I ask again — why are we trying to stop natural selection? I suppose one can try to make the argument that humans are somehow “supernatural”, but just as nature didn’t care what the motivations were for the selective pressures put upon organisms in the past that led to the evolution of the species, should we be trying to eliminate this very important natural process from occurring?
In particular, with black-footed ferrets, isn’t it just as well that they go extinct if they cannot handle the selective pressures they’re subjected to? Regardless of whether the habitat loss was due to human encroachment, or other natural mechanisms, isn’t there actual value in allowing them to go extinct, and allowing their surviving genetic cousins, who may have mutated differently, to continue along their genetic lines?
Now, I suppose you could assert that any intervention we do is “natural”, and thus, even if our intervention to save the black-footed ferret deprives their theoretical white-footed cousin from out competing and thereby surviving and evoloving, it is in fact another form of selection — just one driven by our own particular and arbitrary aesthetics, rather than incidentally due to our normal day-to-day activity.
P.S.: Since migration of organisms happens all the time, what is the definition of “non-native” species? Is there some arbitrary time limit for which a species needs to exist in a location before it is “native”? I’m thinking in particular of my homeland, Hawaii, which grew out of volcanoes in the ocean, and is only home to any species at all because they migrated there from somewhere else (unless, you assert that they became “native” after some amount of genetic drift from their original migratory populations).