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What Are the Warning Signs of Tipping Points?


largemouth bass, ecosystem, Peter LakePredictions of tipping points in ecology, climate change, medical outcomes and other complex systems are a primary goal for many researchers. The pursuit of insights into the timing of critical transitions is no easy way to make a living, particularly because random events can trigger such changes and warning signs are easily missed or misinterpreted.

Perhaps the best approach to studying tipping points is to combine two different approaches—one that explores the architecture of systems that change drastically and another that homes in on telltale signs that a system is on the brink. A team of environmental scientists, ecologists and economists led by Marten Scheffer, an ecologist at Wageningen University in the Netherlands, posit such a two-pronged analysis in the October 19 issue of Science.

In terms of architecture, some systems—be they populations, ecosystems or economies—are made up of parts that are diverse and only marginally connected, which means change tends to occur gradually. Other systems, however, comprised of similar, highly interconnected components may resist change at first and then flip rapidly when pushed to a certain threshold.

Scheffer and his colleagues use coral reefs to illustrate their point about system architecture. Coral reefs are highly connected systems—organisms from one reef are known to repair damage to nearby reefs. If organisms from one reef are constantly saving another reef from destruction over time, it might leave the impression that the damaged reef is highly resilient. However, when many reefs are stricken by, say, a sea urchin disease outbreak such as the one that occurred in the Caribbean in the 1980s, this can leave the entire reef system vulnerable to tropical storm damage and a large-scale, yet unforeseen, ecosystem collapse.

Studying the impact of structure on a complex system's strength or fragility is only half the battle. The researchers argue that so far there are no ways to use architecture alone to measure how close any particular system is to a critical transition. For this, it's helpful to also analyze a system's timeline for clues. Scheffer and his colleagues note that some changes are preceded by a slowdown in activity while others are presaged by an uptick in disorder.

In addition to their own research, Scheffer and his colleagues are contributing to the study of critical transitions in other ways. They are behind the July launch of the Early Warning Signals Toolbox Web site, a clearinghouse for research, case studies and free computer programs dedicated to the early recognition of such tipping points.

The results of critical transitions aren't always bad—i.e. the destruction of a coral reef network or the crashing of financial markets. University of Wisconsin–Madison ecologist Stephen Carpenter has for years studied how critical transitions can be used to keep invasive species from overtaking healthy habitats. Using the food chain at Peter Lake on the Wisconsin–Michigan border as a test bed, Carpenter and his colleagues over several years introduced dozens of largemouth bass into the algae-infested water. In time, the bass ate the once-dominant fathead minnows, pumpkinseeds and other fish that preyed on the lake's populations of water fleas and other tiny, algae-eating animals. With these algae-eaters back in business, the lake water cleared and has remained that way, Carl Zimmer describes in the November 2012 Scientific American article "Ecosystems on the Brink."

Carpenter and his team developed mathematical models of the region's ecological networks using variables including the reproduction rate of individual species and the rate at which they ate one another. The researchers tracked the transition and were able to identify tipping points—15 months ahead of Peter Lake's dramatic transformation.

Image courtesy of James Allred, via

The views expressed are those of the author and are not necessarily those of Scientific American.

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