This paper presents a stylized bioeconomic model of hunter-gatherer foraging effort designed to study the process of intensification on open-access resources. A critical insight derived from the model is that the very success of an adaptation at the level of an individual forager group can create system-level vulnerabilities that subsequently feed back to cause emergent social change. The model illustrates how the intensification of harvest time by individuals within a habitat creates a forager-resource system that becomes vulnerable to perturbations. When the system is vulnerable, it is characterized by two resource harvest equilibria: a sustainable, low-effort equilibrium and a degraded, high-effort equilibrium. In this situation, the forager-resource system can be shocked back and forth between these different equilibria by perturbations, generating considerable risk for foragers. We use the model to isolate the ecological conditions under which the instability of the system generates the risk that foragers will experience a shortfall of resources, and we suggest a mechanism that might lead foragers to adopt social institutions that regulate who can access a habitat as an adaptive response. As an illustration of the potential utility of the insights drawn from the model, comparisons are made with a substantial ethnographic data set.