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 four), nonequilibrium, thermodynamic systems. The treatment is nonrigorous and intuitive, interpreting results for living ecosystems based on parallels between these and the much simpler nonliving ones treated rigorously in thermodynamic theory. If an (open, nonequilibrium) ecosystem receives a boundary flow of energy from its environment, it will use what it can of this energy, the free energy or exergy content, to do work. The work will generate internal hows, leading to storage and cycling of matter, energy, and information, which move the system further from equilibrium. This is reflected in decreased internal entropy and increased internal organization. Energy degraded in the performance of work is exhausted as boundary outputs to the system's environment. This is reflected in decreased organization and increased entropy of the surroundings, the dissipative property (paper three). All properties rest on the conservation principle (paper two). Growth is movement 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 boundary flows increases, which reflects increase in the sum of internal flows, which contribute to throughflow; and (III) when, somewhat coincident with but mostly following upon I and II, system internal organization, reflecting its energy-use machinery, evolves the utilization of information to increase the usefulness for work of the boundary energy supply. These three forms 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 qualitative and subjective attributes inherent in information-based mechanisms that increase the exergy/energy ratio in available energy supplies. The open question of this paper is, which of many possible pathways will an ecosystem take in realizing its three forms of growth? The answer given is that an ecosystem will change in directions that most consistently create additional capacity and opportunity to utilize and dissipate available energy and so achieve increasing deviation from thermodynamic ground. The machinery for this synthesized from the three identified growth processes is reflected in a single measure, exergy storage. Abundant and diverse living biomass represents abundant and diverse departure from thermodynamic equilibrium, and 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 storage will tend to be selected, for these in turn require greatest energy dissipation to establish and maintain, consistent with the second law. Energy storage by itself is not sufficient, but it is the increase in specific exergy, 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 changing 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 models with time-varying parameters. It is also shown how maximization of exergy 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 energy capture (boundary inputs) and low entropy production (dissipative boundary outputs). In middle stages, growing interconnection of proliferating storage units (organisms) increases energy throughflow (Form II growth). This increases endogenous inputs and outputs and, in consequence, throughflow/boundary flow ratios, entropy production, and on balance, biomass. In mature phases, cycling becomes a dominant feature of the internal network, increasing storage and throughflow bath. Biomass and entropy production are maximal, but specific dissipation (as dissipation/storage ratio) decreases, reflecting advanced organization (Form III growth) typified by cycling. Specific exergy (exergy/energy ratio) increases throughout succession to maturity, in early stages mainly due to mass accrual, and in the later stages to gains in information and organization. During senescence, storage, entropy production, specific dissipation, and specific exergy all decrease, reflecting a declining ecosystem returning toward equilibrium. (C) 2126 Elsevier Science B.V. All rights reserved.