A population of 15 villagers is really more than those two food sources can comfortably support. The amount of food available from both the farm and the berry bush is limited, and, once it's depleted, there will be no more available until it regrows.

Your villagers probably won't starve, though, because they can survive a short period famine. To make life easier for them, you really need to research that last level of harvesting so that your villagers will have an unlimited food source.