The bison show what happens when you swap the grazer for the plough.
Tens of millions once roamed the Great Plains, grazing, trampling and fertilising the grass as they moved. The prairie under them built some of the richest topsoil on earth.
By 1889 fewer than 1,000 were left. The plough moved in behind them.
Within fifty years, that topsoil was blowing away in the Dust Bowl.
The herds took thousands of years to build it. The wheat took about two generations to lose it.