Final answer:
The introduction of new diseases was the greatest short-term effect of the Columbian Exchange on American Indians, decimating up to 90 percent of their population.
Step-by-step explanation:
The greatest short-term effect of the Columbian Exchange on American Indians was the introduction of new diseases. European explorers and conquerors brought with them disease-causing bacteria and viruses to the western hemisphere, leading to epidemics among Native American populations who had no natural resistance to these diseases. Consequently, infectious diseases such as smallpox caused the death of an estimated up to 90 percent of Native Americans, having a catastrophic impact on their populations.