After decades of continuous change imposed by human activity, many of the world's natural ecosystems appear susceptible to sudden catastrophic change, an international consortium of scientists reported. Coral reefs and tropical forests are vulnerable, as are northern lakes and forests, the team has found.

Marten Scheffer, an ecologist at the University of Wageningen in the Netherlands, said, "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."

Scheffer is the lead author of the study published Oct. 11 in the scientific journal Nature. He is one of five authors of the paper whose contributors include experts on an array of different ecosystem types.

A gradual awareness is building in the scientific community that stressed ecosystems, given the right nudge, are capable of slipping rapidly from a seemingly steady state to something entirely different, said coauthor Stephen Carpenter, a limnologist at the University of Wisconsin-Madison and immediate past president of the Ecological Society of America.

"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."

An understanding that ecosystems engage in a delicate balancing act has emerged as scientists have become more skillful at assessing entire ecological systems. Studying how catastrophic ecological change has occurred in the past can cast light on how today's ecosystems may be affected.

Six thousand years ago, parts of what is now the Sahara Desert were wet, and its lakes and swamps held crocodiles, hippos, and fish.

"The lines of geologic evidence and evidence from computer models show that it suddenly went from a pretty wet place to a pretty dry place," said Jonathan Foley, a University of Wisconsin-Madison climatologist who is also a coauthor of the Nature paper.

Another drying area is found around Central Asia's Aral Sea. As a result of its shrinking size due to the loss of recharge water and a high rate of evaporation, islands are gaining more surface area. As the sea level continues to drop, more of the sea floor is exposed, and the islands and peninsulas become connected land, the existing Aral Sea could become several separate bodies of water — forming new lakes. Since 1960, most of the fresh water has been diverted for agriculture, and salinity levels have steadily increased.

"Nature isn't linear," Foley said. "Sometimes you can push on a system and push on a system, and finally, you have the straw that breaks the camel's back."

Constant change is a fact of life for most ecosystems, the authors write, whether from increased nutrient levels or human exploitation. Global warming is now adding another destabilizing factor to put ecosystems in a far more precarious situation than scientists had previously imagined.

"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, Wis., that is perhaps the most studied lake in the world. It has seen a steady influx of nutrients such as phosphorus — chemical runoff from farms and suburban lawns — as the land around it has been chemically enriched and then developed.

"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," Carpenter said. 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, and reversing eutrophication is hard because of the phosphorus buildup in soils and sediments.

Similar patterns of ecosystem degradation are evident on coral reefs and in forests. If large enough, forests can influence the weather or even have their own weather systems by facilitating the movement of water from the surface of the earth to the atmosphere. Overexploitation of those forest resources, said Foley and Carpenter, can have profound effects beyond the simple extraction of a resource such as wood.

"The idea that nature can suddenly flip from one kind of condition to another is sobering," said Foley, who suggested that 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."

Says Carpenter, "Although it's possible to push things in the other direction, to a certain extent, restoring a system depends on the art of the possible. What can you do within the constraints of politics and economics to turn back the tide?"