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.

David Hosansky is a freelance writer in the Denver area who specializes in environmental issues. He previously was a senior writer at CQ Weekly and the Florida Times-Union in Jacksonville, where he was twice nominated for a Pulitzer Prize.