Info

Ecosystems have openness (thermodynamic)

Without the Sun, everything on Earth dies! (From the plaintive Ukrainian folksong, ".3 ôa^HB aK Birep...")

2.1 WHY MUST ECOSYSTEMS BE OPEN?

The many lm-trees that we planted more than 30 years ago in our gardens, which may have been open fields at the time, are today more than 30 m tall. They have increased the structure in the form of stems many times and they have more than a thousand times as many leaves and have grown often more than lm in height since last spring. The structures of the gardens have changed. Today they have a high biodiversity - not so much due to different plants, but the tall trees and the voluminous bushes with berries attract many insects and birds. The garden today is a much more complex ecosystem. The biomass has increased, the biodiversity has increased and the number of ecological interactions among the many more species has increased.

When you follow the development of an ecosystem over a longer period or even during a couple of spring months, you are witness to one of the many wonders in nature: an inconceivably complex system is developing in front of you. What makes this development of complex (and beautiful) systems in nature possible?

In accordance to classic thermodynamics all isolated systems will move toward thermodynamic equilibrium. All the gradients and structures in the system will be eliminated and a homogenous dead system will be the result. It is expressed thermodynamically as follows: entropy will always increase in a isolated system. As work capacity is a result of gradients in certain intensive variables such as temperature, pressure, and chemical potential, etc. (see Table 2.1), a system at thermodynamic equilibrium can do no work. But our gardens are moving away from thermodynamic equilibrium with almost a faster and faster rate every year. It means that our gardens cannot be isolated. They must be at least non-isolated; but birds and insects and even sometimes a fox and a couple of squirrels enter from outside the garden—from the environment of the garden, maybe from a forest 1000 m away. The garden as all other ecosystems must be open (see also Table 2.2, where the thermodynamic definitions of isolated, closed, and open systems are presented). Gardens are first of all open to energy inputs from the solar radiation, which is absolutely necessary to avoid the system moving toward thermodynamic equilibrium. Without solar radiation the system would die. The energy contained in the solar radiation

Table 2.1 Different forms of energy and their intensive and extensive variables

Energy form

Extensive variable

Intensive variable

Heat

Entropy (J/K)

Temperature (K)

Expansion

Volume (m3)

Pressure (Pa = kg/s2 m)

Chemical

Moles (M)

Chemical potential (J/moles)

Electrical

Charge (A-s)

Voltage (V)

Potential

Mass (kg)

(Gravity) (height) (m2/s2)

Kinetic

Mass (kg)

0.5 (velocity)2 (m2/s2)

Note: Potential and kinetic energy is denoted mechanical energy.

Note: Potential and kinetic energy is denoted mechanical energy.

Table 2.2 Definitions of various thermodynamic systems

System type

Definition

Isolated Non-isolated Closed Open

No exchange of energy, mass, and information with the environment Exchange of energy and information, but no mass with the environment Exchange of energy and information, but no mass with the environment Exchange of energy, mass, and information with the environment covers the energy needed for maintenance of the plants and animals, measured by the respiration, but when the demand for maintenance energy is covered, additional energy is used to move the system further away from thermodynamic equilibrium. The thermo-dynamic openness of ecosystems explains why ecosystems are able to move away from thermodynamic equilibrium: to grow, to build structures and gradients.

This openness is in most cases for ecosystems a necessary condition only. For example, a balanced aquarium and also our planet are more non-isolated than open; openness is only incidental. One wonders what would be the elements of sufficient conditions. Openness is obviously not a sufficient condition for ecosystems because all open systems are not ecosystems. If a necessary condition is removed, however, the process or system in question cannot proceed. So openness (or non-isolation) as a necessary condition makes this a pivotal property of ecosystems, one to examine very closely for far-reaching consequences. And if these are to be expressed in thermodynamic terms, ecologists need to be aware that aspects of thermodynamics—particularly entropy and the second law—have for several decades been under some serious challenges in physics, and no longer enjoy the solid standing in science they once held (Capek and Sheehan, 2005). So like a garden, science is open too—ever exploring, changing, and improving. In this chapter, we will not take these modern challenges too much into account.

