An implication of las casas’ argument is that a major cause of the decline of the native populations in the americas after 1492 was the:

Epidemics (diseases) brought to the Americas by Europeans.

Explanation:

With the discovery of America by Christopher Columbus in 1492, led the beginning of the Columbian Exchange. Europeans started establishing colonies in the New World. Spanish settlers made trading connections with the natives and exchange goods and materials. The European not only traded good but they also brought diseases in the New World,  that led to the decline of the entire civilizations. Majority of the Native American died with coming connecting with smallpox, chickenpox, cholera, malaria, measles, scarlet fever, etc.