Which of the following is not the type of Ecological pyramid ?

Question

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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.

Why the other options are wrong — (a) 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.  (b) 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'.  (d) 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.

Concept: 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.

State PCS 2023 Q19 — Environment & Ecology PYQ with Answer

State PCS 2023 Q19 — Environment & Ecology PYQ with Answer