And theories came to my mind, like obviously maybe the Dharma people smuggled them there because they had a thing for bears, but then it hit me:
The island can move. Maybe it moved previously, to the Arctic! Maybe right next to an island or an ice floe! And polar bears got on the island! And then it moved again to the South Pacific because, c'mon, IT'S VERY VERY COLD IN THE ARCTIC!
I like this theory. And it is mine.
P.S. I still chuckle at the Lost line "Ahhh, you got a fish biscuit!"