's biological community, one that would affect all of its plants and animals in significant ways.
"We started adding these big ferocious fish and almost immediately this instills fear in the other fish," Carpenter explains. "The small fish begin to sense there is trouble and they stop going into the open water and instead hang around the shore and structure, things like sunken logs. They become risk averse."
The biological upshot, according to the Wisconsin lake expert, is that the lake became "water flea heaven." The system becomes one where the phytoplankton, the preferred food of the lake's water fleas, becomes highly variable.
"The phytoplankton get hammered and at some point the system snaps into a new mode," says Carpenter.
Throughout the lake's three-year manipulation, all its chemical, biological and physical vital signs were continuously monitored to track even the smallest changes that would announce what ecologists call a "regime shift," where an ecosystem undergoes radical and rapid change from one type to another. It was in these massive sets of data that Carpenter and his colleagues were able to detect the signals of the ecosystem's impending collapse.
Ecologists first discovered the signals in computer simulations of spruce budworm outbreaks. Every few decades the insect's populations explode, causing widespread deforestation in boreal forests in Canada. Computer models of a virtual outbreak, however, seemed to undergo odd blips just before an outbreak.
The problem was solved by William "Buz" Brock, a UW-Madison professor of economics who for decades has worked on the mathematical connections of economics and ecology. Brock used a branch of applied mathematics known as bifurcation theory to show that the odd behavior was in fact an early warning of catastrophic change. In short, he devised a way to sense the transformation of an ecosystem by detecting subtle changes in the system's natural patterns of variability.
The upshot of the Peter Lake field experiment, says Carpenter, is a validated statistical early warning system for ecosystem collapse. The catch, however, is that for the early warning system to work, intense and continuous monitoring of an ecosystem's chemistry, physical properties and biota are required.
Such an approach may not be practical for every threatened ecosystem, says Carpenter, but he also cites the price of doing nothing: "These regime shifts tend to be hard to reverse. It is like a runaway train once it gets going and the costs -- both ecological and economic -- are high."
In addition to Carpenter and Brock, authors of the new Science report include Jonathan Cole of the Cary Institute of Ecosystem Studies; Michael Pace, James Coloso and David Seekell of the University of Virginia at Charlottesville; James Hodgson of St. Norbert College; and Ryan Batt, Tim Cline, James Kitchell, Laura Smith and Brian Weidel of UW-Madison.
Journal Reference:
S. R. Carpenter, J. J. Cole, M. L. Pace, R. Batt, W. A. Brock, T. Cline, J. Coloso, J. R. Hodgson, J. F. Kitchell, D. A. Seekell, L. Smith, and B. Weidel. Early Warnings of Regime Shifts: A Whole-Ecosystem Experiment. Science, 28 April 2011 DOI: 10.1126/science.1203672
