So you actually want to put the importance of Africa as a grain supplier waaaay further back: Africa had to supply grain to Italy as early as 146, B.C., it was one of the specific conditions of the province that a tithe was paid on the produce of the grain crop, and Africa, Sicily, and Sardinia provided the vast majority of the city's food supply until Egypt was made a province. Grain shortages within the city were not especially novel: Livy records a number of incidents when bad harvests forced the early city to buy grain from neighboring towns, which were often more fertile than the area around Rome. The need to constantly import grain, however, begins mostly in the latter half of the 2nd Century, B.C., although already in the 3rd Century the Romans were relying on Sicilian imports to some degree: beginning in 263 the Syracusans, though still independent, began supplying the city with grain during shortages, from 210 on the Sicilians paid a provincial grain tax, and Masinissa sent grain to the city from Numidia between 200 and 170. The population of the city, and of the Italian cities in general, should not be understated. By the 1st Century, B.C. Rome had over a million inhabitants, and significantly more seasonal inhabitants during the agricultural off season. The strain that this put on the surrounding agricultural areas was simply impossible to bear--when grain imports were cut off in late antiquity the artificially-inflated population sank back down into the tens of thousands, the size that the suburban rural districts were able to independently feed. Do not, however, make the mistake of thinking that Italian fields were unproductive. They were not necessarily as fertile as Italian or Sardinian fields, but the main obstacle of the Italian grain supply was transport. Campania, Apulia, and the Po river valley are still the most fertile parts of Italy--I can tell you from experience that Apulia is basically wheat fields as far as the eye can see (which is pretty far, because out there it's flat like a pancake)--and their production was significant. In earlier periods such regions had been important for the urban food supply, but by the 2nd and 3rd Centuries, B.C. there were simply better alternatives. It was expensive to haul grain overland from Apulia or the Po--besides the distance land travel was expensive, slow, and inefficient, and in both cases the Appenines made travel more complicated. Sea transport was vastly cheaper, significantly cheaper, and just easier. I believe it's Rickman who has a calculation that hauling a boatload of grain from Egypt into the city by sea was a fraction of the cost of hauling the same amount overland from Apulia, which didn't produce as much as Egypt or Africa anyway--the provinces were as large as all of Italy, and then some, whereas the richest areas of Italian production were minuscule in comparison. Italy is not and was not infertile, but in comparison to what could be extracted from the provinces Italian production was largely insignificant, even if it wasn't cripplingly expensive to transport outside of the immediate Italian district of the harvest. The population of the city rose at the same time as foreign grain came in, but the causality here is hard to pin down--did imported grain result in an increased urban population or did the urban population grow and force the need to import grain? The former seems vastly more likely and is better supported--the urban population did not increase for many years after the beginning of provincial grain taxes from Sicily in 210, and the sudden decrease in the city's population when the grain trade was cut off in late antiquity suggests that it was the grain propping up the population, not the other way around.