2.2 AN ISOLATED SYSTEM WOULD DIE (MAXIMUM ENTROPY)

The spontaneous tendency of energy to degrade and be dissipated in the environment is evident in the phenomena of everyday life. A ball bouncing tends to smaller and smaller bounces and dissipation of heat. A jug that falls to the ground breaks (dissipation) into many pieces and the inverse process, which could be seen running a film of the fall backwards, never happens in nature. Except, of course, the jug did come into existence by the same kind of non-spontaneous processes that make the garden grow. It is instructive to ponder how openness or non-isolation operate here, as necessary conditions. Perfume leaves a bottle and dissipates into the room; we never see an empty bottle spontaneously fill, although the laws of probability do allow for this possibility. There is thus a tendency to the heat form and dissipation. The thermodynamic function known as entropy (S) is the extensive variable for heat and measure therefore to what extent work has been degraded to heat. Strictly speaking, the entropy concept only applies to isolated systems close to equilibrium, but it is often used in a metaphorical sense in connection with everyday far-from-equilibrium systems. We will follow this practice here as a useful way to consider ecosystems; revisions can come later when thermodynamic ecology is much better understood from theory and greater rigor is possible. Transformations tend to occur spontaneously in the direction of increasing entropy or maximum dissipation. The idea of the passage of time, of the direction of the transformation, is inherent in the concept of entropy. The term was coined by Clausius from xponr (transformation) and svtponr¡ (evolution, mutation, or even confusion).

Clausius used the concept of entropy and reworded the First and Second Thermodynamic Laws in 1865 in a wider and more universal framework: Die Energie der Welt ist Konstant (the energy of the world is constant) and Die Entropy der Welt strebt einem Maximum zu (The entropy of the world tends toward a maximum). Maximum entropy, which corresponds to the equilibrium state of a system, is a state in which the energy is completely degraded and can no longer produce work. Well, maybe not literally "completely degraded" but rather, let us say, only "degradiented", meaning brought to a point of equilibrium where there is no gradient with its surroundings, therefore no possibility to do work. Energy at 300 K at the earth's surface is unusable, but can do work after it passes to outer space where the temperature is 3 K and a thermal gradient is re-established. Again, it is a common practice to use the term "degraded" in the sense we have, and "completely" for emphasis; for continuity in communication these practices will be followed here.

Entropy is, therefore, a concept that shows us the direction of events. "Time's Arrow", it has been called by Harold Blum (1951). Barry Commoner (1971) notes that sandcas-tles (order) do not appear spontaneously but can only disappear (disorder); a wooden hut in time becomes a pile of beams and boards: the inverse processes do not occur. The spontaneous direction of an isolated system is thus from order to disorder and entropy, as metaphor, indicates this inexorable process, the process which has the maximum probability of occurring. In this way the concepts of disorder and probability are linked in the concept of entropy. Entropy is in fact a measure of disorder and probability even though for systems like a garden it cannot be measured. Entropy generation can be calculated approximately, however, for reasonably complex systems, and for this one should consult the publications of Aoki (1987, 1988, 1989).

War is a disordering activity, but from such can often arise other levels and kinds of order. For example, a South Seas chieftain once warred on his neighbors and collected their ornately carved wooden thrones as part of the spoils and symbols of their defeat; they came to signify his superiority over his enemies and this enabled him to govern for many years as leader of a well-organized society. This social order, of course, came out of the original disordering activity of warfare, and it was sustained. The captured thrones were stored in a grand thatched building for display on special holidays, a shrine that came to symbolize the chieftain's power and authority over his subjects. One year, a typhoon hit the island and swept the structure and its thrones away in the night. The disordering of the storm went far beyond the scattering of matter, for the social order that had emerged from disorder quickly unraveled also and was swept away with the storm. The remnant society was forced in its recovery to face a hard lesson of the region—"People who live in grass houses shouldn't stow thrones!" In order to understand this order-disorder relationship better, it is useful to describe a model experiment: the mixing of gases.

