Not sure how much I agree with the post at the top of this thread, but overpopulation as a reason for refugees and migration is a bit overplayed, I think.  People don't flee from a particular level of population density, but rather from a lack of economic opportunity, corruption, and political oppression.  The causes of these problems are almost entirely social and political.

Population growth may contribute to the hopelessness and desperation stemming from these root problems, as traditional (pre-modern, subsistence and family oriented) safety nets fail to deal with a larger number of people who need to be saved, but they wouldn't be necessary if the above problems weren't severe.

Sure, there may be more people in the Sahel than can reasonably survive as goat herders, but the real problem isn't that there are too many goat herders, but that they're goat herders to begin with.  There's no money in goat herding, and they know it, but they're powerless to do anything but immigrate.

If it was merely a factor of "too many people in too small a space," which is sort of what "overpopulation" implies, refugees would be coming from places like Tokyo.  

Then again, the way things are going in the US, they may very well start fleeing from New York.