CAMBRIDGE, UNITED KINGDOM-- Coypu, 7-kilogram rodents originally imported here in the 1920s for their fur, once ran rampant through the low-lying fields and wetlands of eastern England, but persistent British biologists zeroed in on the animals9 weaknesses and killed nearly 35,000 of them in 6 years, finally ending the invasion in 1989. Now coypu--called nutria in the United States--are wreaking havoc in sensitive coastal wetlands across the Atlantic, and American researchers have adopted for their own efforts the British strategies, which led to the only successful eradication of widely dispersed vertebrates in recent decades.