Final answer:
The most significant decline in Native American populations during the sixteenth century resulted from European diseases like smallpox, not from direct combat.
Step-by-step explanation:
The greatest decrease in the Native American population during the sixteenth century was predominantly caused not by direct combat, but by the devastating effects of European diseases. These diseases, such as smallpox, diphtheria, and measles, led to catastrophic declines in Native populations because they had no prior exposure or immunity. Evidently, while the threat from European colonizers in terms of warfare and forced relocation was significant, the primary driver of the population reduction was infectious diseases, which, in some cases, wiped out up to 90% of Native American communities. The question as to whether this was a planned genocide remains a matter of debate among historians, although there are instances that suggest deliberate spread of disease, like the infamous smallpox-infected blankets.