Nothing apart from a massive war on the scale never seen on American soil, a massive regional or an even bigger global disaster bigger than anything in recorded history that kills millions and devastates a truly huge area or a pandemic on an astounding scale will cause any significant reduction in population in America. Quoting: Anonymous Coward 28410867
All other economic/drought scenarios although they can have a massive effect on society will not cause a significant reduction in population in America.
I suspect that you have never experienced hunger and what effects that might have upon a population, Australia experiences floods and droughts consider the impact upon your country if supply chains to your country were broken.
Next, consider urban areas whatever country whose population has food for one or two weeks tops if food no longer reached these places what would happen.
Look at the predominately-socialist government dependent in the northeast region of the US during Hurricane Sandy and how these unprepared people wailed for food, water and even underwear while the year prior hurricane Irene hit misstate NY hardest there were no screams for someone to care for them.
When conditions permitted, we ventured out and helped our neighbors and understanding snowstorms we were better prepared it makes a difference.
With urban areas the initial problem is greed, gangs that are not equipped to properly access their situation much rioting, and looting would occur as in any urban area.