NYTimes reports on refugees from Africa coming to Portland, ME. Part of the reason for their selection of Portland is prior immigrants have settled there and say it's safe and welcoming. This is the sort of "chain" immigration pattern which has long been a feature of American life.
When you look at the world today, the countries with the highest birth rates and youngest populations are in Africa. Afghanistan looks to be the first non-African country in the ranking, and it's 23rd. What that means to me is that Africa will be the primary source of migrants over the next few decades. The migrants may go to Europe and the Middle East based on geography (although I saw a discussion of the Nigerian community in China today) but a good number are likely to come to the U.S., since we already have the connections, the first links in the chain.
I wouldn't be surprised in 20 years or so the children of today's Hispanic and Asian immigrants find African immigrants to be a threat. Maybe I'll live that long.