10/10/01 -- One more 'push' may crash some ecosystems

One more 'push' may crash some ecosystems

Man's environmental battering has left some ecosystems in such a fragile state, the slightest alteration, from a dry spell to a fire, may push them into a catastrophic collapse, an international consortium of scientists asserts.

The authors of the review in the British journal Nature recommend rebuilding ecosystem resilience, rather than pursuing the current common practice of controlling an individual disturbance, be it an invasive exotic species or a disease.

Conservation and other policies are based on the long-standing view that ecosystems -- whether lakes, oceans, coral reefs, woodlands or deserts -- respond slowly and steadily to climate, nutrient, habitat and other environmental shifts. In dramatic contrast, 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. At the root of such cataclysmic alterations is the breakdown of resilience of an ecosystem relentlessly pushed away from its natural origins, the study authors suggested.

"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," lead study author Marten Scheffer, an ecologist at the University of Wageningen in The Netherlands, told United Press International.

"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."

While nature has spawned such variations through the ages, a new unnatural force has entered the scene, the scientists said.

"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."

An assessment, over long periods and entire ecological regimes, points to the invisible instability of stressed systems, which, given the right nudge, can suddenly plunge from seeming steadiness into distressed discord, scientists told UPI.

"We realize that there is a common pattern we're seeing in ecosystems around the world," said study co-author Stephen Carpenter, a limnologist at the University of Wisconsin, Madison. "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 findings have important implications for conservationists and policymakers who may base decisions on a misconception, scientists said.

"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, 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," he told UPI. "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."

In one of the more striking examples, a lush, wet playground of hippos, fish and crocodiles that abounded in lakes and swamps suddenly turned into the largest arid area on Earth, the Sahara Desert.

"The sudden catastrophic desertification in the Sahara 5,000 years ago was caused by gradual change in irradiation due to gradual change in the Earth orbit," Scheffer said.

"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," Foley added.

Lake Mendota in Madison provides a more contemporary example. Algae, fat from a steady diet of chemicals from runoff from farms and suburban lawns, have turned the once pristine, crystal-clear waters into a murky cesspool of green slime.

"Over the past 150 years, we've put a huge amount of phosphorus into the mud of Lake Mendota," Carpenter said. "This phosphorus buildup has made it easy for Lake Mendota to go into a eutrophic state."

Reversing the trend will be all the more difficult because the thicket of sun-blocking algae has been suffocating lake-bottom plants that are natural-born cleaners.

"Even if fertilizer runoff is reduced in efforts to clean up the lake, it will take a lot more work to restore the lake without the help of the big plants that had kept the mud from stirring up, keeping the water clear," Foley said.

At most immediate risk are numerous tropical ecosystems, freshwater rivers and lakes and coastal estuaries and other marine systems, Carpenter told UPI.

Evidence supporting the author's assertions spans the globe, 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," he told UPI.

Similar patterns of 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 to the atmosphere, investigators said.

Increasing human population and anticipated global climate changes will bring additional stresses, though some may be somewhat less than projected, if the authors of another Nature study are correct.

Yiqi Luo and his team at the University of Oklahoma in Norman suggest soil microbes may adjust to global warming so their contribution to the release of the notorious greenhouse gas carbon dioxide may be less than predicted.

All models have assumed the CO2 from respiration of bacteria, plant roots and fungi would rise along with Earth's temperature, which is expected to increase between 1.4 degrees Centigrade and 5.8 degrees C (2.5 degrees Fahrenheit to 10.4 degrees F) over the next century as a result of greenhouse gas emissions. But, Luo and crew found that under artificially increased temperatures, soil respiration on the test grassland plots did not go up as much as had been forecast.

"Soil respiration is a major contributor to global carbon cycling, thus only a small decrease could compensate for man-made emissions," said Lindsey Rustad of the U.S. Department of Forest Service in Durham, N.H., who wrote an accompanying News and Views article.

Nevertheless, any climate change may add to a situation that already appears to be more precarious than anyone had imagined, the scientists said.

"We should not be complacent about the response of ecosystems to ongoing global changes in environment," Schlesinger said. "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." Error: Unable to read footer file.