The answer is Both empires used natural resources as the basis of their economies.
Both Songhai and Mutapa empires based their economy mainly on natural resources.
The Songhai Empire (1430-1591) specialized in farming crops such as beans, sheep, onions, millet, and papaya, that they would later sell or trade. Fishing and trading fish were also important, as well as the trading of kola nuts, gold, ivory, spices, palm oil, and precious woods in exchange for salt, cloth, arms, horses, and copper. To trade slaves was very common as well.
As for the Mutapa Empire (1430-1760), its main source of income was the selling and trading of gold, they usually traded it for luxuries like ceramics and beads. Its economy was also based on the exploitation of copper, ivory, salt, iron, tin, and soapstone (a soft rock). They used to grew cotton and produced cotton cloth as well. They also traded other items for livestock which included sorghum (a widely cultivated cereal), millet (a fast-growing cereal plant), ground beans, cow-peas, and bananas from Indonesia.