An Ecological Approach to Biosystem Thermodynamics
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ABSTRACT: The general attributes of ecosystems are examined and a naturallyoccurring "reference ecosystem" is established, comparable with the "isolated" system ofclassical thermodynamics. Such an autonomous system with a stable, periodic input ofenergy is shown to assume certain structural characteristics that have an identifiablethermodynamic basis. Individual species tend to assume a state of "least dissipation"; thisis most clearly evident in the dominant species (the species with the best integration ofenergy acquisition and conservation). It is concluded that ecosystem structure resultsfrom the antagonistic interaction of two nearly equal forces. These forces have theirorigin in the Principle of Most Action ("least dissipation" or "least entropy production")and the universal Principle of Least Action. "Most action" is contingent on the equipar-titioning of the energy available, through uniform interaction of similar individuals. Thetrend to "Least action" is contingent on increased dissipation attained through increasingdiversity and increasing complexity. These principles exhibit a basic asymmetry. Giventhe operation of these opposing principles over evolutionary time, it is argued thatecosystems originated in the vicinity of thermodynamic equilibrium through the resonantamplification of reversible fluctuations. On account of the basic asymmetry the systemwas able to evolve away from thermodynamic equilibrium provided that it remainedwithin the vicinity of "ergodynamic equilibrium" (equilibrium maintained by internalwork, where the opposing forces are equal and opposite).
At the highest level of generalization there appear to be three principles operating: i)maximum association of free-energy and materials; ii) energy conservation (decelerationof the energy flow) through symmetric interaction and increased homogeneity; and iii)the principle of least action which induces acceleration of the energy flow throughasymmetrical interaction. The opposition and asymmetry of the two forces give rise tonatural selection and evolution.
KEY WORDS: Action principles, ecosystem structure, evolution, information, naturalselection, non-equilibrium thermodynamics, teleology.
But ask now the beasts, and they shall teach thee,and the fowls of the air, and they shall tell thee,Or speak to the earth, and it shall teach thee,and the fishes of the sea shall declare unto thee,
Job vii-viii. 12
If science is dependent on progress in subsuming a class of phenomena under
Biology and Philosophy 7: 35-60, 1992. 1992 Kluwer Academic Publishers. Printed in the Netherlands.
laws of a higher order of generality, then the unity of all knowledge is surelydependent on a thermodynamic interpretation of biological phenomena(Landsberg 1987). Nevertheless, the establishment of a coherent thermodynamicbiological paradigm has proved to be extremely difficult on account of theapparent contravention of the Second Law of Thermodynamics by biologicalprocesses. There is no conflict at the level of the universe, organisms exploit anenergy gradient by delaying the energy transformation involved and utilizing thedelay in energy flux for the performance of work. However, at the local level ofthe Earth the tantalizing question remains as to the mechanism whereby orderand organization increase with time and give rise to systems having a very lowprobability of occurrence.
Despite Watt's (1971) vivid warning that "If we do not develop a strongtheoretical core that will bring all parts of ecology back together we shall all bewashed out to sea on an immense tide of unrelated information" only littleprogress toward an accepted synthesis has been achieved in the 20 years thathave elapsed since it was first voiced. This lack of a generally accepted theoryunderpinning biology has had an inhibiting effect on the development of thescience and allowed the development of "invisible colleges" (McIntosh 1980),each following its own suite of ideas.
The problems of unresolved controversy in ecology are compounded by thoseof semantics. One is inhibited from using words such as "stable", "climax","dominance", or "diversity", without several pages of qualification, becausethey have either been appropriated to a specific meaning by individualauthorities, or the basic concept behind the word has been questioned. I believewe cannot allocate these words to specific usage until we have determined thephysical basis on which each expressed concept actually rests.
Less controversial, but of equal difficulty, is that in biology we must talk intrends, presumably caused by underlying forces, without reference to fixedpositions. Biological trends are therefore like vectors, forces having magnitudeand direction but without co-ordinates. Only when these vectors manifestthemselves in a particular ecosystem do they form part of a co-ordinated system,but even here, both magnitude and co-ordinates are virtually impossible toevaluate and only occasionally do we get a firm indication of direction.
