Many, many years ago I saw a piece on the TV, which showed the effects of population.  It was a closed cage, a few feet on a side, such that it fit in a laboratory room.  In it were rats.  The point of the experiment was to document the changes, if any, in the physical and behavioral aspects of the rats as their numbers increased.  Suffice it to say, nothing good came of it to the rats.  I think this guy was the experimenter; certainly he did such experiments.  Wikipedia was down when I wrote this up; now it's back so here's the write up.  There they are.

What got me thinking about the rats was an earlier story about American's appetite for out of season and out of country foods; blueberries in February and the like.  (I didn't save the link; this one looks similar; it's AP so would end up nearly everywhere.)  

Now, put this (imported food dependence) together with the primary, secondary, tertiary breakdown used by economists to describe what an economy makes, and what is a buck worth?  Or put another way, as physical resources become scarce, what will the US have to trade for real stuff?  Who wants CDOs for food?  Have a nice day.