Final answer:
The Native population in Canada declined after European contact primarily because they lacked immunity to European diseases, leading to widespread epidemics and high mortality rates. Warfare, forced displacements, and environmental changes from European colonization also contributed to the decline.
Step-by-step explanation:
The Native population in Canada decreased after European contact mainly due to diseases such as smallpox. Native Americans had no immunity to European diseases, which led to devastating epidemics. Furthermore, European colonization brought about warfare, forced displacements, and environmental changes, which exacerbated the decline of Native populations. The biological warfare of unintentionally introduced pathogens by Europeans caused massive mortality rates because Indigenous peoples had never before been exposed to these diseases.