My impression is that America's birth rate declined in the late 19th Century, in part, as a result of Protestant ministers embracing family planning or "planned parenthood" and as as result the ideal family fell from a dozen kids to around 6 kids. Eventually contraception was accepted as part of normal family life and the ideal family fell from 6 kids to 3 kids. All the while, immigration replaced the dwindling fertility and immigrants eventually accepted the standards of their new country as their own. Of course, contraceptives helped fuel licentiousness. Now we've got 50% divorce rate and 1 in 4 women had at least one abortion. The birth rate among any white population in The World is below replacement. Many Oriental populations are below replacement and, surprisingly, many Muslim countries are also breeding below replacement. Countries like Mexico have seen dramatic declines in birth rate and Puerto Rico, for example, is already below replacement. Haiti is above replacement, but their life expectancy is below 40. As Baby Boomers age and die off, America may experience a real decline in actual population, for the first time, I would think. Of course, America is not unique in that regard. Japan is apparently far ahead of us in this problem, but will probably just open its doors to waves of non-Japanese immigrants, seeking a better life in a rich country like Japan. I would think that countries will have to compete for immigrants and the losers will see their economies get a lot smaller (collapse) and probably experience a lot of blight or "green spaces."
My grandfather's hometown saw half its population get up and go to America and Canada in a relatively short period of time. That town is still on the map and still has lots of people in it, who have my last name. Although, it is a small town and will probably remain that way. If it ever dies, the survivors will probably just move down into the valley and contribute to that areas population.
At some point you might think this will produce a reaction so significant that it actually reverses the trend. I wonder if it will or if the world will eventually decline to one last couple who decide children are too much of a hassle.