Read the passage and answer the question.
The base of a pyramid is broad and it narrows towards the apex. One gets a similar shape whether one expresses the food or energy relationship between organisms at different trophic levels. This relationship is expressed in terms of number, biomass or energy. The base of each pyramid represents the producers or the first trophic level, while the apex represents the tertiary or top level consumer. Any calculation of energy content, biomass or numbers has to include all organisms at that trophic level. No generalisation we make will be true if we take only a few individuals at any trophic level into account. Also, a given organism may occupy more than one trophic level simultaneously. One must remember that the trophic level represents a functional level, not a species as such. In most ecosystems, all the pyramids, of number, of energy and of biomass, are upright. There are, however, exceptions to this generalisation.
Read the passage and answer the question.
The base of a pyramid is broad and it narrows towards the apex. One gets a similar shape whether one expresses the food or energy relationship between organisms at different trophic levels. This relationship is expressed in terms of number, biomass or energy. The base of each pyramid represents the producers or the first trophic level, while the apex represents the tertiary or top level consumer. Any calculation of energy content, biomass or numbers has to include all organisms at that trophic level. No generalisation we make will be true if we take only a few individuals at any trophic level into account. Also, a given organism may occupy more than one trophic level simultaneously. One must remember that the trophic level represents a functional level, not a species as such. In most ecosystems, all the pyramids, of number, of energy and of biomass, are upright. There are, however, exceptions to this generalisation.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Ecological pyramids do not accommodate a food web.
Reason (R): Ecological pyramids assume a simple food chain, something that almost never exists in nature.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
An ecological pyramid places organisms in neat steps: producers at the base, then herbivores, then carnivores. NCERT points out some limits of this model. It does not take into account the same species belonging to two or more trophic levels, and it gives no place to saprophytes.
Assertion (A): Ecological pyramids do not accommodate a food web. — True. A pyramid shows only one straight line of feeding steps, so the many cross-links of a food web are left out.
Reason (R): Ecological pyramids assume a simple food chain, something that almost never exists in nature. — True. In nature, food chains are interlinked, for example through omnivores, so a single simple chain is rarely found.
Why R explains A? A pyramid is built on the idea of one simple food chain, with each organism on a single step. A food web, on the other hand, is a network of interconnected chains. A model that starts from a simple chain cannot show such a network. So R is the correct explanation of A.
NCERT Extract:
Ecological Pyramids – “… However, 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.”