2) Rat Island, The Aleutians
The clue’s in the title here really isn’t it? If you pop a small group of rats on an island and say “GO!” they will take the entire place over in a matter of years. Any wildlife that had been living there previously will either be eaten or out-competed. Rats are machines.
That’s what happened on this small island, previously called Hawadax Island, off the Alaskan coast. Some sailors unwittingly left a few individuals on shore and they took full advantage of the native species. It wasn’t long before the place was literally teeming with the buggers. And they had absolutely no fear of anything, they were the lords of the island.
After a concerted effort by naturalists the island has pretty much been returned to normal now, seabirds are coming back for a snack and a snooze in safety. But if there’s just as few as two or three left it won’t be long until they repopulate the place.
