The ecosystem analogue

An ecosystem is biotic assemblage of plants, animals, and microbes, taken together with their physico-chemical environment (e.g. Kormondv, 1969). In an ecosystem the biological cycling of materials is maintained by three groups: producers, consumers, and decomposers (fig. 1a). The producers are plants and some bacteria capable of producing their own food photosynthetically or by chemical synthesis. The consumers are animals that obtain their energy and protein directly by grazing, feeding on other animals, or both. The decomposers are fungi and bacteria that decompose the organic matter of producers and consumers into inorganic substances that can be reused as food by the producers; they are the recyclers of the biosphere. Nature is capable of sustaining the producer-consumer-decomposer cycle indefinitely, with the sun as the energy source. The smallest such entity that is self-sufficient is an ecosystem.

Fig. 1a The movement of chemicals and materials through the natural ecosystem

Fig. 1b The movement of chemicals and materials through a system resulting from human activity (anthroposystem)

Functionally, human activities that perturb the natural environment can also be divided into three similar components (fig. 1b). Producing activities include energy production (fossil fuels), manufacturing (non-fuel minerals), and growing food. The consumers are humans and their domestic animals. Decomposing or recycling activities include treatment of waste water and recycling of metals. However, whereas an ecosystem relies on its decomposers for a complete recycling of its elements, the system created by human activity lacks such efficient decomposers and recyclers. As such, manufactured materials that are no longer needed and the waste by-products of industrial activity are disposed of into the physical environment. The process of adding unwanted material to the environment is called pollution. The waste products are taken up by the atmosphere and the hydrosphere, and delivered to the biological and geochemical receptors. In this sense, the anthroposystem, as defined above, is more of an open system, as discussed by Ayres in chapter 1 of this volume.

Ecosystem-anthroposystem comparison

The producer-consumer-recycler model provides a convenient framework for comparing ecosystems to anthroposystems. The flow of material in both systems is illustrated qualitatively by the arrows in figures la and lb. In an ecosystem most of the material is transferred from the producers (plants) to the recyclers (bacteria); only a small fraction is passed through the consumers to the recyclers. The decomposes (recyclers) return most of the material to the producers for reuse.

In the anthroposystem the flow from the producers to the recyclers is small (or even non-existent), since it would be pointless to produce (mobilize) material and immediately recycle it without a consumer in the loop. In the anthroposystem much of the mobilized material is transferred to the rest of the external environment by the producer or by the consumer. Hence, it mostly an open system, with recycling accounting for only a small fraction of the mobilized matter.

In an ecosystem, recycling and sustained development (evolution) is facilitated by a close physical proximity and functional matching between the producers and consumers. The physical proximity of producers, consumers, and recyclers in an ecosystem (e.g. plants, animals, and bacteria in a forest) assures that very little energy is required for the physical transport of matter between the plant and its symbiotic bacterial population. Also, the physical proximity allows a reasonably fast mutual adjustment if there is a perturbation in the system.

In the anthroposystem, with consumers playing a more significant role, there is usually a significant physical displacement between the producer and the consumer. The global flow of oil products is the most dramatic example. Accordingly, a significant amount of energy is required to transfer the matter back to the producer or to a recycler. This physical separation of consumers, producers, and recyclers appears to be a major difference between the ecosystem and anthroposystem.

The producer-consumer-receptor-based model is a suitable framework for economic models which study the driving forces of the material flows. It is self-evident that the economics - i.e. the allocation of material resources - will depend on production (availability), consumption (demand), and on the cost at the receptor.

The above ecosystem, i.e. the producer-consumer-receptor-based material flow model, can also be used to formulate physico-chemical models based on mass conservation principles. The next section presents such a formulation.

Ecosystem-based material flow system

The flow of matter from producers to consumers and subsequently to receptors is depicted schematically in figure 2. Most of the production of potential pollutants begins with mining, that is, the removal of a substance from its long-term geochemical reservoir. The amount of pollutant mass, fzM_i, mobilized by mining (tons/yr) is the production rate P_i (tons/yr) of the raw material (coal, oil, smelting ore, etc.) multiplied by the concentration c; (gram/ton) of the impurity (sulphur, mercury, lead, etc.): M_ic_iPi.

Fig. 2 Key matrices in the flow of materials from producer to consumer to receptor

Matter is transferred from the producer to the consumer by transportation, including railroads, trucks, and ships. Functionally, transportation redistributes the mobilized substances over a large geographical area and to a multiplicity of consumers. Any producer, i, may deliver its product to any consumer, j. Mathematically, this producer-consumer transfer is characterized by a surface transfer matrix, s_ij.

The amount of matter, U_ij, originating from producer i and used at consumer j is s_ijM_j. The total amount of matter reaching consumer j is the sum of the matter produced by all producers multiplied by their respective surface transfer matrix elements.

The next transfer occurs between the consumer, or emitter, and the environmental receptors. The consumer is located where the combustion or smelting occurs, and the receptor where the pollutant is deposited following its atmospheric or hydrologic transit. Again, consumer j can transfer matter through the atmosphere to any receptor, k, Hence, the matter received at receptor k that originated at consumer (emitter) j, R_jk, is the product of the use rate U_j times the atmospheric or hydrologic transfer matrix, ajk, from emitter j to receptor k. The total amount of matter deposited at receptor k is the sum of the use rates, Uj, at each emitter weighed by its atmospheric/ hydrologic transfer matrix element.

In this chapter, the numeric values of M_i, c_i, and s_ij are discussed in detail, while discussion of the atmospheric transfer matrix, a_jk, is beyond the scope of this report.

The release of a trace substance at any given emission site U_j (tons/yr) is calculated as follows: U_j = s_ijc_iP_i. It has to be noted that in this simplified formulation, the releases are not broken down by media, i.e. air, land, and water. A general approach to include the transfer through all environmental media is presented in the next section, the Environmental Spheres Analogue. An illustration of such a model is given in Husar (1986), as applied to the mobilization and transfer of sulphur in the United States, from its geochemical reservoirs (by mining) to the consumers at the power plants and subsequently to the receptors and receiving geochemical reservoirs.

Shortcomings of the ecosystem analogue

The ecosystem model of nature and of human activities has a major shortcoming in that it pays little heed to the physical transfer of mobilized matter. It does not answer the question of where the redistribution has occurred. Also, in that model, much of the anthroposystem had to be left open since many of the flows were out of the system as waste products. The next section is an attempt to extend the ecosystem analogue by "closing the system." In such a scheme, material is accounted for regardless of where it goes after leaving a given open system.