Complex sentence: The only population that has increased on the plains is that of various Native American nations, some of which have also started to raise bison, in part for tourism and ecological value, as well as its primary place in their traditional cultures.
Split into simpler sentences: The only population that has increased on the plains is that of various Native American nations. Some of these have started to raise bison, in part for tourism and ecological value, as well as its primary place in their traditional cultures.