Ecology: Biology of Interactions. III-12. Ecological Efficiencies

III-12. Ecological Efficiencies
An excellent diagram for describing the energy flow through a trophic level is the 'Odum square' (Fig. III-12.1). Eugene Odum proposed a clear diagram showing the flow of energy passing through an individual organism, population, or trophic level. This diagram shows which 'arms' the energy flow branches into as it passes through biosystems.

Fig. III-12.1. Distribution of energy flows passing through an organism, population, or trophic level.
For example, from this diagram it is easy to understand the difference between the two main measures of production: gross (A) and net (P): the costs of respiration. The 'Odum square' makes it easy to verify that A=R+P; P=G+S+E, etc.
The following measures of efficiency in energy consumption and processing can be distinguished:
exploitation efficiency E1=Iexploiter/Pprey;
assimilation efficiency E2=A/I;
net production efficiency E3=P/A;
gross production efficiency E4=P/I=E2xE3;
ecological efficiency E5=Pexploiter/Pprey=E1xE2xE3.
The energy flows shown in the diagram are in different ratios for different organisms. Figuratively speaking, the simpler the energy transformation tasks organisms perform, the smaller the losses they incur. Thus, carnivorous animals face a comparatively simple task: they obtain energy from high-quality food that is relatively 'easy' to process and build their own body from. The most complex task is that solved by photosynthetic organisms, which use light as an energy source.
For example, a significant portion of the light falling on plants is reflected from them or absorbed by the soil. A large part of the energy absorbed by plants remains unused. Under favorable conditions, plants can assimilate (consume) about 1% of incident solar energy, with only about 0.5% going into net production (i.e., A = 1%, P = 0.5%). On average for the biosphere, these figures are even worse: plants consume about 0.2% of solar energy, and only about 0.1% goes into net production (A = 0.2%, P = 0.1%). However, even these amounts of energy in absolute terms are very large by human standards.
The feeding efficiency of animals depends substantially on the nature of their food. Assimilation efficiency (ratio of A to I) in carnivorous animals ranges from 60% (in insectivores) to 90% (in strictly meat- and fish-eating animals). In herbivorous animals, assimilation efficiency is 80% in seed-eaters; 60% in those that eat young leaves; 30-40% in those feeding on old leaves; 10-20% or even less in wood-eaters.
Further energy losses depend greatly on metabolic rate. For example, small birds expend more than 99% of assimilated energy in respiration, and less than 1% of the energy they were able to consume goes into net production! In small mammals this figure is 6%, in cattle -- 11%, in pigs -- 20%, and in some poikilothermic animals, especially large fish and reptiles, it reaches 75%!
Compare: a tit, having pecked 1 kg of insects, will gain less than 6 g in weight, while a boa constrictor that ate a kilogram guinea pig will gain more than 660 g of its own weight! To perform these calculations, we equated the energy content per unit weight of insects, tits, guinea pigs, and boa constrictors (this is an acceptable approximation). In both cases we needed to estimate the losses associated with assimilation and net production. In the first case we used the assimilation efficiency value given above for insectivores, as well as the net production efficiency value for small birds (1000 x 0.6 x 0.01 = 6), and in the second -- the corresponding values for carnivores and large reptiles (1000 x 0.9 x 0.75 = 675). Among other things, such is the price of homeothermy...
As energy moves along trophic chains, its quantity decreases while its quality (work potential) increases. A measure of quality is the number of solar energy units that must be dissipated to obtain one unit of energy in a new form available for transfer to higher trophic levels (Table III-12.1). Howard Odum (the brother of Eugene Odum, author of classic ecology textbooks) proposed using a measure he called emergy to denote energy quality. Emergy is a universal measure of the natural resources required -- the measure of solar energy spent to produce a given product.
Table III-12.1. Changes in energy quantity and quality during transformation

	Table 3.12.1. Changes in energy quantity and quality during transformations	Sun	Plants	Consumers I
Consumers II	1000000	10000	1000	100
100	1	100	1000	10000
	Table 3.12.1. Changes in energy quantity and quality during transformations	Sun	Wood	Coal
Consumers II	1000000	1000	500	125
100	1	1000	2000	8000

III-12. Ecological Efficiencies
An excellent diagram for describing the energy flow through a trophic level is the 'Odum square' (Fig. III-12.1). Eugene Odum proposed a clear diagram showing the flow of energy passing through an individual organism, population, or trophic level. This diagram shows which 'arms' the energy flow branches into as it passes through biosystems.

