Why did spanish colonists begin to rely more heavily on the atlantic slave trade by the mid-1500s?

 Sharp decrease in numbers of native Indians.

One  of the main reasons why Spanish conquistadors had to look for slaves from Africa, was the radical decrease in the populations of native Indians. The Indians had either died out of diseases or during the war of conquest. In  addition, inter marriages had also occurred, and usually the mestizos were exempted from months of forced hard labor in the farms and the mines. However the main cause of decline of populations of native Indians was diseases such as small pox which the Spanish colonists had brought with them.

The theory is land mass basically what they said.