This is the simplest possible niche-construction IR controller. It assumes that a population P metabolises Food into Poo when it is above a threshold value Theta, but metabolises Poo into Food (!) when P falls below this threshold. Food and Poo therefore integrate over time P's divergence from Theta, and this information is used to combat that divergence.
Notice that the 'rein' contribution to this controller arises from the fact that P can metabolise Poo into Food, thus also correcting for values of P below Theta. From thermodynamic considerations this is only possible if the system is open to energy input from an external source. Indeed, this model is precisely the structure of the chemotaxis tumbling circuit in E. coli, which relies on an external supply of energy in the form of ATP.
On the other hand, a closed system can only achieve IR control if it has at least two competing species to pull the 'reins' in the two different directions. That is the subject of the next model.