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
"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
Constant change is a fact of life for most ecosystems, the authors write,
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
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
"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?"