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Sustainable exploitation of renewable resources depends on the existence of a reproductive surplus, which is determined by the balance between births, deaths, and somatic growth. The reproductive surplus differs spatially and temporally as environmental conditions vary, and even in the absence of exploitation, change is the rule and constancy is the exception. Sustainable yields may be estimated by direct experimentation, observation of natural systems, or deduction from biological understanding. Each of these approaches has limitations, and for large-scale unique resources, the only way to determine the response of the population to harvesting is by direct exploitation. To find the maximum potential yield, the resource must be overexploited at some time, or very similar resources must have been overexploited. Temporal changes in environmental conditions mean that information on sustainability collected in the past may have limited applicability in the future. The unregulated dynamic of exploiters is to push the resources to overexploitation, and even when regulated, exploiters have been very successful at modifying their behavior so that regulations are less effective than anticipated. The most successful institutions at maintaining sustainability have been small-scale community or pri