The population of the Americas dramatically declined following the arrival of Europeans primarily because of thepolicy of systematic genocide employed by European explorers toward Native Americans. enslavement and brutal treatment of Native Americans by Europeans. lack of natural immunity among Native Americans to European diseases. loss of morale and sense of hopelessness that pervaded Native American societies.

When it comes to the Americas as a whole, the most catastrophic cause of death was disease. Europeans brought disastrous sicknesses like smallpox to Native communities who had no immunity to those diseases. Those diseases traveled along Native trade and communication routes, so even Native communities who had never met or heard of Europeans were still affected by European diseases.

For this reason, disease was more disastrous than even the many, many cases in which Europeans systematically destroyed whole populations of Native peoples, as was seen in a number of Caribbean islands.

(A really fantastic recent bestseller by Charles Mann called 1493: Uncovering the New World Columbus Created, tells some fascinating stories about how this process worked.)