The developing nations that would be the ones bearing the brunt of mass starvations from food shortages are also ones many countries around the world are coming to rely upon for natural resources to maintain developed nation economies, as their natural resource bases are depleted.
Let's say a mass famine whipped across much of Africa. Such a famine would likely cause political upheaval, revolutions, bloody civil wars. As such, countries with deep interests in that part of the world (US, Europe, China, Russia, etc) would start taking sides to make sure the winners would give them the most favorable terms for continuing resource extraction.
What exactly do you think would happen if the world's superpowers got into a pissing match over resources vital to their economies and national security? What if the very intervention by foreign nations caused another terrorist organization to spring forth and start striking targets around the globe?
A new Cold War could very well emerge as poor nations fall and rich nations fight over their bones. And cold wars don't necessarily stay cold forever.
Even without resource wars or terrorism breaking out, the loss of disposable income across the planet as food prices skyrocketed, even in the rich nations, could bring about another recession and/or depression in said rich nations. And once again, this has a tendency to create social and political tension, to the point of civil unrest, riots, and revolutions.
Food shortages won't drive humanity to extinction, but it stands a good shot at driving much of the civilized world back to a state of near-anarchy (think Somalia).
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