Which ethnicity’s population are we going to reduce?

I honestly believe that "we" aren't going to do jack s**t. It's a process which is nearly unsteerable. People are going to live longer and longer, and use resources that would otherwise be used by children they might have had. Society is going to be burdened by caring for the old, and this is going to reduce chances of caring for the young.

In nearly every developed country, population growth is slowing or population has already started decreasing. Only in the least developed regions (some areas of Africa) does the opposite still apply, but UN predictions (made by competent people) suggest the process just reaches there later.

So, every ethnicity's population is going to be reduced. Every ethnicity can also consider if their numbers are adequate, too high or too low. If a nation feels threatened by disappearing from the maps, they can try to reorganize their society. Random ideas: a few laws that give parents various health and social security guarantees regardless of their employment status, especially in case they're single parents, then maybe create a few dating sites that actually try to help their users find people they like, etc...