Final answer:
Europeans turned to Africa, specifically West Africa, for slaves when the Native American population declined due to diseases.
Step-by-step explanation:
When Native Americans died in large numbers from diseases introduced by Europeans, European colonizers turned to Africa for slaves to work in the New World. This transition was largely due to the inadequate labor force left after the indigenous population's decline and the unsuitability of Europeans to the climate and diseases of the New World. West Africa became the most significant region involved in supplying slaves through the Trans-Atlantic slave trade, as part of a brutal and dehumanizing system that lasted for centuries.