Productivity of Ecosystems, Biology tutorial


All the living organisms need energy for the metabolic activities and sustenance. This energy enters the ecosystem via the producers that are capable to produce the organic materials from sunlight.

Concept of productivity:

An ecosystem comprises heterotrophs and autotrophs. Autotrophs comprise of plants, algae and a few bacteria, whereas heterotrophs comprise animals fungi, nearly all protists and bacteria and non-green plants. Autotrophs (that is, plants) are capable to imprison light energy and prepare their own food. Though, heterotrophs should achieve organic molecules that have been synthesized through autotrophs. The energy for survival of the heterotrophs is thus based on the autotrophs.

Primary productivity:

Around 1 to 5 percent of solar energy which falls on a plant is transformed to food or other high quality organic materials. Primary productivity is a word employed to explain the amount of organic matter generated by green plants from solar energy in a given area throughout a given time-period.

1) Gross Primary productivity:

The Gross Primary productivity is the net organic matter produced comprising that employed by the photosynthetic organisms (that is, green plants) for respiration. No subtraction is build up from the net production.

2) Net primary Productivity:

The Net primary Productivity is the net amount of energy fixed per unit of time minus the energy expended through the metabolic actions of the photosynthetic organisms in the community. It equivalents the gross primary productivity minus the quantity of energy expended by the metabolic actions of the photosynthetic organisms.

3) Significance of primary productivity:

Energy enters the ecosystem from the sun as an outcome of the photosynthesis. It is slowly discharged as metabolic processes carry on. The autotrophs first obtain this energy via primary productivity and give all the energy which heterotrophs utilize devoid of which they will not survive. Primary productivity thus becomes necessary to the survival of ecosystems.

Energy Flow:

Energy constantly flows via the biological world in one direction having new energy from the sun constantly entering the system to substitute the energy which is dissipated as heat. Green plants, that is, the primary producers of a terrestrial ecosystem, usually capture around 1 to 5 percent of the energy which falls on their leaves transforming it to food energy. This percentage might be a little higher in particularly productive ecosystems.

When such plants are consumed through herbivores or primary consumers, only a part of the plants accumulated energy is in reality transformed into the bodies of the organisms which consume them. The similar applies to secondary consumers, carnivores that feed on the herbivores. Just some of the potential energy stored in the herbivore tissues is transformed into the body of the carnivore. This reducing trend is maintained from one Trophic level to the next.

Efficiency of energy transfer:

The amount of energy ingested and maintained at each Trophic level goes toward heat production. A great deal of the energy is employed for digestion and work. Generally 40 percent or less of energy goes toward growth and reproduction.

Raven and Johnson in the year 1996 noticed that an invertebrate usually employs around a quarter of the 40% that is, 10% of the food it eats to its own body and therefore into potential food for its predators. The comparable figure differs from around 5% in carnivores to almost 20% for herbivores though, 10% is a good average value for the amount of organic matter which reaches the next Trophic level. Because of the energy lost at each Trophic level are so great, food chains usually comprise of only three or four steps. Much little energy remains in the system as usable energy after it has been incorporated successfully to the bodies at four Trophic levels.

In other words, merely around 10% of the energy fixed in the food is fixed in the body of the animal which eats that food. The Trophic efficiency is in general associated to the formula- exploitation efficiency x assimilation efficiency x production efficiency (Chapman and Reiss, in the year 1995).

Pyramid of Energy:

A pyramid of energy exhibits the flow of energy from one Trophic level of a community to the next and the rate at which energy flows up the food web. The units of the pyramids of energy are energy/area/time example: kcal/M2/yr that is, Kilocalories/Square meter/year and are generally measured over a time-period. Following the law of conservation of energy which states that energy can neither be created nor destroyed however is always conserved; pyramids of energy can never be inverted as can be found by the pyramids of numbers or of biomass. The productivity of herbivores can't exceed the total primary productivity and that of the first level carnivores should be less than the productivity of the herbivores. The productivity of the second-level carnivores should be less than the productivity of the first-level carnivores and so forth.

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