By using this particular eco-exergy based on the same system at thermodynamic equilibrium as reference, the exergy becomes dependent only on the chemical potential of the numerous biochemical components that are characteristic for life. It is consistent with Boltzmann's statement, that life is a struggle for free energy, that is the work capacity in classic thermodynamics.
As observed above, the total eco-exergy of an ecosystem cannot be calculated exactly, as we cannot measure the concentrations of all the components or determine all possible contributions to eco-exergy in an ecosystem. Nor does it include the information of interactions. If we calculate the exergy of a fox for instance, the above shown calculations will only give the contributions coming from the biomass and the information embodied in the genes, but what is the contribution from the blood pressure, the sexual hormones, and so on? These properties are at least partially covered by the genes but is that the entire story? We can calculate the contributions from the dominant biological components in an ecosystem, for instance by the use of a model or measurements, that covers the most essential components for a focal problem. The difference in exergy by comparison of two different possible structures (species composition) is here decisive. Moreover, exergy computations always give only relative values, as the exergy is calculated relative to the reference system. These problems will be treated in further details in Chapter 6. For now it is important to realize that it is the metaphorical quality of the exergy concept, and not its measurability, that is most useful to ecologists. Entropy and exergy can both not be measured for ecosystems. It is not always necessary in science to be able make exact measurements. Ecologists rarely do this anyway. Approximations can yield an approximate science, and that is what ecology is. Modeling in particular approximates reality, not duplicates it, or reproduces it exactly because it is impossible due to the high complexity (see also next chapter). Approximate ecology—it can be quite useful and interesting ecology that can be used to quantify (approximately) for instance the influence of anthropogenic impacts on ecosystems. Often concepts and theories, not only measurements, make science interesting. With all the short-comings presented above, eco-exergy gives an approximate, relative measure of how far an ecosystem is from thermodynamic equilibrium and thereby how developed it is. Such assessment of important holistic ecosystem properties is important in systems ecology as well as in environmental management. This explains how eco-exergy has been applied several times successfully to explain ecological observations (see Jorgensen et al., 2002 and Chapter 8) and as indicator for ecosystem health (see Jorgensen et al., 2004 and Chapter 9).
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