Which development caused a sharp decline in population levels in the Americas beginning in the 15th century?

It was diseases that came from European explorers that caused the decline in population. Many of the Native Americans had no defenses for the disease that the Europeans brought with them. As a result, when they got sick, the results are devastating. Many died from diseases like small pox, measles as well as diseases from Europe.

The sharp decline in population levels in the Americas beginning in the 15th century was caused by the fact that the Europeans brought their own diseases with them which were very deadly for the native Americans of the time. They didn't have the required antibodies and were therefore killed by the foreign invading viruses.