To understand the concept of productivity we must first distinguish the definitions in the production process as shown below:
1. Biomass is the dry weight of all organic matter in plants and animals in an ecosystem.
2. Gross primary productivity (GPP) is the total rate of photosynthesis, including the organic matter used up in respiration during the period of measurement. This is also known as total photosynthesis.
3. NPP is the rate of storage of organic matter in plant tissues that exceeds the respiratory use, R, by the plants during the period of measurement. This is also termed net assimilation. In practice, the amount of plant respiration is usually added to the measurements of NPP to estimate GPP (GPP = NPP + R).
4. Net community productivity is the rate of storage of organic matter not used by heterotrophs (i.e., net primary production minus heterotrophic consumption) during the period under consideration, usually the growing season or a year.
5. Finally, the rates of energy storage at consumer levels are referred to as secondary productivities. Because consumers use only food materials already produced, with appropriate respiratory losses, and convert this food energy to different tissues by one overall process, secondary productivity should not be divided into gross and net amounts. The total energy flow at hetero-trophic levels, which is analogous to the gross productivity of autotrophs, should be designated assimilation and not production.
In all these definitions, the term productivity and the phrase rate of production may be used interchangeably. Even when the term production designates an amount of accumulated organic matter, a time element is always assumed or understood (for instance, a year in agricultural crop production). Thus, to avoid confusion, one should always state the time interval.
In accordance with the second law of thermodynamics, the flow of energy decreases at each step due to the heat loss occurring with each transfer of energy from one form to another. The second law of thermodynamics, or the law of entropy, may be stated as follows: No process involving an energy transformation will spontaneously occur unless there is a degradation of energy from a concentrated form into a dispersed form. For example, heat in a hot object will spontaneously tend to become dispersed into the cooler surroundings. Second, because some energy is always dispersed into unavailable heat energy, no spontaneous transformation of energy (sunlight, for example) into potential energy (protoplasm, for example) is 100% efficient. Entropy (from en = 'in' and trope = 'transformation') is a measure of the unavailable energy resulting from transformations; the term is also used as a general index of the disorder associated with energy degradation. Organisms, ecosystems, and the entire ecosphere possess the following essential thermodynamic characteristics: they can create and maintain a high state of internal order, or a condition of low entropy (a low amount of disorder). Low entropy is achieved by continually and efficiently dissipating energy of high utility (light or food, for example) into energy of low utility (heat, for example). In the ecosystem, order in a complex biomass structure is maintained by the total community respiration, which continually 'pumps out disorder'. Accordingly, ecosystems and organisms are open, non-equilibrium thermodynamic systems that continuously exchange energy and matter with the environment to decrease internal entropy but increase external entropy (thus conforming to the laws of thermodynamics).
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