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Ecological pyramids are nothing but diagrams of data which represents the standing crops at each trophic level. This concept was developed by Charles Ellion in the year 1927.

According to this concept, the trophic structure and functions at each successive trophic level may be graphically shown by means of ecological pyramids. In such pyramids, the first level represents the base of the pyramid and the successive levels are making the apex of pyramids.

The interactions of the food chain phenomenon, concerning loss at each transfer and size metabolism relationship results in communities having a definite trophic structure which is often characteristics of the particular type of ecosystem. Such ecological pyramids are essentially of three types:

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(a) Pyramid of Number :

Which shows the number of individual organisms at each level?

(b) Pyramid of Biomass :

Which shows the total dry weight and other suitable measure of the total amount of living matter?

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(c) Pyramid of Energy :

Which shows the rate of energy flow and productivity of successive levels?

(a) Pyramid of Number :

This deals with the relationship between the producers, herbivores and carnivores at successive levels in terms of their number. It was Charles Ellion (1927) who pointed out the great difference in the number of organism involved in each step of the food chain. As is apparent from the figure- the animals at the lower and or base of the pyramid of the chain are most abundant. Successive links of carnivores decrease rapidly in number until there are very few carnivores at the top. It is worth mentioning that the pyramid of numbers does not indicate the transfer of energy.

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The lake ecosystem provides a typical example of pyramid of numbers.

However, in a tree ecosystem an inverted pyramid is obtained.

The pyramids of numbers do not give a true picture of the food chain as they are not very functional. Also, they do not indicate the relative effects of food chain and size factors in the organisms. In general they vary with different communities with different types of food chains in the same environment. Some times, it becomes very difficult to represent the entire community on the same numerical scale such as in the case of forests.

(b) Pyramid of Biomass :

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The biomass of the numbers of the food chain present at any time forms the pyramid of the biomass. Pyramid of biomass indicates the decrease of biomass in each trophical level from base to apex. For most ecosystems, on land the pyramid of biomass has a larger base of primary producers with a smaller trophic level on the top. This is shown in Fig. 9.

In grass land and forest ecosystem there is a gradual decrease in biomass of organisms at successive levels. However, in a pond ecosystem as the producers are small organisms, their biomass is least but gradually shows an increase towards the apex of pyramid. Thus, the pyramid is inverted in shape.

(c) Pyramid of Energy :

Amongst the three types of ecological pyramids, the energy pyramid is considered to be the best regarding the overall nature of the ecosystem. In this case the rate of energy flow and productivity gradually decreases in the availability of energy from autotrophs to higher trophic levels.

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Pyramid of energy reflects the laws of thermodynamics and is thus always right side up with a large energy base at the bottoms. The base upon which the pyramid of energy is constructed is the quantity of organism produced per unit time or in other words, the rate at which food material passes through the food chains. Some organisms may have a small biomass but the total energy they assimilate and pass on may be considerably greater than that of organisms with a much larger biomass.

The pyramid of energy is shown in the figure. 10. This figure is illustrated with an example that if the deer eats the plants, containing 250 calories of food energy, the deer uses some amount of it for its own metabolism and stores only 25 calories as food energy. Finally a lion that eats deer gets an even smaller amount of energy. In this manner the energy decreases from sunlight to herbivores to carnivores. This is the main reason that energy pyramid is always upright.

Pyramids of Energy Flow and Energy Loss :

At each transfer from one trophic level to another in a food chain or web, work is done, low-quality heat is given off to the environment, and the availability of high-quality energy to organisms at the next trophic level is reduced. This reduction in high-quality energy available at each trophic level is the result of the inevitable energy quality tax imposed by the second law of energy.

The percentage of available high-quality energy transferred from one trophic level to another varies from 2% to 30%, depending on the types of species involved and the ecosystem in which the transfer takes place. In the wild, ecologists estimate that an average of about 10% of the high-quality chemical energy available at one trophic level is transferred and stored in usable form as chemical energy in the bodies of the organisms at the next level.

The rest of the energy is used to keep the organisms alive, and most is eventually degraded and lost to the environment as low-quality heat in compliance with the second law of energy. Some of it is transferred to decomposers, which use a small amount to stay alive and degrade the rest to low-quality heat.

There is a loss of usable-quality energy at each step in a simple food chain. The pyramids of energy flow and energy loss in show that the greater the number of trophic levels or steps in a food chain or web, the greater the cumulative loss of usable high- quality energy.

Living organisms can use energy in several forms, but all can be grouped under one of two headings: radiant and fixed. Radiant energy is in the form of electromagnetic waves such as light. Fixed energy is potential chemical energy bound up in various organic substances which can be broken down or reacted with something else in order to release their energy content.

Only a small fraction of the light energy reaching the earth is trapped; considerable areas of earth have no plants, and plants can utilize in photosynthesis only about 3 per cent of the incident energy. This radiant energy of sun is converted into chemical or potential energy by photosynthesis in plants, the photosynthetic autotrophs:

The reaction is catalyzed by certain pigments (e.g., chlorophylls in the cells). The product of this reaction is carbohydrate such as sugar shown above. This sugar can have several destinations: it can be converted to a relatively inert energy-rich organic substance such as starch, and stored; it can be combined with other sugar molecules to form specialized carbohydrates such as cellulose, which are used by the plants for specific purposes; and it can be combined with other substances-such as nutrient substances such as nitrogen, phosphorus, and sulphur, top build complex molecules like proteins, nucleic acids, pigments, and hormones.

All these type of reactions are necessary for normal growth and maintenance of body tissues and functions of the plants. All required energy which is provided by oxidation so some of the sugar produced by photosynthesis to give CO2, H2O and usable chemical energy:

Oxidation of sugar or any other organic molecule to get usable energy by organism is called respiration. The energy released by respiration is lost permanently to the ecosystem.

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