And if, for instance, a devastating storm surge occurs or the banking system crashes, Alvin Jackson, a jazz musician from New Orleans, wants to be ready. Mr. Jackson, 66, was at the exposition Saturday morning, checking out the Ark 290: a month’s supply of freeze-dried food, helpfully contained in a portable plastic bucket. He had already picked up a brochure for a rain catchment basin designed to be installed on gutters. Whatever the scenario, he planned to be prepared.

“People think that preppers, and I use that term with caution, are guys in beards who live in bunkers and bury ammunition in their yards,” said Mr. Jackson, a dapper man in a pageboy cap who had come to the conference with his wife, Marlane. “But I went through Katrina, and I’m not crazy. I know from experience that things go south, and it can happen just like that.”

Image Heirloom vegetable seeds from White Harvest Seed Company. Credit... Steve Hebert for The New York Times

Mr. Jackson’s cautions notwithstanding, it would be easy to assume that a prepper convention would be peopled with right-wing zealots with a taste for guns and gold, or what survivalists like to call “the bullet-and-bullion set.” But while there was one man standing at a booth handing out business cards for Operation American Spring, a movement to impeach President Obama, there was also a countervailing element of organic gardeners, homeopathic healers and publishers selling books on the commercial uses of hemp.