Conceptual diagram of the model.

We consider a four-variable system of natural/‘semi-natural’ land (N), intensive agriculture (A), unproductive/degraded land (D) and human population (H). The left panel depicts forces on the system, either anthropogenic (in blue) or natural (in black). The right panel represents feedbacks in the model: solid lines are human-driven feedbacks, dashed lines are ecosystem service (ES) feedbacks, and dotted lines are food resource feedbacks. In the right panel, the model has been simplified to a two land variable system, where N + A + D = L. L refers to the total terrestrial land area on Earth. The width of the arrows represent the rate or impact of the parameter. The parameter values and ranges are given in Table A in S1 Text.