What is the threat of invasive species?

How much damage do invasive species cause? What is the difference between an introduced species and an invasive one? What is being done to prevent future invaders? This addition to the Sci-Hi series explains what invasive species are and the numerous problems they have caused. It also examines what scientists and others are trying to do to get rid of invasive species and why some of these methods might cause more problems than they solve ...!