Which is worse? Closing two locks on a waterway that's used to ship millions of dollars' worth of goods from the Great Lakes to the Mississippi basin? Or allowing a voracious Asian carp to deplete the food supply of native fish sustaining a Midwestern fishing industry that nets $7 billion a year?

And how do you put a price tag on the damage caused by the Burmese python and other constrictor snakes that are strangling the precious ecology of the Everglades?

Invasive species, long the cause of environmental hand-wringing, have been raising more unwelcome questions recently, as the expense of eliminating them is weighed against the mounting liability of leaving them be.

Those questions became more urgent Tuesday when a team of scientists led by the University of Notre Dame disclosed that silver carp dominating stretches of the Mississippi River and its tributaries had infiltrated Lake Michigan. The federal government had spent $22 million on electric barriers in the Chicago Sanitary and Ship Canal to keep carp out, but it clearly wasn't enough. An additional $33 million is going into the effort next year.