Ecosystems emerging: 4. growth

This fifth paper in the series on Ecosystems Emerging treats the properties of ecosystem growth and development from the perspective of open (paper fo ur), nonequilibrium, thermodynamic systems. The treatment is nonrigorous an d intuitive, interpreting results for living ecosystems based on parallels between these and the much simpler nonliving ones treated rigorously in the rmodynamic theory. If an (open, nonequilibrium) ecosystem receives a bounda ry flow of energy from its environment, it will use what it can of this ene rgy, the free energy or exergy content, to do work. The work will generate internal hows, leading to storage and cycling of matter, energy, and inform ation, which move the system further from equilibrium. This is reflected in decreased internal entropy and increased internal organization. Energy deg raded in the performance of work is exhausted as boundary outputs to the sy stem's environment. This is reflected in decreased organization and increas ed entropy of the surroundings, the dissipative property (paper three). All properties rest on the conservation principle (paper two). Growth is movem ent away from equilibrium, which occurs in three forms: (I) when there is a simple positive balance of boundary inputs over outputs, which increments storage; (II) when, with boundary inputs fixed, the ratio of internal to bo undary flows increases, which reflects increase in the sum of internal flow s, which contribute to throughflow; and (III) when, somewhat coincident wit h but mostly following upon I and II, system internal organization, reflect ing its energy-use machinery, evolves the utilization of information to inc rease the usefulness for work of the boundary energy supply. These three fo rms of growth are, respectively, growth-to-storage, growth-to-throughflow, and growth-to-organization. Forms I and II are quantitative and objective, concerned with brute energy and matter of different kinds. Form III has qua litative and subjective attributes inherent in information-based mechanisms that increase the exergy/energy ratio in available energy supplies. The op en question of this paper is, which of many possible pathways will an ecosy stem take in realizing its three forms of growth? The answer given is that an ecosystem will change in directions that most consistently create additi onal capacity and opportunity to utilize and dissipate available energy and so achieve increasing deviation from thermodynamic ground. The machinery f or this synthesized from the three identified growth processes is reflected in a single measure, exergy storage. Abundant and diverse living biomass r epresents abundant and diverse departure from thermodynamic equilibrium, an d both are captured in this parameter. It is the working hypothesis of this paper that ecosystems continually maximize their storage of free energy at all stages in their integrated existence. If multiple growth pathways are offered from a given starting state, those producing greatest exergy storag e will tend to be selected, for these in turn require greatest energy dissi pation to establish and maintain, consistent with the second law. Energy st orage by itself is not sufficient, but it is the increase in specific exerg y, that is, of exergy/energy ratios, that reflects improved usability, and this represents the increasing capacity to do the work required for living systems to continuously evolve new adaptive 'technologies' to meet their ch anging environments. Exergy cannot be found for entire ecosystems as these are too complex to yield knowledge of all contributing elements. But it is possible to compute an exergy index for models of ecosystems that can serve as relative indicators. How to compute this index is shown, together with its use in developing mod els with time-varying parameters. It is also shown how maximization of exer gy storage distinguishes between local and global optimization criteria. In ecological succession, energy storage in early stages is dominated by Form I growth which. builds structure; the dominant mechanisms are increasing e nergy capture (boundary inputs) and low entropy production (dissipative bou ndary outputs). In middle stages, growing interconnection of proliferating storage units (organisms) increases energy throughflow (Form II growth). Th is increases endogenous inputs and outputs and, in consequence, throughflow /boundary flow ratios, entropy production, and on balance, biomass. In matu re phases, cycling becomes a dominant feature of the internal network, incr easing storage and throughflow bath. Biomass and entropy production are max imal, but specific dissipation (as dissipation/storage ratio) decreases, re flecting advanced organization (Form III growth) typified by cycling. Speci fic exergy (exergy/energy ratio) increases throughout succession to maturit y, in early stages mainly due to mass accrual, and in the later stages to g ains in information and organization. During senescence, storage, entropy p roduction, specific dissipation, and specific exergy all decrease, reflecti ng a declining ecosystem returning toward equilibrium. (C) 2126 Elsevier Sc ience B.V. All rights reserved.