Nutrias are causing major problems in Louisiana. Someone brought the rodents to the state 40 years ago, hoping to make a fortune from the furs. Nobody wanted to buy the furs, and now the animals roam freely. They're destroying the wetlands. They eat the roots of the water plants along the canals. The plants die, and the banks of the canals begin to erode. The animals are costing the state millions of dollars, and no one seems to know what to do with them. What sentence from the passage best describes the central idea?