Although the enslavement of the indigenous American populations was utilized, you also have to realize the huge, depopulating impact European contact had on them. The introduction of Old World species was extremely destabilizing to the lifestyles of native peoples. The introduction of diseases such as small pox would often decimate populations, leaving only a fraction of those originally there. In fact we have recorded instances of diseases reaching the west coast far before European human contact was made and affecting communities there. The widespread depopulation of Americans coupled with instances of armed resistance hardly made them a competitor for the labor from Africans. I can't access any sources right now but I believe some claim the Indigenous population was reduced by 90% over a period spanning a century and some.

You should check out Ecological Imperialism by Alfred Crosby, he stresses the effects of European disease on the American populations quite well in addition to other species.