Despite these difficulties, there seems little doubt that a ring is closing arounda thermodynamic solution that will remove many of the problems encountered(Weber et al. 1989; Weber et al. 1988; Brooks et al. 1989). Notwithstanding,without major input from both ecologists and more formal thermodynamicistsand a concerted effort on both sides to understand each other, the struggle toresolve the question is likely to be unnecessarily protracted. I would even gofurther and state that unless thermodynamic concepts are now complete andincapable of modification, biology must lead the physics, not the reverse, for theobservations of ecology are usually of great complexity, and are the ones insearch of explanation.
While I believe that the approach of Weber et al. (1989) is the correct one, Ithink that their ecological sources have led them astray. They overlook certain
factors when detailing the general characteristics of ecosystems that are vital toeffecting a satisfactory synthesis. Without a detailed accounting of the"emergent properties" of ecosystems it is unlikely that a satisfactory ther-modynamic solution will be attained.
A major road-block in isolating biological generalities can be traced toLotka's (1922) statement:
In every instance considered, natural selection will so operate as to increase the totalmass of the organic system, to increase the rate of circulation of matter through thesystem, and to increase the total energy flux through the system so long as there ispresented an unutilized residue of matter and available energy.
This statement, while difficult to refute in general terms, conceals rather thanilluminates the problem, for the trends identified are not uni-directional asindicated. The views of Odum (1969) and Schneider (1988), which in turn formthe basis of Weber et al.'s (1989) analysis, are consistent with those of Lotka.The properties of successional or maturing communities, Weber et al. state, canbe divided into five categories:
i. increased energy flow;ii. greater variety of species;iii. more narrow trophic specialization by their members;iv. enhanced amount of cycling;v. longer retention of media in the system.
While it is undeniable that there is evidence for all these statements, none ofthem can be taken to the limit in the manner implied, for trends observed duringearly succession, may be reversed in the terminal stages.
From the ecological evidence, I believe there are essentially two antagonisticforces operating, which, in accordance with Le Chatelier's Law, tend to reachequality at a "climax". I hope to show that these two forces are nearly equal,each dominating system behaviour within a different time-frame and eachhaving an identifiable physical origin.
THE EMERGENT PROPERTIES OF ECOSYSTEMS
The proposition that a mature ecosystem has a greater variety of species thanone at a less mature stage is not universal. For example, McIntosh (1980) statesthat Clements (1905) ("to his eternal credit") was the first ecologist to formulatethe general proposition that over the course of succession:
the number of species is small in the initial stages, it attains its maximum in inter-mediate stages: and again decreases in the ultimate formation, on account of thedominance of a few species. (Emphasis added.)
Over the course of succession species diversity and biomass increase more orless hand-in-hand although alternating stages of dominance and diversity arefrequently evident (Harper 1969). This continues until the final stages which seethe dominant species gaining ascendancy and suppressing certain species. Manyexamples of this decline in species diversity in the terminal stages of successionare listed by Whittaker (1969). This increase and subsequent decline in thenumber of species is compatible with the general pattern of net and grossproduction described by Odum (Figure 1, in Odum 1969).
Similarly, Connell (1978), examining the role of disturbance in tropicalforests through indigenous man's activity, concluded that continuous, relativelyminor disturbance of the climax, leads to an increase in diversity.
However, all ecosystems do not behave in this manner. For example, Paine(1966) in his classic work on Pacific coast marine ecosystems showed thatremoval of the starfish Pisaster ochraceus, the terminal predator, caused areduction in diversity due to the assumption of dominance by the mussel Mytiliscalifornianus. Paine states:
The removal of Pisaster has resulted in a pronounced decrease in diversity asmeasured simply by counting species inhabiting this area, whether consumed byPisaster or not, from a 15 to an eight-species system. The standing crop has beenincreased by this removal and should continue until the Mytilus achieve theirmaximum size.
