I subscribe to a lot of magazines, one of which is National Geographic. I got around to reading the January 2005 issue, and found the article on Alien Invaders to be both interesting and creepy (forgive the pun).
No, the article isn't about E.T. The article is about bio-diversity, and what happens when species are introduced somewhere they have no natural predators. The first example, which you can read in the link above, is about how there is a Burmese python infestation in the Florida Everglades. Burmese pythons in the everglades - how?
It turns out people import them as pets, discover they aren't as cuddly as they thought (?!), and then release them into the Everglades. Many species treated like this die out, but a few, such as the python, are adept generalists - they aren't fussy about what they eat, so they survive and breed.
Ugh. If I move to Florida I guess I won't be doing any backpacking trips there. ;)