Jorgensen and Fath (2004) have discussed eight basic principles or propositions of ecosystems, their properties and processes, and Jorgensen (2006) added two more. These propositions include the thermodynamic laws that are underlying all ecosystem functions, in addition to what is implicitly covered by the general properties of ecosystems in Chapters 2-7. To the extent possible, it will be mentioned below for each of the propositions how they are rooted in the seven ecosystem properties presented in Section 10.2. Interpretation of the propositions has, however, to be subject to the recognition that ecosystems are ontically open—too complex to allow accurate and complete predictions in all details. Nevertheless, let us try to set up the propositions because they can together with the properties presented in Chapters 2-7 and applied in Chapters 8 and 9 suggest new avenues to understand ecosystems:
1. Mass and energy are conserved. This principle is used again and again in ecology since it allows one to write balance equations at the core of ecosystem modeling, such as with a basic box-and-arrow diagram in which: accumulation = input-output.
2. All ecosystem processes are irreversible (this is probably the most useful way to express the second law of thermodynamics in ecology). Evolution and directionality, implicit in autocatalysis, can only be understood in light of the irreversibility principle rooted in the second law of thermodynamics. Evolution is a step-wise development that is based on previously achieved good solutions to survival in a changeable and very dynamic world. Evolution has been proceeded in the direction of ever more complex solutions.
3. All ecosystems are open systems embedded in an environment from which they receive energy-matter input and discharge energy-matter output. From a thermodynamic point of view, this principle is a prerequisite for ecosystem processes. If ecosystems were isolated in the physics' sense, then they would inevitably go to thermodynamic equilibrium without gradients and without life. This proposition is of course completely consistent with Chapter 2. It is noticeable that quantification of openness leads to an understanding of many ecological rates rooted in scaling theory and allometric principles.
4. Thermodynamically, carbon-based life has a viability domain between approximately 250 and 350K. It is within this temperature range that there is a good balance between the opposing ordering and disordering processes: decomposition of organic matter and building of biochemically important compounds. At lower temperatures process rates are too slow and at higher temperatures the enzymes catalyzing the biochemical reactions will decompose too rapidly.
5. Ecosystems have many levels of organization and operate hierarchically. This principle is used again and again when ecosystems are described: atoms, molecules, cells, organs, organisms, populations, communities, ecosystems, and the ecosphere. Hierarchy theory has been presented in Chapters 2, 3, and 7, and has been widely used to explain ecological observations.
6. Carbon-based life on earth has a characteristic basic biochemistry which all organi-sms share. It implies that many biochemical compounds can be found in all living organisms. They have, therefore, almost the same elemental composition derived from approximately 25 elements (Morowitz, 1968). This principle is widely used when stoichiometric calculations are made in ecology, i.e. an approximate average composition of living matter is applied. The proposition is able to give a biochemical explanation of feedback.
7. Biological processes use captured energy (input) to move further from thermodynamic equilibrium and maintain non-equilibrium states of low entropy and high exergy relative to surroundings. This is just another way of expressing that ecosystems can grow. Svirezhev (1992) has shown that eco-exergy of an ecosystem corresponds to the amount of energy that is needed to degrade the system. This proposition is consistent with the properties presented in Chapters 4 and 6.
8. No ecosystem organism exists in isolation but is connected to other organisms and its abiotic environment. Simply put, this states that connectivity is a basic property that, through transactions and relations, binds ecosystem parts together as an interacting and often integrated system. It can be shown by observations and ecosystem network calculations that the network has a synergistic effect on the components: the ecosystem is more than the sum of the components (see e.g. Patten, 1991; Fath and Patten, 1998). The proposition is completely consistent with the content of Chapter 5.
9. After the initial capture of energy across a boundary, ecosystem growth and development is possible by (1) an increase of the physical structure (biomass), (2) an increase of the network (more cycling), and then (3) an increase of information embodied in the system. All three growth forms imply that the system is moving away from thermodynamic equilibrium (Jorgensen et al., 2000), and all three growth forms are associated with an increase of (1) stored eco-exergy and (2) the energy throughflow in the system (power). When cycling flows increase, the eco-exergy storage capacity, the energy use efficiency and space-time differentiation all increase (Ho and Ulanowicz, 2005). When the information increases, the feedback controls and autocatalysis become more effective, specific respiration decreases, and there is a tendency to replace r-strategist species with K-strategists, which means less energy is wasted on reproduction. When earth systems (physical and biological) capture approximately 75% of available solar energy, it is not possible to increase this capture further. The same is true for limiting elements. Under these conditions ecosystems cannot benefit further from growth Form I and must graduate to growth Forms II and III. Thereby, the efficiency of exergy utilization is increased. This description is in accordance with Margalef (1991, 1995): the first stages proceed rapidly with an apparently wasteful use of available energy; later a higher efficiency along a defined direction occurs, because of competition, in the frame of natural selection. Growth Form I is constrained by the conservation of energy and matter, while the two other growth forms are not following the conservation laws. In ecosystem succession the information is transferred from the present to the future and the shift is manifested in a historical way that has many aspects. One of these, the production and accumulation of biomass, prevails at the beginning, and this is often described as "bottom up" control. Later, the high trophic levels take more control, and "top-down-control" becomes more apparent. This proposition has been presented in Chapter 6 and partly in Chapters 4 and 7. It has furthermore been applied several times in Chapters 8 and 9.
10. An ecosystem receiving solar radiation will attempt to maximize eco-exergy storage, ascendency, or maximize power such that if more than one possibility is offered, then in the long-run the one which moves the system furthest from thermodynamic equilibrium will be selected. Eco-exergy storage increases with all three growth forms—see above. When an ecosystem develops it can, therefore, apply all three growth forms in a continuous Darwinian selection process. It is intuitively obvious why the nested space-time differentiation in organisms optimizes thermodynamic efficiency as expressed in the tenth proposition because it allows the organism to simultaneously exploit equilibrium and non-equilibrium energy transfer with minimum dissipation (Ho and Ulanowicz, 2005). This proposition has been touched on in Chapters 4, 6, and 7 and been applied several times in Chapters 8 and 9.
As seen from this short overview of the ten propositions they may be considered a useful organization of the basic system ecology needed to understand the ecosystem reactions and processes. The organization in propositions are different from the basic properties that were applied as the fundament for the presentation of our "New Ecology—A System Approach" shown in Chapters 1-9. It is, of course, not surprising that we need different descriptions of ecosystem processes and responses. Ecosystems are complex systems as touched on several times throughout the book. So, how many different descriptions do we need to describe ecosystems when we consider that a simple physical phenomenon as light requires two descriptions, as waves and as particles?
We, the nine authors, conclude, however, that we do have an ecosystem theory, that can be presented in different ways but under all circumstances can be used to explain and understand ecological observations, properties, and processes (Chapter 8) and even be applied in environmental management (Chapter 9). The theory should be considered one of the first attempt to present an (almost) complete ecosystem theory. It is most likely that it will changed in the coming years as we gain experience by using it, but it can anyhow today as hopefully demonstrated for our readers be applied to explain ecology and to develop a good environmental strategy for ecosystem management. Further development of an ecosystem theory in system ecology will only be possible by trying to use theoretical approaches in ecology and environmental management. We, therefore, encourage all ecologists and environmental managers to apply the presented theory.
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