Fig. 5. Ecologic and economic reserves compete for net primary productivity. The division of net primary productivity is regulated by the extraction effort (“control loop” 1). Increasing extractive effort will reduce ecological “plough-back” and diminish ecological reserves. Reduced ecological reserves will reduce rain-use efficiency (“control loop” 2) and future primary productivity. Economic reserves are bolstered by external subsidies, e.g., government payments. Critical to sustainable management is the nature of the feedback of economic reserves on extractive effort. A positive feedback will lead to overexploitation, a negative feedback may promote system resilience (see text for details).