The correct answer to this open question is the following.
The effect of conquest and settlement on the total population of native civilizations was that the European conquerors brought killer diseases that almost wiped out 80 percent of the Native American Indians tribes.
That was the case. for instance, of the Spanish conquerors in Mesoamerica, the Caribbean, and South America. Spanish people brought diseases such as cholera, smallpox, chickenpox, typhus, influenza, and measles. And of course, Native Indians had no defenses in their immune systems to that unknown diseases for them.