During late Roman times the provinces Africa were major grain and olive oil exporters to the Western Roman Empire. For that reason it was a major loss when they fell to the Vandals. However, in the modern day I've never had the sense that Algeria or Tunis have much of an agricultural surplus. Is that just a rising population eating up the surplus or has the amount of food produced really declined. And if it has declined, when and how did it do so?
Recently read a really wonderful book called "The Inheritance of Rome". Memory is failing me a bit at the moment, but Wickham covers this area with depth and vigor:
The Vandals invaded pretty much lived off what the Romans had set up. This coincided with a fairly large localization of trade in the lands of the (most would say former) Western Empire. Some part of this had to do with the Vandals becoming some pretty hardcore pirates, sacking so many Eastern and Western Roman cities that the old English word for the Mediterranean means Sea of the Vandals. Therefore, while trade still existed, the grain was disrupted by piracy and not as highly desired (it played a big part in the Roman Empire's tax system to feed the Army, so it was actually overgrown in most areas compared to a "normal" market) therefore there was a decrease in trade / production. The Eastern Romans reconquered the area under Justinian I, but soon after the Muslims swept through the area that part of the Mediterranean became quite often unsafe for Roman traders.
Routes to Sicily (bread basket in it's own right during this period), Egypt (huge breadbasket too) and Rome (still had significance to Constantinople, though wasn't nearly as important anymore) were still defensible and well patrolled by the Eastern Roman Navy. Egypt continued to be the Breadbasket for the Eastern Roman Empire and extremely valuable as a province until they lost it to the Muslim conquest during the 600s. Egypt's loss was a huge blow to the Eastern Romans, and contributed quite significantly to their crisis century. They only really recovered some of their former glory when they substituted lands in the Crimea and the northern edge of the Black Sea around 800.
Hope this helps, I'd very much recommend the Inheritance of Rome, it jumps around a bit but gives a really wonderfully unassuming and wide-ranged view of the former Roman sphere of influence from 600-1000.