Valleys, unlike islands, are rarely (if ever) isolated enough from the surrounding countryside that natural predators can not get there, and British Columbia has plenty of natural wolves and cougars, I don't see how introduced domestic dogs and cats (not to mention that few domestic cats would take on anything the size of an eagle) would do anything, should they exist... while introduced animals can cause a lot of problems, generally it is caused when predatory animals are introduced to areas with no natural predators or where a new animal that is better at using the same ecological niche is introduced.
That was my idea too: an island, or even a valley in some island.
But this is part of the quote: "Interior of B.C. NA". That would be the mainland, right?