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By means of a systemic analysis of the relationships among components of an ecosystem's web, the flows of energy and other resources converging to produce the output (biomass, biodiversity, assets, etc.) can be evaluated on a common basis, i.e. the content of solar equivalent energy (emergy). Indices and ratios based on the emergy flows can be calculated and used to evaluate the behaviour of the whole system. In this paper, one of these indices, the emergy yield ratio η (total yield emergy per unit of emergy invested) is evaluated and suggestions made to modify it to account for present and future environmental damages due to the use of a given resource. The meaning of this index, with and without the proposed modification, is stressed illustrating the long-term effects of environmental pollution as well as some key uncertainty factors that are very often not taken into account. Odum, 1993

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