Hmm... do you mean creatures brought in from off-world who are unaffected by Shifts? I... to be honest, no one really gives them much thought. Not that they don't matter - they certainly do, I have a flying ferret at home - they just haven't been a priority to the science teams.

I believe that as they are drawn in by the residents of the worlds they come from, once those people are no longer on this world, the creatures will go back, as well since there'll be nothing to keep them here. Does that help?