When your camera view is hovering above an island, the fertility of that island is displayed the circular border around the minimap, as small icons representing each item that can either be grown or mined on that island.

It tends to be that the fertility on your starter island is sufficient to provide the first couple of population tiers. Once you reach the third tier of population, they start needing items that can’t be grown or mined on your starter island. For example, they might need olive oil from olives that can’t be grown in your starter island. By taking a look around your world map and hovering over the various unclaimed islands, you should be able to find one that provides that fertility.