CORVALLIS - A group of studies by an aquatic entomologist at Oregon State University suggest that at least some of the problems facing streams in the American West may relate to their loss of extreme water flows, ranging from severe droughts to flash floods.
The same dams that have tamed the violent or extreme nature of these streams may also 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, researchers say.
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 in the West 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," said David Lytle, an OSU entomologist. "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 suggest they are critical to many native ecosystems."
The concept is not new, Lytle said. But its implications are significant.
Just as forest scientists have discovered in recent decades the critical role of fire in maintaining healthy forest ecosystems in many areas, so too are stream ecologists now 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 also have similar effects.
But recent research done by Lytle and his colleagues in this area, published in several professional journals including Ecology and American Naturalist, is revealing what he calls the "footprint of evolution" in some stream systems, in which 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, caddisfly larvae are almost completely scoured out of the stream by the floods. About 96 percent disappear. But through generations of evolution, a significant amount of the insects metamorphose into their flying adult phase during a period that's timed exactly with the most common flood season, keeping them out of the stream while the waters sweep by.
Research has been done on cottonwood trees that once grew thickly along the banks of many western streams and rivers, providing shade, nutrients and woody debris that further aided the health of the ecosystem. These trees can experience some mortality due to floods. But it has also been learned that cottonwoods need bare, mostly scoured banks, the types of conditions common after a flood, to germinate their seeds and reproduce. And cottonwoods are now in serious decline in many areas.
In the Colorado River, loss of flooding following construction of the Glen Canyon Dam has caused a wholesale shift in fish and fauna, allowing invasive species to displace native ones. The problem is bad enough that "simulated floods" have been attempted with rapid water releases - so far with mixed ecological results.
"We've seen the ecology of many western streams change dramatically," Lytle said. "Some fish species have declined or disappeared, possibly relating to the change in flow regime or other factors. And the removal of these floods and droughts, which native species could handle but many others cannot, opens the door to a whole range of new, invading competitors."
Lytle's research documented another interesting example of adaptation to extreme conditions which appears to go back 150 million years. There are species of giant waterbugs that thrive in some desert streams. During a major rainstorm of the type that can cause flash floods, Lytle and his colleagues once observed these water bugs do a mass exodus from the stream, literally marching up the canyon wall for protection just before a flood burst through the area. They came back within a day.
Later, in a controlled experiment that simulated heavy rain, the scientists were able to trigger the same behavior. The insects thought a flood was coming and headed for cover.
"If you look carefully for adaptation to extreme events, you tend to find it," Lytle said. "This includes adaptation by plants, insects, fish, trees, all the components of a stream ecosystem."
The research suggests that loss of extreme events is a major factor in the problems being experienced across much of the West in stream ecology, Lytle said. At this point, aquatic organisms, including fish, are among the most imperiled fauna in North America, he said, with problems often far surpassing those of their terrestrial neighbors.