The role of starfish in this situation appears to be equivalent to that of man inmaintaining higher diversity in tropical forest regions as noted above (Connell1978).
From such evidence Stanley (1973) developed a "cropping principle" toaccount for the sudden flowering of many new multicellular species in the LatePrecambrian. Nevertheless, an increase in species richness continues only up toa certain level of cropping intensity; beyond this point "overgrazing" reducesdiversity. Slobodkin et al. (1967) believe that the overgrazed condition occursonly through the effects of man-protected herbivores or introduced species. Forexample, when rabbits (Oryctolagus cuniculus) (a species introduced intoEngland in the 12th century) were eliminated from English downland by thedisease Myxomatosis, a number of species previously thought to have beeneliminated from the region suddenly re-appeared (Harper 1969).
The above observations can be interpreted most readily on the basis of twoantagonistic forces driving the ecosystem in different directions, with somewhatdifferent end-points being reached in different environmental settings: one forceinduces increased diversity, the other increased biomass. Over most of the serethese two processes go hand-in-hand with continuous jockeying for dominanceby one or the other, with immediate success depending on the physical environ-ment and the nature of species present. Only at the theoretical end point or"climax" does the system attain a degree of stability when the two underlyingforces approach equality.
In many cases it is evident that, in the terminal stages, the system behaviour is
dominated by the force tending to reduce diversity. This reduction in diversity isusually small relative to the maximum diversity attained in the penultimatestage. In the case of the starfish-dominated community studied by Paine, it maybe presumed that the climax is reached when a balanced state is attained close tothat of maximum diversity. This would be consistent with the fact that there hasbeen a much longer period for evolution in the marine environment than interrestrial ecosystems.
Similarly with biomass, it is evident that Paine's removal of the starfishpermitted an increase in biomass, yet conversely it is equally apparent thatremoval of the dominant trees in a forest will reduce biomass. In general, it maybe said that removal of a dominant predator will tend to increase biomass,whereas removal of dominant vegetation will reduce biomass. Removal of apredator eliminates one energy transformation stage. Because each energytransformation has a very high energy demand, biomass can increase eventhough total energy input remains constant. This implies that the specific energyflux (the energy to support unit biomass) will decline as biomass increasesbecause of longer retention time within the system of each unit of energyassimilated.
It seems impossible to define the "climax" or the "mature" ecosystem in preciseterms of diversity or biomass, even in relatively stable systems. Plant-dominatedand animal-dominated systems have similarities and they have differences. Inthe animal-dominated system the primary producer initiates a food-chain, orfood-web, which is terminated by the dominant animal species, whereas in theforest system plants are both primary producer and dominant. In either case,only in autonomous systems with a constant environmental cycle, will theclimax state be attained.
Irrespective of its successional stage, any perturbation of a system will set itback to a new position, from which it will begin, once more, to move toward theclimax, possibly by a different route since conditions rarely repeated themselvesprecisely. If extermination of part of the species complement occurs, resultingfrom the evolution or immigration of a new species, or a major environmentalcatastrophe, the system, over the course of time, either through further immigra-tion or evolution, will again proceed toward increasing diversity. Thus, theclimax is rather an elusive state, somewhat akin to a perspective point on thehorizon toward which a system converges, but one that changes and recedes asthe immediate conditions or species change. Ecosystems may therefore beregarded as dynamic entities whose major characteristic is stability-seekingrather than stability.
"More narrow specialization by members of a mature ecosystem" is subject tosimilar restrictions. In general, as Whittaker (1965) states, species tend to bepartial rather than direct competitors, tending
...to evolve also toward habitat differentiation, toward scattering of their centers ofmaximum population density in relation to environmental gradients so that fewspecies are competing with one another.
It is evident that over successional time, species will so distribute themselves asto ensure a more specialized division of the total "ecological space," but thistrend may be reversed when a forest tree or large animal assum...