Open systems and their energy balance

Ecosystems are open systems in the sense that they are open for mass and energy transfer. Ecosystems receive energy from solar radiation and water from precipitation, dry deposition from the atmosphere, inputs by wind and flows of various types plus migration of species. A system that is closed for in- and outputs of energy and mass is called an isolated system, while a system that is closed to in- and outputs of mass, but open to energy transfers, is named a closed system. A non-isolated system is a closed or open system. If an ecosystem is isolated, it would inevitably move toward thermodynamic equilibrium and become a dead system with no gradients to do work—or, as expressed in Chapter 2, dG = 0 and dS = 0 at a maximum S value. The openness explains why an ecosystem can maintain life and stay far from thermodynamic equilibrium because maintenance of life requires input of energy, which of course is only possible if an ecosystem is at least non-isolated.

The use of the Second Law of Thermodynamics for open systems is crucial. At first glance, it looks like ecosystems violate the Second Law because they are moving away from thermodynamic equilibrium by formation of a biological structure. Ecosystems receive, however, energy as solar radiation, which can compensate for the steady transfer of work to heat. Several proposals on how to apply the Second Law of Thermodynamics on systems far from thermodynamic equilibrium have been given, as will be demonstrated in Section 3.3, but before we turn to this central issue

Towards a Thermodynamic Theory for Ecological Systems, pp. 41-67 © 2004 Elsevier Ltd. All Rights Reserved.

for open systems, what openness implies for the properties of ecosystems and how it is possible to quantify openness will be discussed.

In ecosystem steady states, the formation of biological compounds (anabolism) is in approximate balance with their decomposition (catabolism). The energy captured by an ecosystem can, in principle, be any form of energy (electromagnetic, electrical, magnetic, chemical, mechanical, etc.) but for the ecosystems on Earth the short-wave energy of solar radiation (electromagnetic energy) plays the major role. One must keep in mind that there are only four types of energy in our physical world: gravitational, electromagnetic, and energies of strong and weak interactions (in microphysics). All biosphere processes use only electromagnetic energy. Until humans were a part of the biosphere and used only an electromagnetic form of energy, we should hope that all so-called "ecological crises" could be resolved. But as soon Homo sapiens began to use the two latter forms of energy (nuclear power plants) and, moreover, intends to use the thermonuclear synthesis, she enters into a principal contradiction with the biosphere. As a consequence, these new crises apparently become unresolved. However, this is a speculative hypothesis, too.

The following reaction chain summarises the consequences of energy openness (J0rgensen et al., 1999): source: solar radiation ! anabolism (charge phase): incorporation of high quality energy, with entrained work capacity (and information), into complex bio-molecular structures, entailing anti-entropic system movement away from equilibrium ! catabolism (discharge phase): deterioration of structure involving release of chemical bond energy and its degradation to lower states of usefulness for work (heat) ! sink: dissipation of degraded (low work capacity and high entropy) energy as heat to the environment (and, from Earth, to deep space), involving entropy generation and return toward thermodynamic equilibrium.

This same chain can also be expressed in terms of matter: source: geochemical substrates relatively close to thermodynamic equilibrium! anabolism: inorganic chemicals are moulded into complex organic molecules (with low probability, it means that the equilibrium constant for the formation process is very low, low entropy, and large distance from thermodynamic equilibrium)! catabolism: synthesised organic matter is ultimately decomposed into simple inorganic molecules again; the distance from thermodynamic equilibrium decreases, and entropy increases ! cycling: the inorganic molecules, returned to near-equilibrium states, become available in the nearly closed material ecosphere of Earth for repetition of the matter chargedischarge cycle.

Input environments of ecosystems serve as sources of high quality energy whose high contents of work and information and low entropy raise the organisational states of matter far from equilibrium. Output environments, in contrast, are sinks for energy and matter lower in work capacity, higher in entropy, and closer to equilibrium. Since, in the organisation of ecosystems, output environments feed back to become portions of input environments, living systems operating in the ecosphere, which is energetically non-isolated but materially nearly closed, must seek an adaptive balance between these two aspects of their environmental relations in order to sustain their continued existence.

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