Info

Source: Morowitz (1968).

Source: Morowitz (1968).

pursue the same lines as those mentioned in context with Eq. (6.3), where the mass conservation principle was applied. The conversion of biomass to chemical energy is illustrated in Table 2.2. The energy content per 1 g ash-free organic material is surprisingly uniform, as illustrated in Table 2.2 (see also the discussion in Chapter 7). Notice that the energy content in the table is the energy of "dead matter" applied as fuel due to its elementary composition. Table 2.2d indicates DH, which symbolises the increase in enthalpy, defined (see Section 2.4) as H = U — pV. The energy is originated from the following chemical process: organic matter + oxygen = carbon dioxide + water + other inorganic compounds as nitrate, sulphate and phosphate.

Biomass can therefore be translated into energy, and this is also true of transformations through trophic chains. This implies that the short trophic chains of "grain to human" should be preferred to the longer and more wasteful "grain to domestic animal to human". The problem of food shortage cannot, however, be solved so simply, since animals produce proteins with a more favourable amino acid composition for human food (lysine is missing in plant proteins) and eat plants that cannot all be used as human food today. But food production can, to a certain extent, be increased by making the trophic chains as short as possible.

These relationships (see, for instance, J0rgensen, 2001b) can also be illustrated by means of so-called ecological pyramids that can either represent the number of individuals, the biomass (or energy content) or the energy flows on each level in the trophic chains or trophic networks. Only the energy flow forms a true pyramid due to the loss of heat by respiration. The pyramids based on numbers are affected by variation in size and the biomass pyramids by the metabolic rates of individuals.

However, as will be shown in the next chapters, energy in ecosystems cycles like matter, if we consider the chemical energy carried by biomass. It will make the interpretation of trophic levels more complicated.

Ecological efficiency should also be mentioned here; see Table 2.3 (J0rgensen, 2002b), where some useful definitions are listed, and Table 2.4, where the efficiency values are

Table 2.3

Ecological efficiency

Table 2.3

Ecological efficiency

Concept

Definition

Lindeman's efficiency

Ratio of energy intake level n to n — 1: F„/F„_i

Trophic level assimilation efficiency

Asn/ Asn —1

Trophic level production efficiency

Pn/Pn— 1

Tissue growth efficiency

Pn/Asn

nn

Assimilation efficiency

Asn/Fn

Utilisation efficiency

Fn/UDn — 1

Variables: F, trophic input; UD, undigested food; As, assimilated food; R, respiration; P, net production; E, excretion; G, growth; n, trophic level. F = As + UD, As = P + R and P = G + E.

Variables: F, trophic input; UD, undigested food; As, assimilated food; R, respiration; P, net production; E, excretion; G, growth; n, trophic level. F = As + UD, As = P + R and P = G + E.

exemplified (Table 2.4). All these definitions use the energy balance of the ecosystem or organisms as its active component, i.e. they are based on the First Law. The items of the balance are similar to the ones used in equality (6.4): the trophic input F = As + UD where As is an assimilated fraction and UD is an undigested food.

There is a close relationship between energy flow rates and organism size (denoted as the allometric principle) and some of the most useful of these relationships are illustrated in J0rgensen (1988, 1990, 1994) (see also Peters, 1983). Since many rate parameters are closely related to the rate of energy exchange, it is possible to find unknown parameters for various organisms on the basis of knowledge about the same parameters for other organisms, providing that the sizes of the organisms are known. This is illustrated in the four references mentioned above. The size or what also may be called the openness, which is a ratio of the surface area relative to the volume, will be further touched on in the next chapter.

Any self-sustaining ecosystem will contain a wide spectrum of organisms ranging in size from tiny microbes to large animals and plants. The small organisms account in most cases for most of the respiration (energy turnover), whereas the larger organisms comprise most of the biomass. It is therefore important for the ecosystem to maintain both small and large organisms as it will mean that both the energy turnover rate and the energy storage in the form of biomass are maintained.

The developments and reactions of ecosystems in general are not only a question of the energy flow, as will be touched upon many times in this volume. Matter and information also play a major role. So, matter, energy and information form the "triangle of freemasons". No transfer of energy is possible without matter and information and no matter can be transferred without energy and information. The relationships between energy and information will be discussed in more detail in Chapters 4 and 5. The higher the levels of information are, the higher is the utilisation of matter and energy for the development of ecosystems farther from the thermodynamic equilibrium; see also Chapter 5.

E.P. Odum has described the development of ecosystems from the initial stage to the mature stage as a result of continuous use of the self-design ability (Odum, 1969, 1971a,b). See the significant differences between the two stages of systems listed in Table 2.5 and notice that the major differences are on the level of information. The content of information

Table 2.4a

Assimilation efficiency (As/F) for selected organisms (after various authors)

Taxa As/F value

Internal parasites

Entomophagous Hymenoptera Ichneumon sp. 0.90 Carnivores

Amphibian (Nectophrynoides occidentalis) 0.83

Lizard (Mabuya buettneri) 0.80

Praying mantis 0.80

Spiders 0.80-0.90 Warm- and cold-blooded herbivores

Deer (Odocoileus sp.) 0.80

Vole (Microtus sp.) 0.70

Foraging termite (Trinervitermes sp.) 0.70

Impala antelope 0.60

Domestic cattle 0.44

Elephant (Loxodonta) 0.30

Pulmonate mollusc (Cepaea sp.) 0.33

Tropical cricket (Orthochtha brachycnemis) 0.20 Detritus eaters

Termite (Macrotermes sp.) 0.30

Wood louse (Philoscia muscorum) 0.19

Table 2.4b

Tissue growth efficiency (P/As) for selected organisms (after various authors)

Taxa P/As value

Immobile, cold-blooded internal parasites

Ichneumon sp. 0.65 Cold-blooded, herbivorous and detritus-eating organisms

Tropical cricket (Orthochtha brachycnemis) 0.42

Other crickets 0.16

Pulmonate mollusc (Cepaea sp.) 0.35

Termite (Macrotermes sp.) 0.30

Termite (Trinervitermes sp.) 0.20

Wood louse (Philoscia muscorum) 0.16 Cold-blooded, carnivorous vertebrates and invertebrates

Amphibian (Nectophrynoides occidentalis) 0.21

Lizard (Mabuya buettneri) 0.14

Spiders 0.40 Warm-blooded birds and mammals

Domestic cattle 0.057

Impala antelope 0.039

Vole (Microtus sp.) 0.028

Elephant (Loxodonta) 0.015

Deer (Odocoileus sp.) 0.014

Savanna sparrow (Passerculus sp.) 0.011

Shrews Even lower values

Table 2.4c

Ecological growth efficiency (P/F = (As/F)(P/As)) for selected organisms (after various authors)

Table 2.4c

Ecological growth efficiency (P/F = (As/F)(P/As)) for selected organisms (after various authors)

Taxa

P/F value

Herbivorous mammals

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