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A three-year experiment in which scientists manipulated the ecology of a small lake in northern Wisconsin has provided some of the first evidence that major changes in ecosystems, so-called “regime shifts,” are preceded by warning signs. The scientists argue that learning to identify these signs could help predict rapid ecological changes that harm fisheries, forests and water supplies.

The experiment was begun in 2008 by a research team lead by Stephen Carpenter, director of UW-Madison’s Center for Limnology (the study of bodies of freshwater). The team selected a lake with a relatively balanced food chain. Largemouth bass in the lake fed on smaller fish in the lake, which in turn fed on phytoplankton (small green organisms) and zooplankton (“water fleas”) in the water.

The researchers sought to throw the miniature ecosystem out of whack. They began stocking the lake with new predators – more bass. As their numbers rose, the smaller fish, which previously dominated the lake, began to group together in shoals. In an attempt to hide from the new bass, they congregated in shallow water, where there were more rocks and logs.

Gradually, with predation on the rise, the numbers of small fish dwindled, which was good news for the phytoplankton and water fleas. As life for the water fleas got safer, they began appearing in larger forms, especially in the lake’s open waters, where smaller fish were relatively scarce.

The experiment succeeded in up-ending the lake’s ecosystem. Once dominated by phytoplankton-nibbling peace lovers, it became a pacifist’s nightmare ruled by the predatory bass.

 As the lake underwent this revolution, the researchers collected reams of data. They found early warning signs of the impending regime shift, including wide variations of chlorophyll levels in the water. Chlorophyll is the green substance that plants and phytoplankton use during photosynthesis to create sugar or other organic sources of energy.

The wide variations preceded the bass’ rise to dominance. According to New York’s Cary Institute of Ecosystem Studies, which participated in the study, similar wide fluctuations are measured in brain waves before seizures and the Dow Jones index before a market crash.

“Warnings may be evident well before the regime shift is complete,” says the research team’s paper, which appeared recently in the journal Science. “The early warning indicators appear to be useful even in cases where the form of the potential regime shift is not known.”

It adds, “Early warning signs are potentially useful for managing ecosystem services that are subject to catastrophic change, such as unwanted shifts in freshwater quality or production of fisheries and rangelands.”

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