Overpopulation

For background, see the post and ensuing conversation on the possible folly of attempting to live a modern lifestyle that is quote-unquote sustainable. The map above is supposed to show population density - concentration of red - around the world. I can't verify it, but it seems reasonable to me.

Overpopulation and environmental issues: how intertwined are they? And how different is the relationship between these issues in different parts of the world?

I ask because I often hear the subject of overpopulation come up in environmental discussions; but aren't the environmental issues related to overpopulation vastly differently in sub-Saharan Africa than in a suburb of Cincinnati? In what locales/situations is overpopulation enough of an issue (in terms of both impact and tractability) to warrant problem-solving action?