Usame's Archipelago, Novel Setting

Monteleone says the setting is like words: one of the most important things in a novel.

So, for my novel, I am thinking about a setting--this is just a rough draft:

Usame's Archipelago is unique: some islands lay north of the Tropic of Cancer, like Usame, and hence, have a temperate climate where coffee trees can not be grown.

On the other hand, many islands lay south of the Tropic of Cancer, and therefore, coffee trees, sugar cane, and other tropical crops can be grown.

Usame is the richest and largest island, so the rest of the archipelago depends economically on Usame. They also have the strongest army, so no one wants to play with Usame.

If coffee is forbidden, many islands won't be able to survive because their whole island is planted with coffee trees: it is not easy to plant something else when your trees are old, and especially when you do not know how to plant anything else.