Humans travel with a veritable army of hitch-hikers: marine plants and animals that float in a ship's ballast water; rats, bugs, and other creepers (and slitherers) that stow away in cargo planes. When some of these alien critters disembark in a foreign land, the ecosystem is never the same. Sometimes people deliberately introduce alien species, only to see the aliens crowd out the natives. Here we present eight of the most fascinating battles between humanity and invasive species.

In South Florida, Burmese pythons released in the swamps by pet owners who no longer feel like caring for their 20-foot-long snakes have established a firm colony in Everglades National Park. It's gotten so bad, wildlife officials are considering placing a bounty on the snakes. Meanwhile, four African rock pythons have been found in the region, and biologists say these extra-vicious snakes will lunge at humans. If the two species interbreed, they could create a hybrid hissing terror.

Status: Time to Get Serious