Migration

Migration I think is hard coded. You can just junk your population, so that there are only native breathers on the planet.

SEV, more than a feeling.

From the settings.txt;

Race Starting Percent Reproduction := 10
Race Starting Percent Migration := 5
Race Migration Added 1 Percent Per Population Amount := 1000

I think they allow you to change the rate, but not to have a negative migration.

SEV, more than a feeling.

Did you start a new game?
It seems that these values are read only at the beginning of a new game.

I am not sure what is really the effect of the "Race Starting Percent Migration". However the "Race Migration Added 1 Percent Per Population Amount" seems to work and if you lower the amount (standard game = 1000) to lets say 100 you get a great increase of the migration from heavily populated colonies to less populated colonies.

OK, I understand now what you try to do.

Try a less radical number like:

Race Starting Percent Migration := 25

As soon as migration is higher than reproduction it should have the effect you want.
But even if this works you will pay a price: probably all you domed colonies will stay at 1M population, if they don't have mixed races.

The solution Kwok made for the AI in Balance Mod with scrapping the non-breathing population is highly effective for a human player too!

The other way around this is to make the population get VERY unhappy about populations being scrapped. As one can imagine...

SEV, more than a feeling.