Efficiency of Vegetation

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If the working process creating a new biomass is photosynthesis, then the working machine is plant. Therefore, it is natural to say about efficiency of plant (efficiency of vegetation) or 'green leaf than about efficiency of photosynthesis. The rate of photosynthesis depends on the amount of light reaching the leaves, the temperature of surrounding air, and the availability of water and other nutrients such as nitrogen and phosphorus. One of these factors ('limiting factors') already limits the rate, so that the real efficiency of vegetation is lower than its

Table 1 The NPP and the efficiency of utilization

Continents

(gCm2yr1)

Efficiency (%)

Europe

365

0.54

Asia

421

0.38

North America

353

0.40

South America

899

0.49

Africa

443

0.25

Australia with

370

0.19

Oceania

Land on average

408

0.37

Efficiency of solar energy utilization by vegetation can be defined as the ratio of enthalpy, contained in the NPP, to the solar radiation, reaching to the Earth's surface and integrated over the vegetation period. The corresponding values for continents and for land overall are shown in Table 1.

One square meter of the terrestrial vegetation on average utilizes in the course of 1 year about 17 million joules of solar energy, but this gigantic number constitutes only 0.37% of the total solar energy that comes into the Earth's surface.

The total annual production of terrestrial vegetation is about 60 gigatons (Gt, 1 Gt = 109 t) of carbon, while for the ocean this value is estimated as 25 Gt with NPP = 75gCm"2yr_1. Thus, the mean global NPP is 186 gC m"2 yr"1, and the mean efficiency of global vegetation is about 0.1%. However, it is necessary to take into account that much energy is consumed in the process of forming and maintaining of the thermostat for vegetation. It is very similar to the situation with greenhouse, where the most part of energy is used for its heating.

Energy Transfers, Trophic Chains, and Trophic Networks

If we look at a global pattern of pathways on which the solar energy stored in biomass is flowing within the gigantic (and unique) ecosystem (often associated with the biosphere), we see the network entangling the Globe. It is named a 'trophic network or a food web', and as a rule subdivided on local networks. The trophic network is described by an oriented graph with vertices corresponding to species that constitute the ecosystem, and links indicating trophic interaction between them (their directions show the energy flowing, for instance, prey ! predator). In the network structure the 'trophic levels' are naturally distinguished, that is, groups of species having no direct trophic interactions; however, species of one level usually either compete for life resource or cooperate in its utilization. It is natural that some part of energy is spent (and later on dissipated as heat) in such kind of interactions - this is a payment by means of energy for stability of the network structure. Another significant part of consumed energy (from 30% to 70%) is spent for maintaining life in the process of 'metabolism' (respiration).

In any trophic network a structure, in which every two adjacent species form a prey-predator pair and which is described by a linear graph, can be distinguished. It is called a 'trophic (food) chain'; their interlacing and branching form a trophic network. Since the energy dissipation along the chains is very high they are usually short (their length measured in the number of links is about 4-6).

The basic trophic species of chain usually are 'producers' (plants, autotrophic organisms that accumulate the solar energy and 'nutrients' - carbon, nitrogen, phosphorus, etc.); the next species are 'primary consumers' (herbivorous, heterotrophic organisms), and 'secondary consumers' (carnivores, predators, preying on herbivorous). Really, the chain may be longer. It is not necessary that the chain be originated by an autotroph: it may be any species considered as a resource for consequent ones. For instance, if a resource is 'detritus' (faeces, dead organic matter) then a special 'detritus chain' can be considered. At last, trophic chains could be 'open' and 'closed'; as a rule they are open in relation to the energy flowing through an ecosystem and carbon that is accumulated in the process of photosynthesis and spent in the respiration, and closed in relation to nutrients turning in the ecosystem.

In order to start a 'biogeochemical machine' we have to 'close' the chain by species named 'decomposers' (protozoa, bacteria, fungi, scavengers, and carrion eaters), which in the course of their vital activity split complex organic compounds into simpler mineral substrates (nutrients) for autotrophs. A principal scheme of such kind of 'biogeo-chemical machine' is shown in Figure 3.

Really, the closure is not complete: about 1% of dead organic matter is deposited in the deep ground ('kero-gen'), and has accumulated over long periods of geologic time (oil and coal repositories).

A small amount of the energy passes from one trophic level to another; for instance, only from 5% to 25% of plant biomass is consumed by herbivores, the rest falls out and becomes a resource for decomposers (detritophages). Efficiency of this passing is called 'ecological efficiency';

Figure 3 Flows of mass and energy in an elementary biogeochemical machine.

DOM 0.8

AM 197

AM 1182

AM 8.9 231

Producers 17 GGG

DOM 6.12

DGM 146

DOM 727

Inflow 2773

Figure 4 Trophic chain of the ecosystem of Silver Springs. Energy flows and biomasses are measured in mWm"2 and kJ m"2, respectively. DOM, dead organic matter; AM, assimilation and metabolism.

The result of such kind of consequent energetic transitions is a pyramid of energy, with most energy concentrated by autotrophs at the bottom of trophic chain and less energy at each higher trophic level. As an example the trophic chain and the pyramid of biomass of the concrete ecosystem of warm Silver Springs in Florida are presented in Figure 4. Note that it is a classic object that has been studied by H. T. Odum. The ecosystem has four trophic levels: (1) producers (phytoplankton), (2) herbivores (zooplankton), (3) carnivores (fish), (4) higher predators (predacious fish), and one special level, decomposers, with biomass equal to 105 kJ m"2. Since the system is through-flowing, that is, described in the terms of energy flows, therefore the chain may be considered open, without decomposers.

Conclusion

As described above biosphere machines from the anthro-pocentric point of view are badly made, with very low efficiency. They dissipate the solar energy by heating the environment more than perform some useful work. Nevertheless, they are significantly reliable. Since it is necessary to pay for their reliability and stability, they pay by high dissipation of energy that in turn decreases their efficiency.

on average, it equals 10%. The rate at which these consumers use the chemical energy of their food for growth and reproduction is called 'assimilation efficiency'. For instance, assimilation efficiency of herbivores lies in the interval from 15% to 80%, while the interval for carnivores is from 60% to 90%.

It is easy to estimate that such a predator as Homo sapiens (the third trophic level) gets only 1% of solar energy stored by plants. Unfortunately, the situation has not improved; if he would be a vegetarian, by winning in the ecological efficiency, he would lose in the assimilation efficiency.

See also: Radiation Balance and Solar Radiation Spectrum.

Further Reading

Budyko MI (2001) Evolution of the Biosphere (Atmospheric and

Oceanographic Sciences Library), 444p. Berlin: Springer. Jorgensen SE and Svirezhev YuM (2004) Towards a Thermodynamic

Theory for Ecological Systems, 366p. Amsterdam: Elsevier. Morowitz HJ (1978) Foundations of Bioenergetics. New York: Academic Press.

Smile V (2002) The Earth's Biosphere: Evolution, Dynamics, and Change. Cambridge, MA: MIT Press.

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