Conservation

Specific exergy = exergy / biomass

Figure 6.11 Holling's four phases of ecosystems, described terms of biomass versus specific exergy. The presentation is inspired by Ulanowicz (1997).

C. Ecosystems organize to maximize exergy degradation (Kay, 1984). Living systems transform more exergy to heat at the temperature of the environment, or said differently, produce more entropy, than their non-living complements. Living systems, therefore, increases the destruction of exergy but at the same time living systems increase the order and organization.

D. A system that receives a throughflow of exergy will have a propensity to move away from thermodynamic equilibrium, and if more combinations of components and processes are offered to utilize the exergy flow, the system has the propensity to select the organization that gives the system as much stored exergy as possible (see Section 6.5, Jorgensen and Mejer (1977, 1979), Jorgensen (1982, 2002), Mejer and Jorgensen (1979)).

E. Ecosystems have a propensity to develop toward a maximization of the ascendency (Ulanowicz, 1986; see also Section 6.5).

The usual description of ecosystem development illustrated for instance by the recovery of Yellowstone Park after fire, an island formed after a volcanic eruption, reclaimed land, etc. is well covered by E.P. Odum (1969): at first the biomass increases rapidly which implies that the percentage of captured incoming solar radiation increases but also the energy needed for the maintenance. Growth Form I is dominant in this first phase, where exergy stored increases (more biomass, more physical structure to capture more solar radiation), but also the throughflow (of useful energy), exergy dissipation and the entropy production increases due to increased need of energy for maintenance. Living systems are effectively capturing (lower the albedo) and utilizing energy (exergy) (both the exergy decomposed due to respiration and the exergy stored in the living matter are increased) from the ambient physical systems.

Growth Forms II and III become, in most cases, dominant later, although an overlap of the three growth forms takes place. The smaller the ecosystem is, which implies that it has a high relative openness (see Chapter 2), the faster will Growth Forms II and III, particularly Growth Form III, contribute to the development of the ecosystem (Patricio et al., 2006). The recovery of small inter-tidal rocky communities has been examined in this paper. It was found that biodiversity and specific eco-exergy (= eco-exergy/biomass) recover much faster than biomass and eco-exergy, i.e. the Growth Forms II and III are dominant in the initial phase of recovery.

When the percentage of solar radiation captured reaches ~80 percent it is not possible to increase the amount of captured solar radiation further (due in principle to the second law of thermodynamics). Further growth of the physical structure (biomass) does, therefore, not improve the exergy balance of the ecosystem. In addition, all or almost all the essential elements are in the form of dead or living organic matter and not as inorganic compounds ready to be used for growth. The Growth Form I will and can therefore not proceed, but Growth Forms II and III can still operate. The ecosystem can still improve the ecological network and can still evolve novel, more complex organisms and environments. One tendency is to increase the occurrence of larger, long-lived organisms (Cope's law: the later descendent may be increasingly larger than their ancestors; for instance, the horse today is much bigger than the horse fossils from 20 to 30 million years ago) and less developed with more developed and more non-nonsense genes. Growth Forms II and III do not require, however, more exergy for maintenance. Exergy degradation is, therefore, not increasing but is maintained at a constant level (see Figures 6.9 and 6.10). The accordance with the five descriptors plus specific entropy production and the three growth forms based on this description of ecosystem development is shown in Table 6.6

Table 6.6 Accordance between growth forms and the proposed descriptors

Hypothesis

Growth Form I

Growth Form II

Growth Form III

Exergy storage

Up

Up

Up

Power/throughflow

Up

Up

Up

Ascendency

Up

Up

Up

Exergy dissipation

Up

Equal

Equal

Retention time

Equal

Up

Up

Entropy production

Up

Equal

Equal

Exergy/biomass = specific exergy

Equal

Up

Up

Entropy/biomass = specific entropy

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