Cleita, you poor stupid neo-Romantic hippy-brained moron...North America might support something on the order of a grand total of 3 million hunter-gatherers, nomadic semi-pastoralists, and no-tech subsistence farmers (Depending on the region), once the wildlife had a chance to restore and reestablish itself, and the fences rotted and rusted away. Of course that means a maximum life expectancy somewhere south of the big Five-Oh, and women popping out babies from the age of about 15, half of which would die before age 5 (Probably more for quite awhile, with all the bad genes modern medicine has preserved in the gene pool). Given a present US population of over 350 million, not even counting Canada and Mexico, that means even less than one out of a hundred now living would be able to survive such a switch.