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Margari Hill, while a graduate student at Stanford, wrote a paper on this topic, "The Spread of Islam in West Africa: Containment, Mixing, and Reform from the Eighth to the Twentieth Century.". She divided the spread of Islam in West Africa into three stages: containment, mixing, and reform. To answer your question, though, while Arabs initially spread Islam, eventually native West Africans did.
Another scholar explains: "Islam was introduced into North Africa by the Arabs during their meteoric sweep across the whole of North Africa--from Egypt to Morocco--between A.D. 641 and 708. Among the soldiers and freebooters were missionaries who were bent on converting the peoples of the Sahara and the interior of Africa to Islam." Adu Boahen, Topics in West African History (Longman 1966), 9-10. Some leadership in West Africa was converted, but not the kings of Ghana, who nevertheless "allowed complete freedom of religion and worship and even employed some of the Muslims as civil servants." Id. at 10. In the mid-11th century, the Almoravid movement, "dedicated to the spread of Islam by means of the jihad," took Ghana's capital for a period of time and weakened the state, eventually contributing to its downfall. Id. at 10-13. The Almoravids were Berbers. Flash forward to the 18th century, though, and you will encounter the Fulani jihad against the Hausa kingdoms. See Mervyn Hiskett, The Sword of Truth: The Life and Times of Shehu Usuman Dan Fadio (OUP 1973).