[a fool in paradise]

The gap in population in the West will be filled by (Islamic) immigration.

[RayChuang88]

Interestingly, the really rich Islamic countries in the Middle East, Malaysia and even Indonesia is experiencing one thing that has kind of surprised Muslim cultural scholars: the population growth is slowing down. Indeed, the more affluent the population is, the less likely the population will have an explosion of child bearing.

[Ben Ficklin]

In rounded off numbers, there are about 7 billion on earth with 4 billion in Asia, 1 billion in Africa, 1 billion in Europe and Russia, and 1 billion the Americas.

By 2050, Asia will add a 5th billion and Africa a 2nd billion. By 2100 Africa will add a 3rd and 4th billion, for an 11 billion total.

Ray is correct about the falling fertility rates.

CIA Factbook Country Comparison of Fertility Rates in descending order