Introduction

Ecology has given many examples of numeraires that can be used as indicators of performances in ecosystem analysis. With the same scope thermodynamics and general system theory have developed functions that have been widely used as holistic indicators. In the intention of those who invented or adapted these concepts, these functions are 'orientors' because they show tendencies in complex, adaptive, hierarchical systems: according to H. T. Odum, toward a maximization of the emergy flow; while according to S. E. J0rgensen toward a maximization of eco-exergy content. These two approaches are not necessarily in contrast. On the contrary, they describe the possible behavior of a system at different stages of its development.

Exergy-oriented researchers root themselves in the terrestrial specificity defining, as a reference state, the mean composition of the Earth's crust, or of the atmosphere, or of a peculiar local context, considered in a steady state and without introducing further assumptions about the datum.

Emergy-oriented scientists, on the other side, base their descriptions on a previous knowledge of the biosphere that has a very general tendency in concentrating energy in more and more condensed forms through trophic chains and webs, metabolism of organisms, as well as through bio-geochemical cycles.

With exergy we have a measure of a system's distance from thermodynamic equilibrium, with a snapshot of our environment (or of a more restricted, local bulk), identified with its mean values.

On the other hand, the emergy description is more dependent on the actual metabolism of the biosphere and its evolutionary history, building its transformities - the coefficients used to express the ecological 'value' of a material, a flux, or a specific good - on that background.

An exergetic potential, or an emergy storage, could be either a resource or a toxic, depending on the specific ecological meaning that it will express when it comes in contact with a specific organism, or an ecological association in the environment.

The ratio of eco-exergy to empower can indicate the trend of an ecosystem development, because it links information coming from exergy with the one coming from empower.

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