Quality is a system property, which means that an 'absolute' scale of quality cannot be made, nor can the usefulness of a measure of quality be assessed without first defining the structure and boundaries of the system. Self-organizing systems (be they the biosphere or an ecosystem) are organized with hierarchical levels (Figure 3) and each level is composed of many parallel processes. This leads to two possible definitions of quality: (1) parallel quality, and (2) cross quality. The first, 'parallel quality', is related to the efficiency of a process that produces a given flow of energy or matter within the same hierarchical level (comparison among units in the same hierarchical level in Figure 3). For instance, for any given output, say biomass, there are almost an infinite number of ways of producing it (all the various species in the myriad of ecosystems in the biosphere; each with slightly different efficiencies). A recent compilation of transformities for ecosystem-level gross primary production yielded transformities from 1.0E3 to 9.2E4sejJ~1 depending on species composition, community type, and driving energies. Since each individual process has its own efficiency, the output from the process has a distinct UEV. Quality as measured by UEV relates to the emergy required to make like products under differing conditions and processes. For the most part, UEVs of like products are within the same order of magnitude.
The second definition of quality, 'cross quality', is related to the hierarchical organization of systems. In this case, UEVs are used to compare components or outputs from different levels of the hierarchy, accounting for the convergence of emergy at higher and higher levels (comparison of different hierarchical levels, in Figure 3). At higher hierarchical levels, a larger convergence of inputs is required to support the component (i.e., much plant biomass to support 1° level consumer, many 1° level consumers to support 2° level consumers, etc.). Also, higher feedback and control ability characterize components at higher hierarchical levels. Therefore, higher UEVs, as equated with higher level in the hierarchy, often exhibit greater flexibility and greater spatial and temporal effect. With this definition of quality, the higher the UEV the higher the quality of the process or product. UEVs of products from different hierarchical levels usually differ by at least 1 order of magnitude.
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