First off, the North American grasslands were a managed ecosystem. The Native American agriculture of the time looked very different from how Europeans did it,

First off, the North American grasslands were a managed ecosystem. The Native American agriculture of the time looked very different from how Europeans did it, but it was still a form of agriculture. Second, it's 90 million cattle crammed into a much, much smaller space than the bison occupied. And, unlike how it worked with the bison, it's largely supported by intensive monocultures that leave little room for native ecosystems or biodiversity. Even the pastures at my friend Dan's small family organic dairy farm are something of a disaster in the biodiversity department, compared to a proper prairie.