And by increasing perceptions of shortages, the restrictions have led to hoarding around the world, by farmers, traders and consumers.

“People are in a panic, so they are buying more and more  at least, those who have money are buying,” said Conching Vasquez, a 56-year-old rice vendor who sat one recent morning among piles of rice at her large stall in Los Baños, in the Philippines, the world’s largest rice importer. Her customers buy 8,000 pounds of rice a day, up from 5,500 pounds a year ago.

The new restrictions are just an acute symptom of a chronic condition. Since 1980, even as trade in services and in manufactured goods has tripled, adjusting for inflation, trade in food has barely increased. Instead, for decades, food has been a convoluted tangle of restrictive rules, in the form of tariffs, quotas and subsidies.

Now, with Australia’s farm sector crippled by drought and Argentina suffering a series of strikes and other disruptions, the world is increasingly dependent on a handful of countries like Thailand, Brazil, Canada and the United States that are still exporting large quantities of food.

On a recent morning here in Bangkok, sweaty and heavily tattooed dock workers took turns grabbing 120-pound sacks of rice from a conveyor belt and carrying them on their heads to cranes that whisked the sacks deep into the hold of a freighter bound for the Philippines. Most of the one million tons of rice that leaves the dock here each year follows the same spine-crushing routine.