Here is an essay on ‘Ecological Pyramid’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Ecological Pyramid’ for school and college students.
Essay # 1. Definition of Ecological Pyramid:
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.
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The idea of ecological pyramids was developed by Charles Elton (1927), so the ecological pyramids are also called Eltonian 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.
Essay # 2. Types of Ecological Pyramids:
On the basis of ecological parameters, ecological pyramids are of three types:
(a) Pyramid of Number:
It is the graphic representation showing the arrangement of 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 (Fig. 14.15A), the pyramid of number is a straight 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 grassland ecosystem and from phytoplanktons 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 (Fig. 14.15C), single large-sized tree (T1) is attacked by numerous minute plant-eating insects (T2) preyed upon by fewer spiders and carnivorous insects (T3) which are further preyed upon by a lesser number of small-sized birds (T4) which are finally preyed upon by only a few large sized birds of prey (T5), the ecosystem is spindle-shaped. Similarly, the pyramid of number in forest ecosystem is spindle-shaped.
But in a parasitic food chain e.g. an oak-tree (Fig. 14.15B); 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. Hyperparasites like bacteria; fungi, etc. are the greatest in number and occupy the top of inverted pyramid of numbers.
(b) Pyramid of Biomass:
The total amount of weight of 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.
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Pyramid of biomass is a straight or upright pyramid (Fig. 14.16) in a terrestrial habitat which shows that biomass is maximum at the level of producers and there is progressive decrease in biomass from lower to higher trophic levels. It is found that about 10-20% of the biomass is transferred to next trophic level in a food chain.
But in aquatic habitats, 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 phytoplanktons 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 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 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 per cent of energy available at 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 10 per cent law. It was proposed by Lindemann (1942).
Essay # 3. Limitations of Ecological Pyramids:
1. These do not take into account those species which belong to two or more trophic levels on the basis of their feeding relationship with others e.g. a sparrow is said to be primary consumer when it eats seeds, fruits, peas, etc., while it is referred as secondary consumer when it eats insects and worms.
2. These do not accommodate a food web.
3. Though saprophytes play important role in the material cycling in an ecosystem, yet they are not taken into account in ecological pyramids.