Suppose we have two gases, one red and one yellow, in two containers separated by a wall. If we remove the wall we see that the two gases mix until there is a uniform distribution: an orange mixture. Well, a uniformly mixed distribution, anyway; in a statistical sense the distribution is actually random. If they were originally mixed they would not be expected to spontaneously separate into red and yellow. The "orange" state is that of maximum disorder, the situation of greatest entropy because it was reached spontaneously from a situation of initial order—the maximum of which, by the way, is the uniform distribution. Random, uniform; one must take care in choice of wording. Entropy is a measure of the degree of disorder of the system (notice that the scientific literature presents several definitions of the concept of entropy). The disordered state occurred because it had the highest statistical probability. The law of increasing entropy expresses therefore also a law of probability, of statistical tendency toward disorder. The most likely state is realized, namely the state of greatest entropy or disorder. When the gases mix, the most probable phenomenon occurs: degeneration into disorder—randomness. Nobel Prize winner for physics, Richard Feynman, comments that irreversibility is caused by the general accidents of life. It is not against the laws of physics that the molecules rebound so as to separate; it is simply improbable and would not happen in a million years. Things are irreversible only in the sense that going in one direction is probable whereas going in the other, while it is possible and in agreement with the laws of physics, would almost never happen.

So it is also in the case of our South Sea islanders. Two populations kept separate by distance over evolutionary time could be expected to develop different traits. Let one such set be considered "red" traits, and the other "yellow." Over time, without mixing, the red traits would get redder and the yellow traits yellower—the populations would diverge. If a disordering event like a storm or war caused the islanders to disperse and eventually encounter one another and mix reproductively, their distinctive traits would over a long period of time merge and converge toward "orange." A chieftain governing such a population would not be able to muster the power to reverse the trend by spontaneous means; eugenic management would be required. A tyrant might resort to genocide to develop a genetically pure race of people. Without entropy such an extreme measure, which has over human history caused much misery, would never be needed. Spontaneous de-homogenization could occur, re-establishing the kind of thermodynamic gradient (red vs. yellow) that would again make possible the further ordering work of disordering war. No entropy, no work or war—necessary or sufficient condition?

The principle of increasing entropy is now clearer in orange molecules and people: high-entropy states are favored because they are more probable, and this fact can be expressed by a particular relation as shown by Boltzmann (1905): S = — k log p, where S is entropy, k Boltzmann's constant, and p the probability of an event occurring. The logarithmic dependence makes the probability of zero entropy equal to one. The universality of the law of entropy increase (we speak metaphorically) was stressed by Clausius in the sense that energy is degraded ("de-gradiented") from one end of the universe to the other and that it becomes less and less available in time, until "Warmetode", or the "thermal death" of the universe. Evolution toward this thermal death is the subject of much discussion. It has been shown (Jorgensen et al., 1995) that the expansion of the universe implies that the thermodynamic equilibrium is moving farther and farther away. In order to extend the theory from the planetary to the cosmic context it is necessary to introduce unknown effects such as gravitation. Current astrophysics suggests an expanding universe that originated in a great primordial explosion (big bang) from a low-entropy state, but the limits of theoretical thermodynamic models do not allow confirmation or provide evidence.

The study of entropy continues: this fundamental concept has been applied to diverse fields such as linguistics, the codification of language and to music and information theory. Thermodynamics has taught us many fascinating lessons, particularly that (I) energy cannot be created or destroyed but is conserved and (II) entropy of isolated systems is always increasing, striking the hours of the cosmic clock, and reminding us that both for man and for energy-matter, time exists and the future is distinct from the past by virtue of a higher value of S.

The second law of thermodynamics, still upheld as one of nature's fundamental laws, addresses the pathways we should avoid in order to keep life on Earth. It shows the universal, inescapable tendency toward disorder (in thermodynamics, the general trend toward an entropy maximum), which is also, again metaphorically, a loss of information and of usable energy availability. This tendency to the Clausius' "thermal death", speaks to the thermo-dynamic equilibrium, namely the death of biological systems and ecosystems, through the destruction of diversity. There are two ways to achieve such a condition when:

(a) through energy exchanges as heat fluxes, there are no more differences in temperature and nothing more can be done, because no exchange of usable energy is allowed;

(b) a system, becoming isolated, consumes its resources, reaching a great increase in its internal entropy and, at the end, to self-destruction.

