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An ecological pyramid is a graphical representation of an ecological parameter, like number or biomass, or accumulated energy at different trophic levels in a food chain in an ecosystem.

The idea of ecological pyramids was developed by Charles Elton (1927), so the ecological pyramids are also called Estonian pyramids. An ecological pyramid may be upright (tapering towards the tip) or inverted (widens towards the tip) or spindle-shaped (broader in the middle and narrow above and below). An upright ecological pyramid indicates that the producers outnumber or outweigh the herbivores which, in turn, outweigh or outnumber the carnivores.

A pyramid of energy shows how much energy is retained in the form of new biomass at each trophic level, while a pyramid of biomass shows how much biomass (the amount of living or organic matter present in an organism) is present in the organisms. There is also a pyramid of numbers representing the number of individual organisms at each trophic level. Pyramids of energy are normally upright, but other pyramids can be inverted or take other shapes.

Ecological pyramids begin with producers on the bottom (such as plants) and proceed through the various trophic levels (such as herbivores that eat plants, then carnivores that eat flesh, than omnivores that eat both plants and flesh, and so on). The highest level is at the top of the food chain.

It is the graphic representation showing the arrangement of the number of individuals (population size) of different trophic levels in a food chain in an ecosystem.

In a predatory food chain e.g. food chain in a grassland ecosystem or a pond-ecosystem, the pyramid of number is a square pyramid. Here, the size of organisms increases with each trophic level from the producer level to the top carnivore level while their number decreases in a food chain (from grasses to predatory birds in the grassland ecosystem and from phytoplankton to large fish in pond ecosystem).

So, the producers are smallest sized but maximum in number while the top carnivores are largest in size but least in number so these cannot be used as prey by another species. In a tree-dominated ecosystem, a single large-sized tree is attacked by numerous minute plant-eating insects preyed upon by fewer spiders and carnivorous insects that are further preyed upon by a lesser number of small-sized birds which are finally preyed upon by only a few large-sized birds of prey, the ecosystem is spindle-shaped. Similarly, the pyramid of the number in the forest ecosystem is spindle-shaped.

But in a parasitic food chain e.g. an oak-tree; a pyramid of number is an inverted pyramid in which a single oak tree (producer) supports a large number of fruit-eating birds which, in turn, support a still larger number of parasites like lice and bugs. Hyper parasites like bacteria; fungi, etc. are the greatest in number and occupy the top of the inverted pyramid of numbers.

 

(b) Pyramid of Biomass:

The total amount of weight of the fresh or dry weight of living or organic matter in an ecosystem in a unit area at any time is called biomass. Pyramid of biomass is a graphic representation of biomass present per unit area in different trophic levels.

Pyramid of biomass is a straight or upright pyramid in a terrestrial habitat which shows that biomass is maximum at the level of producers and there is a progressive decrease in biomass from lower to higher trophic levels. It is found that about 10-20% of the biomass is transferred to the next trophic level in a food chain.

 

But in aquatic habitats, the pyramid of biomass is inverted (Fig. 14.17) or spindle-shaped where the biomass of a trophic level depends upon the reproductive potential and longevity of the members because the biomass of phytoplankton is less than that of zooplanktons in which it is less than that of secondary consumers.

 

(c) Pyramid of Energy:

It is a graphic representation of the amount of energy trapped per unit time and area in different trophic levels of a food chain.

The unit of measurement of energy is kcal/m2/year. According to the second law of thermodynamics, there is a gradual decrease in energy at successive trophic levels. It is so because at each transfer, about 80-90 percent of the energy available at a lower trophic level is used to overcome its entropy and to perform metabolic activities and only 10% (in herbivores) to 20% (in carnivores) is available to next trophic level.

So, the pyramid of energy is always upright or straight pyramid (Fig. 14.18). The law governing the retaining of only 10% of chemical energy at each trophic level is called a 10 percent law. It was proposed by Lindemann (1942).

 

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