Thermodynamics and Ecological Efficiency

There are many books and papers that use arguments about entropy in ecological systems. While we can focus on particular predators and consider the ecological efficiency of a particular focus population from the standpoint of particular predators, detritovores, etc., the boundary conditions for analyses of entropy and enthalpy in closed systems are certainly not met by ecological systems. 'Closing' an ecological system in the sense of elementary thermodynamics results in almost immediate death of all organisms in the system. They are not at equilibrium, nor is it generally possible to control temperature properly, and certainly they are not at chemical equilibrium. Ecological efficiencies are dimensionless ratios of rates defined in units of energy per time over some interesting area but they have not been measured within the boundary conditions for thermodynamics or steady-state thermodynamics.

Ecological efficiency involves the idea of a population or trophic level being fed upon by a predator or parasite population while feeding on some lower trophic level, but without the benefit of the farmer's attention. This seems to be a basically unstable arrangement. Why does not a higher trophic level completely eat out the lower one, or why cannot the lower trophic level evolve properties that make it immune to predation by the members of the higher tropic level?

Estimates of ecological efficiency vary widely. Some of this variance is due to actual biological differences but some is due to a variety of technical problems. We generally cannot directly measure transfers of energy in the field. Therefore, ecological efficiency measures require that more readily available measures, such as census data, stomach contents analyses, or some measure of biomass, are converted to calories by use of different conversion constants derived from measure of the caloric content of different categories of organisms in the system combined with field census data.

With the possible exception of relatively rare and large organisms in controlled situations, census evaluations in nature have relatively large and sometimes poorly known errors. Typically, there are not replicates available for the situation being examined.

Also, while steady-state conditions are not required for efficiency measurements, the meaning of non-steady-state evaluations is limited.

Ecological efficiency is not a constant across ecological situations, although this was, for a while, a tempting hypothesis. The same systems examined over different timescales may have different ecological efficiencies.

Attempts to circumvent the problem of sampling, repetition, and lack of a steady state can be made by using laboratory populations, in which food to the populations and yield from the populations can both be controlled. Of course, the relevance of these studies to field values remains uncertain.

When ecological efficiency was evaluated in laboratory populations of Daphnia and hydra, they approximately agreed with each other and were within the range of values reported from nature. It can be demonstrated that any apparent constancy of ecological efficiency cannot be the result of evolutionary selection. In fact, it is an epiphenomenon.

Ecological efficiency has a curious relation to natural selection, which may shed light on other evolutionary questions. Using the now classical metaphor of the spandrels of San Marco, Gould and Lewontin elegantly underscored the question ofwhether there exist biological properties that are not in themselves selected for or against by the evolutionary process but are carried along by selection in the process of selection for some other properties. Ever since, there have been arguments about whether or not such properties exist and whether or not there are any properties of the biological world that are ignored by selection.

Ecological efficiency is certainly a real, measurable, quantity, but it seems to be immune to natural selection of any simple kind.

An elementary tenet of modern evolutionary theory is the natural selection for differences in fitness among members of a population. Sometimes, the fitness advantages of the selected organisms are not obvious. This can generally be resolved by further study or suitable reex-amination of details of the process. If necessary, the problem can often be resolved by considering the effects of selection not only on individuals but also on their kin. A mother whose probability of individual survival and longevity is materially reduced by reproduction may be producing a sufficient number and quality of young so that the total fitness of mother and young may be higher than that of some other mother and young, thereby providing raw material for selection.

In order for natural selection for ecological efficiency to achieve any particular value, the selective process must somehow work in a coordinated way on at least three populations within that community. There is no readily describable mechanism for that to occur. Possibly a massive group selection process acting on entire trophic systems might raise or lower the rates and quantities of energy transfer, but what is much more likely is that one or more of the populations in the trophic system will be eliminated or greatly reduced while ecological efficiency continues to be set by multispecies relationships that have no direct selective meaning.

The actual value of ecological efficiency seems to be an epiphenomenon of selection, operating on all of the components of energy transfer in a trophic system, rather than being in any way the direct target of selection.

This implies that there is at least one property of ecological systems that is real and measurable but not related in any clear way to natural selection. Ecological efficiency values require coordination among parts of ecological systems, each of which are themselves subject to natural selection without particular coordination between the different selective forces.

How much of the appearance and properties of landscapes is similarly free of direct natural selection? Are there other properties of ecological systems that are epi-phenomenal in this sense?

See also: Maximum Sustainable Yield; Trophic Index and Efficiency.

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