Ecology: The Biology of Interactions. 3.12. Ecological Efficiencies

Ukrainian language (latest version) / Russian language (updates stopped)

3.11. Trophic relationships and levels

D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 3. Biogeocenology and Ecology of Communities

3.13. Ecological Pyramids

3.12. Ecological Efficiencies A remarkable scheme for describing energy flow through a trophic level is the “Odum square” (Fig. 3.12.1). Eugene Odum proposed a visual diagram showing the flow of energy through an individual organism, a population, or a trophic level. The diagram shows into which “branches” the energy flow splits as it passes through biosystems. [IMG_1] Fig. 3.12.1. Distribution of energy flows through an organism, population, or trophic level For example, this scheme makes it easy to see the difference between the two main measures of production: gross (A) and net (P), namely respiratory costs. The “Odum square” makes it easy to verify that A = R + P; P = G + S + E, etc. The following measures of energy use and conversion efficiency can be distinguished: exploitation efficiency E1 = I_exploiter / P_prey; assimilation efficiency E2 = A / I; net production efficiency E3 = P / A; gross production efficiency E4 = P / I = E2 × E3; ecological efficiency E5 = P_exploiter / P_prey = E1 × E2 × E3. The energy flows shown in the scheme are related differently in different organisms. Figuratively speaking, the simpler the task of energy conversion, the lower the losses. Carnivores solve a relatively simple task: they obtain energy from high-quality food that is comparatively “easy” to process and to use for building their own bodies. The most difficult task is solved by photosynthetic organisms, which use light as their energy source. For example, a substantial part of the light falling on plants is reflected or absorbed by soil. Most of the absorbed energy remains unused by plants. Under favorable conditions, plants can assimilate about 1% of incident solar energy, and only about 0.5% is converted into net production (i.e., A = 1%, P = 0.5%). On average for the biosphere, these values are even lower: plants assimilate about 0.2% of solar energy, and only about 0.1% becomes net production (A = 0.2%, P = 0.1%). Still, in absolute terms these quantities are very large by human standards. The efficiency of animal feeding strongly depends on food type. Assimilation efficiency (A/I) in carnivores ranges from 60% (in insectivores) to 90% (in meat- and fish-eaters). In herbivores, assimilation efficiency is about 80% in seed-eaters; 60% in feeders on young leaves; 30-40% in feeders on old leaves; and 10-20% or even less in wood-eaters. Further energy losses depend greatly on metabolic rate. For example, small birds expend over 99% of assimilated energy on respiration, and less than 1% of consumed energy goes into net production. In small mammals this value is 6%, in domestic cattle 11%, in pigs 20%, and in some poikilotherms, especially large fish and reptiles, it reaches 75%. Compare: a titmouse that eats 1 kg of insects gains less than 6 g in mass, while a python that eats a 1-kg guinea pig gains over 660 g of its own mass. In these calculations we assumed equal energy content per unit mass of insects, tits, guinea pigs, and pythons (an acceptable approximation). In both cases, we estimated losses from assimilation and net production. In the first case we used the assimilation efficiency given above for insectivores and the net production efficiency for small birds (1000 × 0.6 × 0.01 = 6), and in the second case the corresponding values for carnivores and large reptiles (1000 × 0.9 × 0.75 = 675). Among other things, this is the price of homeothermy. As energy moves along trophic chains, its quantity decreases while its quality (work potential) increases. A quality indicator is the number of units of solar energy that must be dissipated to obtain one unit of energy in a new form available for transfer to higher trophic levels (Table 3.12.1). Howard Odum (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 required natural resources, i.e., the amount of solar energy spent to obtain a given product. Table 3.12.1. Changes in energy quantity and quality during transformations Sun Plants Consumers I Consumers II Energy quantity 1000000 10000 1000 100 Energy quality (emergy) 1 100 1000 10000 Sun Wood Coal Electricity Energy quantity 1000000 1000 500 125 Energy quality (emergy)

Sun

Plants

Primary consumers

Secondary consumers

Consumers II

1000000

10000

1000

100

100

1

100

1000

10000

Sun

Wood

Coal

Electricity

Consumers II

1000000

1000

500

125

100

1

1000

2000

8000

Another consequence of matter and energy transfer from level to level is biological accumulation: increasing concentrations of many substances selectively retained by biomass. The measure of concentration in a trophic chain (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 because of selective uptake from the environment (radioactive iodine after Chernobyl, strontium substituting for calcium, cesium for potassium). Xenobiotics accumulate due to the absence of excretion mechanisms (chloroform in membranes, DDT and its breakdown products in adipose tissue). Sometimes accumulation begins even at the abiotic level (DDT and heavy-metal ions selectively accumulate on detrital particles). Suspended-detritus filter feeders are among the most powerful toxin accumulators.

3.11. Trophic relationships and levels

D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 3. Biogeocenology and Ecology of Communities

3.13. Ecological Pyramids