Here is a video from 2009 laying out how Islam is becoming a dominant world religion through higher fertility rates.
While Europe’s European population has hit unsustainably low levels level, the Islamic immigrants are growing exponentially faster.
According to the video, the US is even with a fertility rate of 2.1 children for every couple for White & Hispanic due more so to the Hispanic population. Muslims are described as having a 8.0 rate or 8 children per couple (though this may be lower as some Muslim families are polygamous with more than 1 wife).
Even without immigration a replacement rate of 4 children for every adult the 1% of the US population will grow exponentially. Within a few generations a democratic republic will not be able to hold off a change to a theocracy such as Islam.
I just thought it was an interesting concept since it is so much in the news.