The entire population of an island needs food. Without food, the peasants will starve to death, but they'll cause lots of problems before they do. It is the banker's responsibility to see that everyone is fed.
The people of your realm will grow food on their own, and harvest it, without intervention. However, it rarely does much good in the region in which it's grown: cities consume more food than they produce, while rural regions produce a surplus. Region lords and traders are able to trade food.
Sometimes harvests will fall short, for various reasons:
- Poor weather can flood, freeze, or scorch crops in the fields
- Enemy looting can destroy both crops in the fields, the means of production, and stores of food
- Reduced production can cause good crops to go to waste if not enough people are available to work the harvest
- Traders buying food can deplete food reserves
These things can cause a realm to no longer be able to feed itself. Sometimes realms cannot feed themselves even when operating at full capacity, in peacetime. If your realm does not have enough food, it will have to trade for food. Region lords and traders can often make up the shortfall by trading with friendly realms who are willing to sell their surpluses. If there are no such willing partners, sometimes other arrangements can be made ...
Seasons multiply the normal harvest amounts by a flat percentage.
Summer: Normal production level. This is what is reported on the region pages.
Autumn: 200% of summer production levels
Winter: 25% of summer production levels
Spring: 75% of summer production levels
Weather changes in a determined weather area affects the food production of all the regions in it.