For this reason living systems cannot be at the conditions of the thermodynamic equilibrium, but keep themselves as far as possible from that state, self-organizing due to material and energetic fluxes, received from outside and from systems with different conditions of temperature and energy.

To live and reproduce, plants and animals need a continuous flow of energy. This is an obvious and commonly believed truism, but in fact organisms will also readily accept a discontinuous energy inflow, as life in a biosphere, driven by pulsed energy inputs that the periodic motions of the planet provide, demonstrates. The energy of the biosphere that originates in the discontinuous luminous energy of the sun, is captured by plants and passes from one living form to another along the food chain. This radiant pathway that provides us with great quantities of food, fibers, and energy, all of solar origin, has existed for over 4 billion years, a long time if we think that hominids appeared on the earth only 3 million years ago and that known history covers only 10,000 years. The ancestors of today's plants were the blue-green algae, or cyano bacteria, that began to practice photosynthesis, assuming a fundamental role in biological evolution.

All vegetation whether natural or cultivated, has been capturing solar energy for millennia, transforming it into food, fibers, materials and work, and providing the basis for the life of the biosphere. The vast majority of the energy received by the Earth's surface from the sun is dispersed: it is reflected, stored in the soil and water, used in the evaporation of water and so forth. Approximately 1 percent of the solar energy that falls on fertile land and water is fixed by photosynthesis by primary producers in the form of high-energy organic molecules: solar energy stored in chemical bonds available for later use. By biochemical processes (respiration) the plants transform this energy into other organic compounds and work.

The food chain considered in terms of energy flows has a logic of its own: the energy degrades progressively in the different phases of the chain (primary producers and secondary consumers including decomposers), giving back the elementary substances necessary to build again the molecules of living cells with the help of solar energy.

The organization of living beings in mature ecosystems slows the dispersal of energy fixed by plants to a minimum, using it completely for its complex mechanisms of regulation. This is made possible by large "reservoirs" of energy (biomass) and by the diversification of living species. The stability of natural ecosystems, however, means that the final energy yield is zero, except for a relatively small quantity of biomass that is buried underground to form fossils. Relatively small, true, but in absolute terms in some forms enough to power a modern civilization for centuries.

Photosynthesis counteracts entropic degradation insofar as it orders disordered matter: the plant takes up disordered material (low-energy molecules of water and carbon dioxide in disorderly agitation) and puts it in order using solar energy. It organizes the material by building it into complex structures. Photosynthesis is, therefore, the process that by capturing solar energy and decreasing the entropy of the planet paved the way for evolution. Photosynthesis is the green talisman of life, the bio-energetic equivalent of Maxwell's demon that decreases the entropy of the biosphere. On the Earth, living systems need a continuous or discontinuous flow of negative entropy (i.e. energy from outside) and this flow consists of the very solar energy captured by photosynthesis. This input of solar energy is what fuels the carbon cycle. The history of life on the Earth can be viewed as the history of chemotropic life, followed by the photosynthesis and the history of evolution, as the history of a singular planet that learned to capture solar energy and feed on the negative entropy of the universe for the creation of complex self-perpetuating structures (living organisms).

Compared to us, the sun is an enormous engine that produces energy and offers the Earth the possibility of receiving large quantities of negative entropy (organization, life), allowing a global balance that does not contradict the second law of thermodynamics. Every year, the sun sends the Earth 5.6 X 1024 J of energy, over 10,000 times more energy than humans consumes in a year.

Was this article helpful?

0 0
Solar Power Sensation V2

Solar Power Sensation V2

This is a product all about solar power. Within this product you will get 24 videos, 5 guides, reviews and much more. This product is great for affiliate marketers who is trying to market products all about alternative energy.

Get My Free Ebook


Post a comment