Brian Ward-Perkins in his book The Fall of Rome and the End of Civilization argues that the economy of the Roman empire was highly sophisticated and depended on circulating goods throughout the Mediterranean. As the empire grew, regions were able to specialize more and more on the goods they were best at. As long as Gaul and Italy could rely on African and Egyptian grain, they could focus on grapes and olives.

These trade systems faltered staring with the crisis of the third century and really got bad after the Vandals took Africa in 429. The west was particularly affected because they relied on other parts of the empire for basic goods like grain and their trade products were not particularly valuable compared to the goods from Syria and Egypt (spices from the far east, and Arabia for example).

WP's book is pretty good, if totally one-sided. It's also short and approachable. I suggest you give it a read if you are really interested.