MPSC 2023 Prelims Paper-I · Question 19 of 100

Correct — option (3), 'Pyramid of food web'. Read the stem twice before answering, because it is a negative question and nothing on the page marks it: the word 'not' is printed in exactly the same weight as the rest of the line, and it asks for the item that is NOT a type of ecological pyramid. Three types exist and they are the other three options. The pyramid of number counts individuals at each trophic level; the pyramid of biomass measures the dry weight of living matter at each level; and the pyramid of energy measures the energy that flows through each level in a given time. There is no fourth type called a pyramid of food web, and there could not be one, because a food web is a different kind of object altogether. A pyramid is a way of representing quantities level by level, taking the trophic levels as horizontal tiers and drawing each in proportion to whatever is being measured. A food web is a network — the many food chains of a community drawn together, showing which organism eats which, with a single species often appearing at more than one level. The one describes how much there is at each step; the other describes who eats whom. You cannot stack a network into tiers without first collapsing it into trophic levels, at which point you are no longer drawing the web. So the item that does not belong is the food web, and option (3) is the answer. It is worth remembering that the ecological pyramid goes back to Charles Elton, who set out the pyramid of numbers in the 1920s, and that of the three types only the pyramid of energy is invariably upright.

• (1)Pyramid of number — The pyramid of number is a genuine type — indeed the original one, described by Charles Elton in the 1920s, which is why ecological pyramids are sometimes called Eltonian pyramids. It plots the count of individual organisms at each trophic level, and in a grassland it takes the expected shape: very many grass plants, fewer grasshoppers, fewer frogs, fewer snakes, a single hawk. It can also invert, and that is what makes it interesting: a single large tree may support thousands of insects feeding on it and a smaller number of birds feeding on them, so the producer level is narrower than the level above it. Being a real type of pyramid, it cannot be the answer to a question asking which is NOT one.
• (2)Pyramid of biomass — The pyramid of biomass is also a genuine type. It replaces the head-count of the pyramid of numbers with the dry weight of living material at each trophic level, which is a fairer measure when the organisms differ enormously in size — one tree and one aphid count equally in a pyramid of numbers but not in a pyramid of biomass. It too can be inverted, most famously in open water, where the standing crop of phytoplankton at any given moment may weigh less than the zooplankton feeding on it, because the phytoplankton reproduce and are eaten so rapidly that a small standing biomass supports a larger one above it. A candidate who chooses this option has usually confused 'can be inverted' with 'is not a real type'.
• (4)Pyramid of energy — The pyramid of energy is a genuine type and is the most informative of the three, because it measures a rate — the energy passing through each trophic level in a given period — rather than a snapshot. It is the only one of the three that is always upright, and the reason is thermodynamic: energy is lost as heat at every transfer, so roughly a tenth of the energy at one level becomes available to the next, the proportion generalised as Lindeman's ten per cent law. That is precisely why the pyramid of energy resolves the paradoxes of the other two. Choosing it as the odd one out inverts the truth, since it is the type that best deserves the name.

An ecological pyramid is a graphical device for showing how a quantity is distributed across the trophic levels of an ecosystem: producers at the base, then primary consumers, secondary consumers and so on, each level drawn as a horizontal bar whose length represents the measured quantity. Charles Elton set out the first of these in the 1920s, counting individuals, and two further versions followed, one measuring biomass and one measuring energy flow. Each answers a different question and each has its own limitations. The pyramid of numbers ignores size, so it can invert in a tree ecosystem where one producer supports many small consumers, and it becomes absurd where parasites are involved, since parasites on a host outnumber the host many times over. The pyramid of biomass corrects for size but records a standing crop at one instant, so it inverts in aquatic systems where the producers turn over very fast. The pyramid of energy measures flow over time and is always upright, because the second law of thermodynamics guarantees losses at every transfer — the basis of Lindeman's ten per cent law and of the fact that food chains rarely exceed four or five links. A food web, by contrast, is not a pyramid at all but a map of feeding relationships, and its value is that it shows the alternative pathways that give a community its stability.

