To me it just seems to be survival of the strongest. Whether the seeds get there by mail or carried by winds across oceans or on the backs of rats on cargo ships, in time all life will find its way across the planet. If a bird drops a seed from another continent on soil it's considered natural but if a human drops the seed all of a sudden it's an evasive species. Nature is a cruel hearted bitch. Species go extinct all the time- it's just the way the world works. Species adapt or they perish.