information in the model network that we use to describe ecosystems is minor compared to the information in the genes, but we cannot exclude the fact that the real, much more complex network may contribute considerably to the total exergy of a natural ecosystem.
3. Approximations are made in our thermodynamic calculations. They are all indicated in the calculations and are, in most cases, negligible.
4. All the components in a natural (complex) ecosystem are not known. Therefore, these calculations can only be used to determine exergy indices of our simplified images of ecosystems, for instance, of models.
5. The exergy indices are, however, useful, as they have been successfully used as goal functions (orientors) to develop structural dynamic models. The 'difference' in exergy by 'comparison' of two different possible structures (species composition) is decisive here. Moreover, eco-exergy computations always give only relative values, as the exergy is calculated relative to the reference system.
It is, in general, required that a proper holistic ecosystem health indicator should cover the following system properties (attributes): homeostasis, absence of diseases, diversity or complexity, stability, resistance and buffer capacity, vigor or scope for growth, and a suitable balance between system components. Eco-exergy meets this requirement as follows:
1. Homeostasis is required to ensure survival and eco-exergy measures survival directly.
2. Absence of disease is needed to ensure growth. All three growth forms (growth of biomass, network complexity, and information; see also Fundamental Laws in Ecology) implies increase of eco-exergy.
3. Diversity or complexity : High exergy must imply that all or almost all ecological niches are occupied and that requires of course high diversity and complexity. That all ecological niches are occupied implies, furthermore, that a number of higher organisms with a high level of information are present, that is, the specific eco-exergy is high.
4. Stability and resilience is partially covered by buffer capacity, which is shown (see above) to be proportional to exergy by a statistical analysis of model results.
5. Vigor or scope for growth — Again all three growth forms involve increased eco-exergy.
6. Balance between system components means that there are both higher and lower organisms and many different organisms present, which is the only property that is hardly covered by eco-exergy.
As eco-exergy increases with all three growth forms (see Fundamental Laws in Ecology): growth of biomass, development of the network, and growth of information - the application of eco-exergy as a holistic indicator seems justified. Moreover, all the attributes of E. P. Odum that are widely used to distinguish between an early ecosystem and a mature ecosystem are covered by the three growth forms (see Fundamental Laws in Ecology).
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