Extreme conditions help nature thrive.

• Position: Ecosystems

At least some of the problems facing streams in the western U.S. may relate to their loss of extreme water flows, ranging from severe droughts to flash floods, suggests a group of studies by David Lytle, an aquatic entomologist at Oregon State University, Corvallis. The same dams that have tamed the violent or extreme nature of these streams also may be disrupting aquatic ecosystems that depend on such events to favor native species, keep out invasive plants or animals, and maintain a natural ecological balance that evolved over millennia.

Studies ranging from the unusual evolution of a giant waterbug in high mountain streams of Arizona to the mysterious disappearance of cottonwoods on river banks across much of the West all point to the same conclusion--that streams and rivers have evolved with regular floods, droughts, and everything in between, and any disruption of those patterns may pose a risk to native ecosystems. "Right now in the American West, there are more than 15,000 dams," indicates Lytle. "They remove the extreme flow events that used to exist, preventing both the major floods and the extremely low flows during summer months. But the increasing level of knowledge we're gaining about these extreme disturbances suggests they are critical to many native ecosystems."

The concept is not new, he explains. Its implications, however, are significant. Just as forest scientists have discovered in recent decades the critical role of fire in maintaining healthy forest ecosystems in many areas, stream ecologists now are learning more about the nature and extent to which streams have been disrupted by efforts to tame their extreme events. Many other natural disturbances--windstorms, insect outbreaks, terrestrial droughts--may have similar effects.

Lytle's research has revealed what he calls the "footprint of evolution" in some stream systems, whereby certain species are fully adapted to extreme events and may even depend upon them for survival. In one mountain stream system in Arizona that is periodically blasted by flash floods, for instance, caddisfly larvae are almost completely scoured out of the stream by the floods and about 96%...