ECOLOGICAL PRODUCTIVITY

The basic or primary productivity of an ecological system is defined as the rate at which energy is stored by photosynthetic and chemo-synthetic activities of producer organism in the form of organic substances which can be used as food material. The producer organisms are cheaply green plants. If photosynthesis were the only process occuring in plants, we could measure production by accumulation of carbohydrates but unfortunately, at the same time plants perspire using energy for maintenance activities.

At equilibrium, photosynthesis will be equal to respiration and this is known as compensation point and if plants were to exist at compensation point, they would be no production of food materials for animals. 

PRIMARY PRODUCTIVITY

Two kinds of primary productivity could be distinguished into:-

1. Gross primary productivity (GPP) and

2. Net primary productivity (NPP)

GPP:- This is the total rate of photosynthesis including the organic matter used up in respiration during the measurement period. This is also known as total photosynthesis or total assimilation.

NPP:- This is the rate of storage of organic matter in plant tissues in excess of the respiratory utilization by the plants during the period of measurement. This is also referred to as apparent photosynthesis or net assimilation.

The keyword in the definition is rate so the time element must be considered i.e. the amount of energy fixed in a given time and this means that biological productivity differs from the yield in chemical or industrial sense. In chemical or industrial sense; reaction ends with the production of a given amount of material but in biological communities the process is continuous in time. So it is important to designate a time unit e.g. amount of food manufactured per day or per year.

In a more general term, productivity of an ecosystem refers to its richness. We should however note that while a rich or productive community may have a larger quantity of organisms than a less productive community. This is by no means always the case. So standing biomass or standing crop present at any given time shouldn't be confused with productivity.

A fertile pasture which is being grazed by livestock will likely have a much smaller crop or grass than a less productive pasture at the time of measurement.

The grazed pasture situation is to be effected in a wide variety of natural communities where all trophic levels are present and active with the result that consumption occurs more or less simultaneously with production.

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SECONDARY PRODUCTION

Secondary production is the generation of biomass of heterotrophic (consumer) organisms in a system. This is driven by the transfer of organic material between trophic levels, and represents the quantity of new tissue created through the use of assimilated food. Secondary production is sometimes defined to only include consumption of primary producers by herbivorous consumers (with tertiary production referring to canivorous consumers), but is more commonly defined to include all biomass generation by heterotrophs.

Organisms responsible for secondary production include animals, protists, fungi and many bacteria.

Secondary production can be estimated through a number of different methods including increment summation, removal summation, the instantaneous growth method and the Allen curve method. The choice between these methods will depend on the assumptions of each and the ecosystem under study. For instance, whether cohorts should be distinguished, whether linear mortality can be assumed and whether population growth is exponential.

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