array9

This is surely a question about granularity of space and/or time.

Create a completely new ( & therefore sterile) habitat, and everything is alien, showing how small the granularity could be. But the species must be able to live ( say for a season) within the area.

If something died out, and then made a return, can it be alien? The granularity for history is more complicated and depends on whether the other species are adapted to its future presence already. Do they still know how to eat it, or run from it?

Aliens provide new young blood, unhindered by previous predators or parasites, and are by reputation often more successful.

Could they be overall more productive? Could a 'slash and burn' policy for habitat be more successful than a preserved habitat? The flourishing after the Yellowstone fire? The rebirth of various London boroughs?

Change will create aliens, and some initial 'flourish'. Whether it produces more diversity is more open to question. Though it is obvious that changes to a monoculture will tend to increase it, almost by definition.

Unchanging habitats near to a 'steady state' universe may be less open and adventurous, and more difficult to observe, as defensive and territorial strategies are fine tuned.

But it can be that some species positively like being aliens, moving to new habitats regularly. Moving to new 'terneries' when predators start getting their act together, moving campsite as it gets more difficult to hunt.

It may be sensible to make sure that one does a proportion of work to promote / produce aliens, though many may have difficulty with the idea of creating them on purpose. Forcible transporting and releasing of 'red kite' is perhaps an example to consider carefully.

MartinW