For some important resources the almost nent within the next few decades. Estimates not be based on current consumption rate must account for the probable increase of tion of' "dynamic life time", which can be share will accelerate the

exhaustion of stocks is immi- "life time" of resources must a "static" life time index) but rate. This leads to the calcula-shorter than the static life time. Calculation of static and dynamic life time can at best serve to determine the bounds of actual life time of a resource. As a resource becomes scarce, its consump- tion must approach zero thus lengthening the calculated life time. The relative amount of remaining resources, i.e. scarcity, will therefore determine the development of the consumption rate. If material is recycled, it is important to know how quickly a product is scrapped and material is returned to the production process. A model de-scribing the dynamics of nonrenewable resource use must account for these processes.

Fisheries represent an interaction between ecological and economic systems. All else being equal, fish populations can sustain fishing indefinitely if extraction rates are below renewal rates, but above this, catch starts to fall with increasing fishing effort. Economic pressure makes it difficult to stay below those limits. 

It is necessary for fishers to meet their costs to keep fishing over time, so a minimum profit must be met; as more people join the fishery, there is less available for each person fishing. Unmanaged fisheries are often over-exploited so that catch is much lower than it could be. Proper management often means putting limits on people - limiting the number of boats or number of fish that can be caught.