Witness the migration of species across the Bering Straits and the Isthmus of Panama. Great exchange of species has occurred between both oceanic and continental biota in these areas as they have been repeatedly submerged and exposed, alternately being corridors for aquatic and terrestrial life. In response to the Ice Ages, great movement of species has occurred, Even now, I understand that the armadillo is extending his range north from his native Mexico. I have seen a grassy meadow and a field of star thistles side by side, with only barbed wire separating them. The fence can't stop the thistle seed, yet it does not invade the intact meadow, showing the thistle to be an antibody-like response of the prairie ecosystem to overgrazing by cattle.
New species create niches for more species, further increasing potential diversity.
Many species are extinct in their original habitat, existing only where they have been introduced to new areas by man. We are changing the world through our destruction, pollution, and now possible climate change. Local ecosystems need the infusion of new species to help their adaptation to a changed environment. You stay, I go. It is ironic to me to hear people of European ancestry accuse other organisms of being 'invasive exotics, displacing native species'.