The shift toward more terrestrial resources in later Buckeye Knoll populations might reflect several things.

Population growth may have put more pressure on the most productive aquatic resources, driving groups to increase their reliance on terrestrial game.

Climate change may have altered the productivity of the river system.

Or the shift may reflect changes in where groups were spending their time — if later populations were ranging more widely across the uplands and less focused on the river corridor, their diet would naturally reflect more terrestrial resources.

The isotope record cannot by itself distinguish among these possibilities.