Why natives over introduced? Like Keith said, they evolved here under local conditions of sudden flood, extended drought and long brutal winters. They're tough bastards if they ain't being out competed. From rainbow competition it looks like the cutts can hide behind a temperature barrier, cutts typically like colder water than the rainbows but with brookies no such barrier exists and they aggressively breed with virtually no competition until the entire ecological niche is filled with stunted brook trout. Much the same as yellow perch when they have a decent feed source and no competition.
You mentioned whitetailed deer, whitetails followed agriculture west and pretty much thrived but they still use eastern winter survival tactics. The winter of 2002-3 (I believe??) was a bad one in Southern Saskatchewan , long and cold. The area around Val Marie had a large population of deer, about 70/30% whitetail to mule deer. When it got bad the deer did what deer evolved to do to survive. The whitetails yarded up in sheltered area like they do in the east and the native muleys disappeared into the windswept areas. It was ugly, starving dying deer everywhere there was shelter, 3-400 hd dead right in the village of Val Marie itself. When it finally breaks in the spring SERM (Saskatchewan Environment and Resource Management) does an assessment and they estimate that 96% of the whitetails and 60% of the mule deer died that brutal winter.
Sorry guy but as much as I liked huntin' wiley whitey or fishin' for bows with their aerial displays I'll take native over introduced, they have evolved the tactics to survive out west