Fig. III-12.1. Distribution of energy flows passing through an organism, population, or trophic level.
For example, from this diagram it is easy to understand the difference between the two main measures of production: gross (A) and net (P): the costs of respiration. The 'Odum square' makes it easy to verify that A=R+P; P=G+S+E, etc.
The following measures of efficiency in energy consumption and processing can be distinguished:
exploitation efficiency E1=Iexploiter/Pprey;
assimilation efficiency E2=A/I;
net production efficiency E3=P/A;
gross production efficiency E4=P/I=E2xE3;
ecological efficiency E5=Pexploiter/Pprey=E1xE2xE3.
The energy flows shown in the diagram are in different ratios for different organisms. Figuratively speaking, the simpler the energy transformation tasks organisms perform, the smaller the losses they incur. Thus, carnivorous animals face a comparatively simple task: they obtain energy from high-quality food that is relatively 'easy' to process and build their own body from. The most complex task is that solved by photosynthetic organisms, which use light as an energy source.
For example, a significant portion of the light falling on plants is reflected from them or absorbed by the soil. A large part of the energy absorbed by plants remains unused. Under favorable conditions, plants can assimilate (consume) about 1% of incident solar energy, with only about 0.5% going into net production (i.e., A = 1%, P = 0.5%). On average for the biosphere, these figures are even worse: plants consume about 0.2% of solar energy, and only about 0.1% goes into net production (A = 0.2%, P = 0.1%). However, even these amounts of energy in absolute terms are very large by human standards.
The feeding efficiency of animals depends substantially on the nature of their food. Assimilation efficiency (ratio of A to I) in carnivorous animals ranges from 60% (in insectivores) to 90% (in strictly meat- and fish-eating animals). In herbivorous animals, assimilation efficiency is 80% in seed-eaters; 60% in those that eat young leaves; 30-40% in those feeding on old leaves; 10-20% or even less in wood-eaters.
Further energy losses depend greatly on metabolic rate. For example, small birds expend more than 99% of assimilated energy in respiration, and less than 1% of the energy they were able to consume goes into net production! In small mammals this figure is 6%, in cattle -- 11%, in pigs -- 20%, and in some poikilothermic animals, especially large fish and reptiles, it reaches 75%!
Compare: a tit, having pecked 1 kg of insects, will gain less than 6 g in weight, while a boa constrictor that ate a kilogram guinea pig will gain more than 660 g of its own weight! To perform these calculations, we equated the energy content per unit weight of insects, tits, guinea pigs, and boa constrictors (this is an acceptable approximation). In both cases we needed to estimate the losses associated with assimilation and net production. In the first case we used the assimilation efficiency value given above for insectivores, as well as the net production efficiency value for small birds (1000 x 0.6 x 0.01 = 6), and in the second -- the corresponding values for carnivores and large reptiles (1000 x 0.9 x 0.75 = 675). Among other things, such is the price of homeothermy...
As energy moves along trophic chains, its quantity decreases while its quality (work potential) increases. A measure of quality is the number of solar energy units that must be dissipated to obtain one unit of energy in a new form available for transfer to higher trophic levels (Table III-12.1). Howard Odum (the brother of Eugene Odum, author of classic ecology textbooks) proposed using a measure he called emergy to denote energy quality. Emergy is a universal measure of the natural resources required -- the measure of solar energy spent to produce a given product.
Table III-12.1. Changes in energy quantity and quality during transformation

Another consequence of the transfer of matter and energy from level to level is biological accumulation -- the increase in concentration of many substances selectively retained by biomass.
A measure of concentration in the trophic chain (a measure of biological accumulation) is the accumulation coefficient (substance content in tissues / substance content in the environment). The accumulation coefficient of radioactive Phosphorus in goose tissues is 2 000 000. Biogens and their substitutes accumulate through selective uptake from the environment (radioactive Iodine after Chernobyl, Strontium instead of Calcium, Caesium instead of Potassium). Xenobiotics accumulate due to the absence of excretion mechanisms (chloroform in membranes, DDT and its breakdown products in fatty tissue). Sometimes accumulation begins even at the abiogenic level (DDT, heavy metal ions selectively accumulate on detritus particles). Detritus suspension filter feeders are the most powerful accumulators of toxins.