Ghana was a very powerful empire at a certain point. Ghana became powerful by gaining control of the trade routes, and wealth through trade in salt and gold. Soon, this empire had to come to an end when they got invaded, overgrazed and, eternal rebellion by the Almoravids.
Then, came along Mali. They gained power by conquering nearby kingdoms including Ghana. They took over many of Ghana's riches, like the gold and salt trade route. Soon enough, it was time for the great empire to fall this empire fell because of its great amount of people and as soon as the greatest ruler of time died "Mansa Musa" they had weak rulers to come.
Lastly, came along Songhai. In the 1300's the lands of songhai lay within the empire of Mali, which was at the height of its power. As mali weekend a people called the songhai grew in strength.