Why did the spanish and portuguese increasingly rely on dark-skinned africans to serve as slaves in their colonies in the atlantic and the new world by the 16th century?

The Spanish and Portuguese began to strong rely on dark skinned Africans to be slaves in the Atlantic colonies and the New World by the 16th century because so many of the people who were native to the area had been worked so hard they had died, some had perished from disease brought over from Europe. The Europeans needed a cheap labor source and the dark skinned Africans fit the bill.