what was the reason for the population decline during that time?
After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly.
Population decline
After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly.
the culture of the aztec empire was based around which geographic form Central Mexico this culture became popular between which time period? Post-classic period from 1300 to 1521 which language was spoken by the people of that time? Nahuatl language
Arrival of the Europeans in Mexico and the conquest. After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly. This was largely the result of the epidemics of viruses brought to the continent. After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly.
Aztecs Aztecs Aztecs
Population decline Population decline Population decline
After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly.  After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly. This was largely the result of the epidemics of viruses brought to the continent against which the natives had no immunity.  After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly. 
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Aztecs [SEP] Population decline
After the arrival of the Europeans in Mexico and the conquest, indigenous populations declined significantly. This was largely the result of the epidemics of viruses brought to the continent against which the natives had no immunity. In 1520–1521, an outbreak of smallpox swept through the population of Tenochtitlan and was decisive in the fall of the city; further significant epidemics struck in 1545 and 1576. There has been no general consensus about the population size of Mexico at the time of European arrival. Early estimates gave very small population figures for the Valley of Mexico, in 1942 Kubler estimated a figure 200,000.