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.
According to the passage, why must all the organisms at a trophic level be included while working out an ecological pyramid?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → No generalisation will be true if only a few individuals at a trophic level are taken into account.
The passage states that any calculation of energy content, biomass or numbers has to include all organisms at that trophic level, and that “No generalisation we make will be true if we take only a few individuals at any trophic level into account.” A few individuals picked from a level do not stand for the whole level, so the pyramid built from them would be misleading.
Option (1) Organisms at each trophic level depend on those at the lower trophic level for their energy demands. — Incorrect. This is true in an ecosystem, but it explains how energy passes up the levels. It is not the reason given for counting every organism.
Option (2) No generalisation will be true if only a few individuals at a trophic level are taken into account. — Correct. This is the reason the passage itself gives.
Option (3) The amount of energy decreases at successive trophic levels. — Incorrect. Energy does fall at each higher level, but that explains the upright shape of the energy pyramid, not why every organism must be counted.
Option (4) A given species may occupy more than one trophic level in the same ecosystem. — Incorrect. The passage does say that an organism may occupy more than one trophic level, but it says this about the trophic level being a functional level, not about counting all organisms.