yes, but what you are talking about is the natural progression of one dominant species to another.....caused by some sort of 'natural disaster'. (Fire, climate change, meteor strike, etc.) I doubt that mammals would have taken over any ecosystem in the Age of Dinos if something hadn't have happened to cause the demise of the dinos in the first place. mammals just happened to be int he right place at the right time, and were able to take advantage of the opened ecosystems (that they already inhabited).

Evolution is the natural mutation of species.

There is nothing natural about introducing a species into an ecosystem that it hasn't evolved with.

I doubt that if there was a fire that wiped out the forests of the Rockey Mountains, that somehow, a tree from Australia would miraciously appear out of nowhere to repopulate the now denuded slopes.