After decades of gradual change by humans, many of the world's natural ecosystems - from coral reefs and tropical forests to northern lakes and forests - appear susceptible to sudden catastrophic ecological change, according to an international consortium of scientists.
"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," says Marten Scheffer, an ecologist at the University of Wageningen in the Netherlands.
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, says Stephen Carpenter a limnologist at the University of Wisconsin-Madison.
"We realize that there is a common pattern we're seeing in ecosystems around the world," says Carpenter. "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."
The recognition that many of the world's ecosystems engage in a delicate balancing act has emerged as science has become more adept at assessing entire ecological systems and by a better understanding of how catastrophic ecological change has occurred in the past. For example, 6,000 years ago, swaths of what is now the Sahara Desert were wet, featuring lakes and swamps that teemed with crocodiles, hippos and fish.
"The lines of geologic evidence and evidence from computer models shows that it suddenly went from a pretty wet place to a pretty dry place," says Jonathan Foley, a UW-Madison climatologist. "Nature isn't linear. Sometimes you can push on a system and push on a system and, finally, you have the straw that breaks the camel's back."
Most ecosystems, the authors write, face a steady diet of change, whether it be from increased nutrient levels or a ratcheting up of human exploitation. Moreover, anticipated changes in global climate are expected to add to what now seems to be a far more precarious situation than scientists had previously imagined.
"All of this is set up by the growing susceptibility of ecosystems," Carpenter says. "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."
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, says 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," says Foley, who suggested that changes can be irreversible.
Carpenter sees two management messages: "One, you can't see the change unless you have a view of the entire ecosystem over a long period of time and, two, there are slowly changing variables" that can lay a foundation for catastrophic change.