Marginal Value Theorem

The marginal value theorem is a formal model in behavioral ecology that predicts when a forager should leave a resource patch. Formulated by Eric Charnov in 1976, it states that a forager should depart a patch when the instantaneous rate of resource intake in that patch drops to the average intake rate available across all patches. The theorem assumes that resources are patchily distributed, that travel between patches incurs time costs, and that the forager has perfect information about the environment. It has been tested across taxa from bees to human shoppers. In practice, real foragers often stay longer than predicted — a deviation that reveals the importance of risk aversion, information uncertainty, and the non-stationarity of real environments. The theorem remains foundational despite these deviations because it isolates the logic of patch exploitation from the noise of implementation.