The amount of energy fixed in a given time, the time element, must be considered. Biological productivity thus differs from yield in chemical or industrial terms. In industry, the reaction ends with the production of a given amount of material, whereas in biological communities, the production process is continuous in time, so a time unit must be designated (e.g., the amount of food manufactured per day or per year). Although a highly productive community may have more organisms than a less productive community, this is not so if organisms in the productive community are removed or 'turned over' rapidly. For example, a fertile pasture being grazed by livestock is likely to have a much smaller standing crop of grass than a less-productive pasture not being grazed at the time of measurement. Biomass or standing crop present at any given time should not be confused with productivity. Usually, one cannot determine the primary productivity of a system or the production of a population component simply by counting and weighing the organisms present at any one moment, although NPP can be estimated from standing crop data when living materials accumulate over a period of time (growing season) without being consumed (e.g., as in cultivated crops).
Only about half of the total radiant energy of the sun is absorbed, and at most about 5% (10% of the energy absorbed) can be converted by gross photosynthesis under the most favorable conditions. In turn, plant respiration appreciably reduces the food available for heterotrophs, usually by about 20-50%. At the peak of the growing season, especially during long summer days, as much as 10% of the total daily solar input may be converted into gross production, and from 65% up to 80% of this may remain as net primary production during a 24 h period. Even under the most favorable conditions, however, these high daily rates cannot be maintained over the annual cycle, nor can they achieve such high yields over large areas of farmland.
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