The study, published in Nature, found that human impacts on many of the world's ecosystems could cause them to abruptly shift with little or no warning from their apparently stable natural condition to very different, diminished conditions far less able to support diversity of life, including human. "Models have predicted this, but only in recent years has enough evidence accumulated to tell us that resilience of many important ecosystems has become undermined to the point that even the slightest disturbance can make them collapse," said Marten Scheffer, an ecologist at the University of Wageningen in the Netherlands and lead author of the study.

Conventional scientific and conservation thinking has been that ecosystems such as lakes, oceans, coral reefs, woodlands or deserts respond slowly and steadily to climate change, nutrient pollution, habitat degradation and other human environmental impacts. But the new study shatters this paradigm, finding instead that, after decades of continuous change imposed by human activity, many of the world's natural ecosystems are now susceptible to sudden catastrophic change. In dramatic contrast to conventional environmental thinking, the investigators paint a picture of unexpectedly sudden, drastic switches of state, from lush, lake-dotted forests teeming with plants and animals to scorching, parched deserts devoid of all but the hardiest of lifeforms, for example.

"In approaching questions about deforestation or endangered species or global climate change, we work on the premise that an ounce of pollution equals an ounce of damage," said co-author Jonathan Foley, a University of Wisconsin-Madison climatologist and director of the Center for Sustainability and the Global Environment at the Institute for Environmental Studies at UW-Madison.

"It turns out that assumption is entirely incorrect. Ecosystems may go on for years exposed to pollution or climate changes without showing any change at all and then suddenly they may flip into an entirely different condition, with little warning or none at all.

"The idea that nature can suddenly flip from one kind of condition to another is sobering," said Foley, who said that such changes can be irreversible. "For hundreds of years, we've been taught to think in very linear ways; we like to think of nature as being simple. But now we know that we can't count on ecosystems to act in nice simple ways."

This new awareness of the nonlinearity of ecological change - that stressed ecosystems, given the right nudge, are capable of slipping rapidly from a seemingly steady state to something entirely different - is building in the scientific community, according to co-author Stephen Carpenter, a limnologist at the University of Wisconsin-Madison and immediate past president of the Ecological Society of America. An understanding that ecosystems engage in a delicate balancing act has emerged as scientists have become more skillful at assessing entire ecological systems. "We realize that there is a common pattern we're seeing in ecosystems around the world," said Carpenter, an authority on lakes. "Gradual changes in vulnerability accumulate and eventually you get a shock to the system, a flood or a drought, and boom, you're over into another regime. It becomes a self-sustaining collapse."

"We systematically alter conditions on the earth, such as temperature and nutrient levels. We usually assume that things are okay if nature is not changing too strongly and assume that we may always reverse change by taking 'a step back' if things seem to become too bad," said Scheffer. "Our article shows that this does not hold. We may see little effect until the breakpoint. Once the catastrophic change has occurred, the way back is typically very difficult."

The study found that these cataclysmic alterations result from the breakdown of resilience of ecosystems being relentlessly degraded by human activities. The implications of losing ecosystem resilience, the paper's authors say, are "profound" in light of current resource management. "Regime shifts," they write, "can imply a drastic loss of biodiversity as well as utility for humans."

"We are now witnessing a human-induced, tremendously rapid change in conditions, compared to what happened in most of the ancient past," Scheffer said. "None of the changes ahead will stop nature from functioning in one way or another. However, some of the rapid switches may take us by surprise and cause not only a tremendous loss of biodiversity but also play havoc with human use of nature in an economic sense as well as in a wider sense."

"All of this is set up by the growing susceptibility of ecosystems," Carpenter said. "A shock that formerly would not have knocked a system into another state now has the potential to do so. In fact, it's pretty easy."

Carpenter cited Lake Mendota, an urban lake in Madison, Wisconsin, that is perhaps the most studied lake in the world. It has seen a steady influx of nutrients such as phosphorus from chemical runoff from farms and suburban lawns as the land around it has been developed and artificially enriched. "Over the past 150 years, we've put a huge amount of phosphorus into the mud of Lake Mendota, and it's prompted a lot of algae growth in a lake that was once very clear." In 1993, scientists watched nutrient levels rise sharply after a single heavy rain washed nutrients into the lake. "This phosphorus buildup has made it easy for Lake Mendota to go into a eutrophic state," characterized by green surface scums, Carpenter said.

Reversing eutrophication is difficult because of the phosphorus buildup in soils and sediments. Evidence supporting the author's assertions spans the globe, other scientists said. "I find that my own work in southern New Mexico, where we have seen a widespread change from semiarid grassland systems that were productive rangelands to arid shrublands, substantiates what these authors describe in the desert regions," said William Schlesinger, James B. Duke professor of biogeochemistry and dean of the Nicholas School of the Environment and Earth Sciences at Duke University in Durham, N.C. "Abrupt environmental change has affected these ecosystems worldwide.

"We should not be complacent about the response of ecosystems to ongoing global changes in environment. What may seem gradual and unimportant could produce big, undesirable changes in ecosystems and the productivity of food and forestry systems upon which we all depend."

- Rad-Green