严重干旱能引起生态系统永久性损害

An eight-year study has concluded that increasingly frequent and severe drought, dropping water tables and dried-up springs have pushed some aquatic（水生的） desert ecosystems into "catastrophic regime change," from which many species will not recover. The findings, just published in the journal Freshwater Biology, raise concerns that climate change, over-pumping of aquifers（蓄水层） for urban water use, and land management may permanently affect which species can survive.

"Populations that have persisted for hundreds or thousands of years are now dying out," said David Lytle, an associate professor of zoology at Oregon State University. "Springs that used to be permanent are drying up. Streams that used to be perennial are now intermittent（间歇的） . And species that used to rise and fall in their populations are now disappearing."

The research, done by Lytle and doctoral candidate Michael Bogan, examined the effect of complete water loss and its subsequent impact on aquatic insect communities in a formerly perennial desert stream in Arizona's French Joe Canyon, before and after severe droughts in the early 2000s.

The stream completely dried up for a period in 2005, and again in 2008 and 2009, leading to what researchers called a rapid "regime shift" in which some species went locally extinct and others took their place. The ecosystem dynamics are now different and show no sign of returning to their former state. Six species were eliminated when the stream dried up, and 40 others became more abundant. Large-bodied "top predators" like the giant waterbug disappeared and were replaced by smaller "mesopredators" such as aquatic beetles.

"Before 2004, this area was like a beautiful oasis（绿洲） , with lots of vegetation, birds and rare species," Lytle said. "The spring has lost a number of key insect species, has a lot less water, and now has very different characteristics."

The phenomena, the researchers say, does not so much indicate the disappearance of life -- there is about as much abundance as before. It's just not the same.

"Our study focused on a single stream in isolation, but this process of drying and local extinction is happening across the desert Southwest," Bogan said. "Eventually this could lead to the loss of species from the entire region, or the complete extinction of species that rely on these desert oases."

Small streams such as this are of particular interest because they can be more easily observed and studied than larger rivers and streams, and may represent a microcosm of similar effects that are taking place across much of the American West, the researchers said. The speed and suddenness of some changes give species inadequate time to adapt.