Beating your Neighbor to the Berry Patch

Foragers often compete for resources that ripen (or otherwise improve) gradually. What strategy is optimal in this situation? It turns out that there is no optimal strategy. There is no evolutionarily stable strategy (ESS), and the only Nash equilibrium (NE) is unstable: strategies similar to the NE can always invade. But in spite of this instability, the NE is predictive. If harvesting attempts are costly or there are many competitors, the process tends to remain near the unstable NE. In this case, the resource often goes unharvested. Harvesting attempts—when they happen at all—usually occur when the resource is barely ripe enough to offset costs. The more foragers there are, the lower the chance that the resource will be harvested and the greater its mean value when harvested. This counterintuitive behavior is exhibited not only by theoretical models and computer simulations, but also by human subjects in an experimental game.