Lovely but lethal, the water hyacinth can double in size in as little as 12 days, crowding out native plants and preventing fish from spawning by covering spawning beds. Imported from South America as an ornamental, the species now afflicts fresh waters in Hawaii, California and the Southern United States from Texas to the Carolinas. (Willie Durden/U.S. Department of Agriculture)

Thousands of non-native species have become established in the United States, often causing devastating environmental and economic problems. Although many imported plants and animals provide the nation with food and other products, foreign organisms from pepperweed and cheatgrass to zebra mussels and Asian tiger mosquitoes clog waterways, fuel wildfires, devastate rangelands and even threaten human health. The annual cost has been estimated at up to $138 billion. Controlling the invaders poses such a challenge that scientists believe the most effective solution is to stop the organisms at the nation's borders. But some experts say the government doesn't provide enough resources for adequate inspections. Next year, Congress is set to consider legislation that could tighten restrictions on shippers and others who may inadvertently transport non-native species.