Feral pigs lead to the loss of island fox
This is probably better in the environmental protection page, but since feral pigs were mentioned here.
http://www.pnas.org/content/99/2/791.full.pdf
Not just interesting because of the pigs, but because it had multiple indirect impacts throughout the system. This is the kind of stuff people mean when they talk about the things that are difficult to study, and we can't know, when they tell people not to do x, y, or z on their property. Or like people up here that are live baiting with southern crayfish, and those crayfish are now wrecking the banks and causing all sorts of problems.