When a species stakes a claim in new surroundings, it often spells bad news for the native flora and fauna. This program presents invasive species case studies and highlights methods for ousting such unwanted guests. Viewers visit Florida’s Gasparilla Island, where a burgeoning black iguana population of Guatemalan ancestry has wrecked ecological havoc. Diving into the Great Lakes, the program looks at another intruder, the zebra mussel—now a target of cavitation, or cleansing that rids a ship’s ballast water of exotic stowaways. Finally, viewers learn the ins and outs of the tapeworm, which has recently been linked to epilepsy, and the hookworm—a creature with medical pros and cons. (23 minutes)