Ecology questions in MPSC papers are built on a small number of clearly defined lists — the types of ecological pyramid, the kinds of species interaction, the categories of biodiversity, the trophic levels of a food chain — and the Commission tests them by inserting one plausible-sounding item that does not belong to the list. That makes the negative stem the natural format, and this paper carries seven such stems, none of them marked in bold or in any other way. The habit that protects a candidate is mechanical: read the stem, find the negating word, and write it down or underline it before looking at the options, because the commonest failure in this format is not ignorance of the subject but an answer given to the question the candidate expected rather than the one printed. Here, three of the four options are genuine types and a candidate who has read the stem as a positive question will find three defensible answers and no way to choose among them — which is itself a signal that the stem has been misread.

• This is a negative stem: the word 'not' decides the question and is printed in the same weight as the rest of the line, with nothing to mark it in either language column.
• There are three types of ecological pyramid — the pyramid of numbers, the pyramid of biomass and the pyramid of energy; there is no pyramid of food web.
• A food web is a network of interconnected food chains showing which organism eats which, not a level-by-level representation of a quantity, so it is a different kind of object from a pyramid.
• Charles Elton set out the pyramid of numbers in the 1920s, which is why ecological pyramids are sometimes called Eltonian pyramids.
• The pyramid of numbers can invert in a tree ecosystem and the pyramid of biomass in open water, but the pyramid of energy is always upright because energy is lost as heat at every transfer — the basis of Lindeman's ten per cent law.

• Missing the negation in the stem, which in this paper is printed in ordinary type with nothing to mark it, and answering the positive question instead
• Treating the pyramid of biomass or of numbers as not a real type because either can be inverted; invertibility is a property of a real pyramid, not a disqualification
• Confusing a food web, which is a network of feeding relationships, with a pyramid, which is a level-by-level representation of a quantity
• Failing to notice the warning sign that three options all look defensible, which usually means the stem has been read the wrong way round

Ecological pyramids, food chains and food webs are staple MPSC material and are asked in two shapes: the definition question, which asks what a named pyramid measures or which of them is always upright, and the odd-one-out question used here, which inserts a term that sounds like a member of the list but is not. The Commission also asks about inverted pyramids, since the tree ecosystem and the pond ecosystem are the standard examples and they discriminate well between candidates who have learnt the list and those who have understood it. Negative stems are common in this section and are not signalled typographically in this paper, so the reading discipline matters as much as the ecology. Expect the same concepts to reappear in questions about the ten per cent law and about why the number of trophic levels in a food chain is limited.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following ecological pyramids is always upright, and for what reason ?

  • (a)Pyramid of numbers, because producers always outnumber consumers
  • (b)Pyramid of biomass, because producers always weigh more than consumers
  • (c)Pyramid of energy, because energy is lost as heat at every transfer
  • (d)All three are always upright

  Answer(c) Pyramid of energy, because energy is lost as heat at every transfer — the second law of thermodynamics guarantees that only a fraction of the energy at one trophic level, generalised as about ten per cent in Lindeman's law, becomes available to the next. The pyramid of numbers can invert in a tree ecosystem and the pyramid of biomass in open water, where a small standing crop of rapidly reproducing phytoplankton supports a larger biomass of zooplankton.

• practice — not a real PYQ

  In which of the following ecosystems is the pyramid of biomass typically found to be inverted ?

  • (a)A grassland ecosystem
  • (b)A forest ecosystem
  • (c)An open-water or pond ecosystem
  • (d)A desert ecosystem

  Answer(c) An open-water or pond ecosystem — the phytoplankton reproduce and are consumed so rapidly that their standing biomass at any given instant may be smaller than that of the zooplankton feeding on them, giving an inverted pyramid. The energy pyramid for the same water body remains upright, because it measures the flow of energy over a period rather than the mass present at one moment.