There's something cool about Canada, I just found out that Alberta is the only large region of permanently inhabited human territory which lacks brown rats. One thing you have to remember is that the brown rat only began spreading within the last 1,000 years (in the process displacing the black rat), and it seems to have arrived in the British Isles only within the last two to three centuries. North America did not have the rat until Europeans arrived, and it didn't show up in Alberta until 1950. At that point the government attempted an eradication program. Apparently this can work because there aren't ecologically congenial corridors for the rats to constantly reappear through migration.