Why did Britain change its strategy to focus on the Southern colonies in the summer of 1778?

The French allied with the colonials, and the British believe they had supporters in the south.

Explanation:

The French signed a treaty with the colonials in 1778 which forced the British to rethink their tactics.  They were now having to fight both the newly formed Americans and the French.  The British falsely believed they had more colonial supporters in the south, so began to focus their efforts there in attempt to subvert the French involvement.  This lead to the colonials fighting the British in the south with guerrilla warfare tactics instead of proper field combat, as the British were use to.  For America, the French helped a ton when they joined the war (Thanks Benjamin Franklin).

they remained in rural areas and worked at jobs such as lumbering or farming.

interdependence is when all participating groups and individuals depend on each other. if you depend on one anothers goods, you end up trading in order to get the items you're looking for. (for ex: if i have food and you have water, we trade in order to depend on each other to live)