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Ecosystem — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYEcosystem4 Marks
Question
Define ecological pyramids and describe with examples, pyramids of number and biomass.
✓

Answer

→ The trophic structure and function at successive trophic levels, i.e. (producers, herbivores, carnivores), may be graphically represented by means of ecological pyramids where the first or producer level constitutes the base of the pyramid and the successive levels, the tiers, making the apex.
→ Ecological pyramids are of three general types.
(A) Pyramid of Number
(B) Pyramid of Biomass
(C) Pyramid of Energy
→ Trophic level represents structure of an ecosystem is one kind of producer consumer arrangement.
(A) Pyramid of number:
→ It shows the number of individual organisms at each trophic level. Producers occur in the largest number and top level carnivores are in the smallest number. Normally, the pyramids of numbers is upright.
Image
→ There are exceptions to this generalisation.
→ If you were to count the number of insects feeding on a big tree, the number pyramid is inverted. → For example, 50 parrot live on a banyan tree and 50 insects on each parrot.
(B) Pyramid of biomass:
→ It shows the total dry weight or the average biomass of organisms at a particular trophic level.
Image
→ The pyramid of biomass in sea is generally inverted.
→ Example, the biomass of fishes far exceeds that of phytoplankton.
(C) Pyramid of energy:
→ It shows the rate of energy flow and/or productivity at successive trophic levels. Pyramids of energy are always upright.
→ Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.
Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time or annually per unit area.
→ According to law of thermodynamics amount of available energy decreases during utilization.
Image
→ There are certain limitations of ecological pyramids.
→ Such as it does not take into account the same species belonging to two or more trophic levels.
→ It assumes a simple food chain, something that almost never exists in nature; it does not accommodate a food web.
→ Sparrow, when eats seed, pea and fruits then it is a primary consumer but when it eats insects and worms, its called secondary consumers. Saprophytes are not given any place in ecological
→ pyramids even though they play a vital role in the ecosystem.

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