新托福阅读材料：8WildWaystoCombatInvasiveSpecies（6）
外来入侵物种对本地区的生态系统会造成许多不良影响，科学家们用自己的聪明才智研究出了许多对抗这些“外来侵略者”的有趣方法。接下来我们就一起来看看吧!Snuffing out Smelt in Northern Wisconsin
This giant disk of black rubber may seem harmless, but don't be fooled. This gizmo—called a GELI, short for Gradual Entrainment Lake Inverter—is designed to kill. It's the newest weapon in the war against invasive rainbow smelt, a native of the North Atlantic states.
Rainbow smelt were brought to the Midwest in 1912 as food for fish farm salmon. But the tiny fish soon escaped into Lake Michigan and then moved into Wisconsin's inland lakes, perhaps by hitchhiking in fishermen's bait buckets. In Wisconsin alone smelt have already invaded about 25 lakes, "and every year they tend to pop up in another lake," says Jake Vander Zanden, a biologist at the University of Wisconsin–Madison's (U.W.) Center for Limnology. Smelt compete with other species for food and feed on baby fish, including trout, walleye, yellow perch and cisco.
How will the GELI help to get rid of smelt? The goal, says Jordan Read, an environmental engineering graduate student at U.W.–Madison, is to "flip the lake on its head." Temperate lakes tend to stratify, with cold water sinking to the bottom and warm water floating on top. Smelt like the cold water. If the cold water disappears, so will the smelt—or at least that's the hope.
The researchers plan to test this idea out in Crystal Lake, an 84-acre lake in northern Wisconsin. In 2011 they will put about a dozen of these devices in the middle of the lake. Each disc has an inflatable tube running around the rim that is connected to an air compressor. Once the device has sunk to the bottom of the lake, the researchers can start the compressor, fill the tube, and the GELI will rise slowly to the surface, bringing up a large volume of cold water from the lake bottom. Once the GELI hits the surface, the tube will deflate and the GELI will sink again, bringing warm surface water to the bottom. "Eventually we erode that stratification," Read says.
The baby smelt don't mind warm water, so to wipe out the entire smelt population the researchers will have to mix the lake for several years. According to Vander Zanden and Read, the mixing shouldn't have an impact on native fish, which can tolerate warmer waters. The hope is that once the smelt are gone, native fish like yellow perch will rebound. "One of the big ideas here," Vander Zanden says, "is to test of whether the ecosystem can really be brought back to something resembling its original state after eliminating this nasty invasive."
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