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Ecosystem productivity
Dec 14, 2013

Tansley has defined ecosystem as “the system resulting from the integration of all the living and non-living factors of the environment.”

The productivity of an ecosystem refers to the rate of production i.e. the amount of organic matter accumulated in any unit time. Productivity is of following types:

1. Primary Productivity. It is associated with the producers which are autotrophic, most of which are photosynthetic, and to a much lesser extent the chemosynthetic microorganisms. These are the green plants, higher macrophytes as well as lower forms, the phytoplanktons and some photosynthetic bacteria. Primary productivity is defined as “the rate at which radiant energy is stored by photosynthetic and chemosynthetic activities of the producers.” Primary productivity is further distinguished as:

(a) Gross Primary Productivity. It is the total rate of photosynthesis including the organic matter used up in respiration during the measurement period. This is also sometimes referred to as Total (Gross) Photosynthesis or Total Assimilation. It depends on the chlorophyll content. It is estimated in terms of either chlorophyll content as, Chl/g dry weight/unit area, or photosynthetic number i.e. amount of CO2 fixed/g Chl/hour.

(b) Net primary Productivity. It is the rate of storage of organic matter in plant tissues in excess of the respiratory utilization by plants during the measurement period. This is thus the rate of increase of biomass and is also known as Apparent Photosynthesis or Net assimilation. Thus net primary productivity refers to balance between gross photosynthesis and respiration and other plant losses as death etc.

2. Secondary Productivity. It refers to the consumers or heterotrophs. These are the rates of energy storage at consumer level. Since consumers only utilize food material (already produced) in their respiration, simply converting the food matter to different tissues by an overall process, secondary productivity is not divided into ‘gross’ and ‘net’ amounts. Thus, some ecologists prefer to use the term assimilation rather than production at this level. Secondary productivity actually remains mobile (i.e. keeps moving from one organism to another) and does not live in situ like the primary productivity.

3. Net Productivity. It refers to the rate of storage of organic matter not used by the heterotrophs (consumers) i.e. equivalent to net primary production minus consumption by the heterotrophs during the unit period, as a season or year etc. It is thus the rate of increase of biomass of the primary producers which has been left over by the consumers. Net productivity is generally expressed as production of C g/m2/day which may then be consolidated on month, season or year basis.

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