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:
Statement I: The pyramid of energy is upright because, when energy flows from a particular trophic level to the next trophic level, some energy is lost as heat at each step.
Statement II: Each bar in the energy pyramid indicates the amount of biomass present at each trophic level in a given time or annually per unit area.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Statement I is correct but Statement II is incorrect.
Energy passes from one trophic level to the next, and at every transfer a part of it is lost as heat. So each higher level has less energy than the one below it. This is why the pyramid of energy keeps its upright shape in every ecosystem, unlike the pyramid of biomass in the sea.
Statement I: The pyramid of energy is upright because, when energy flows from a particular trophic level to the next trophic level, some energy is lost as heat at each step. — True. Heat loss at each step leaves less energy for the next level, so the bars get narrower towards the apex.
Statement II: Each bar in the energy pyramid indicates the amount of biomass present at each trophic level in a given time or annually per unit area. — False. Each bar shows the amount of energy, not biomass. Biomass is shown by the bars of the pyramid of biomass.
NCERT Extract:
Ecological Pyramids – “… The pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton. … 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.”