Invasive species--don't let them in the door.

• Position: On First Reading

Slithering invaders hit the island of Guam three decades ago and have flourished: now up to 26,000 per square mile. Brown tree snakes have gobbled their way through the island's bird species, bitten babies and tangled up in power lines, causing frequent electric blowouts at a cost of $4 million in damages annually.

Native to New Guinea, the Solomon Islands, eastern Australia and Indonesia, the snakes showed up in Guam in the 1940s, probably hitching rides on military shipments at the end of World War II.

But scientists have finally found a way of getting rid of the pests ... with Tylenol (or at least with acetaminophen, its active ingredient). U.S. Department of Agriculture researchers figured out that dead mice stuffed with acetaminophen make a final snack for the snakes, while few other island animals find the rodents tasty.

The tree snake is just one example of an invasive species, which include plants, animals, insects and microbial pests not native to the United States that have an adverse effect on their environment, like reducing the amount of grazing land or choking out native flora and fauna. Invasive weeds and other species cause an annual $136 billion in economic damages, according to a recent study by Cornell researchers.

Each state hosts hundreds of nonnative species. Percentages range from 7.9 percent in New...