How did the success of West African kingdoms contribute to the development of kingdoms in western and southern Africa?

As the populations of western kingdoms grew, overpopulation caused citizens to migrate east and south to build new empires.
As people from other continents came to find jobs in western empires, native citizens moved east and south to escape foreign influence.
As trade for goods from distant lands flourished, native citizens moved east and south to discover new goods and people with whom to trade.
As more foreigners arrived so did new diseases, leading citizens to migrate east and south to save their families from suffering and possible death.