There were several factors that contributed to long-term population decline among Native American communities in the 18th century. One of the most significant factors was the impact of European colonization and the spread of European diseases, which had a devastating effect on Native American populations. Many Native American communities were devastated by outbreaks of diseases such as smallpox, influenza, and measles, to which they had no immunity. These diseases spread rapidly through Native American communities, leading to high mortality rates and long-term population decline.
Other factors that contributed to Native American population decline in the 18th century included land loss and displacement, as European settlers increasingly encroached on Native American territories, and conflicts and wars with European powers. Additionally, Native American communities were often disrupted and displaced by the expansion of the fur trade, as well as by the introduction of new forms of economic and social organization that were brought by European colonizers.
It is important to note that the causes of population decline among Native American communities in the 18th century were complex and varied, and that the impact of these factors was different for different communities and regions.