Comment by goodcanadian

Part of the reason that they can be controlled in Alberta is that they can't survive the winter outside of human structures. Combined with the fact that the border areas are relatively sparsely populated farmland, it is more a case of eradicating small isolated populations than controlling rats en masse. The rats can do a bit of "island hopping" in the summers, so an active campaign has to be maintained to keep them out.

The other reason that it is possible to maintain, of course, is the fact that the decision was made BEFORE rats invaded.