That’s an incorrect hypothesis. Tall grass prairie, while definitely manipulated by indigenous people, doesn’t really require management; it’s the climax community for the biome. Further, fringe areas, like parkland, actually encroach on grasslands, not the other way around.

Grasses are disturbance specialists, and prairie has a natural and short fire cycle that maintains this disturbance. Take away the disturbance and you get woody species coming in on the fringe areas. In this regard, First Nations would burn parkland to create more area for grassland. If their population were declining, the lack of management would result in less bison habitat, not more.

E: I’m hilariously lost with the original comment - everyone point and laugh please. Lmao.