UPPSC 2019 Prelims GS Paper-I · Question 20 of 150

Correct — C, 'Producers are more than primary consumers.' Energy enters an ecosystem only through the producers, and at every feeding step most of it is lost as heat in respiration — as a rule of thumb only about 10% passes to the next trophic level (Lindeman's ten per cent law). The producer level therefore has to form the broad base of the ecological pyramid: it captures and holds far more energy, and in the great majority of ecosystems more biomass and more individuals, than the herbivores it feeds. That upward tapering is the standard structure of an ecosystem, which makes (c) the statement that is generally true.

• (a)Primary consumers are least dependent upon producer — The exact opposite is true. Primary consumers are the herbivores — they feed directly on producers and on nothing else, so no trophic level is more dependent on producers than this one.
• (b)Primary consumers are out-number producers — Not the general rule — producers normally outnumber the herbivores they support. Numbers can be inverted in special cases (a single large tree feeding thousands of insects, or a parasitic food chain), but as a blanket statement about ecosystems it is wrong, and the energy pyramid is never inverted.
• (d)Secondary consumers are the largest and most powerful — Wrong on both counts. Energy dwindles as it moves up the chain, so higher consumer levels are the fewest in number and the smallest in total biomass, not the largest. Body size also does not track trophic position — the blue whale, among the biggest animals alive, feeds near the base of the chain on krill.

An ecosystem is organised into trophic levels: producers (green plants and other autotrophs) fix solar energy, primary consumers (herbivores) eat them, secondary and tertiary consumers eat the level below, and decomposers return nutrients to the soil. Energy flow is one-way and lossy, so the quantity available shrinks sharply at each step; nutrients, by contrast, cycle. Ecological pyramids — of numbers, of biomass and of energy — are simply pictures of this trophic structure.

Decide this question from the energy pyramid, which is always upright, and do not get dragged into the exceptions. Three of the four options contradict the direction of energy flow outright, which leaves (c) standing. Be honest about the caveat, because examiners exploit it elsewhere: the pyramid of numbers is inverted in a single-tree ecosystem and in parasitic chains, and the pyramid of biomass is inverted in the open ocean, where a small standing crop of phytoplankton supports a larger mass of zooplankton and fish at any given instant.

• Energy flow is unidirectional: sun → producers → herbivores → carnivores, with decomposers returning nutrients
• Ten per cent law (Lindeman, 1942): roughly 10% of the energy at one trophic level passes to the next; the rest is lost mainly as respiratory heat
• The pyramid of energy is always upright; the pyramid of numbers can be inverted (single tree, parasitic chains) and the pyramid of biomass is inverted in oceans
• Because so little energy survives each transfer, food chains rarely extend beyond four or five trophic levels
• Primary consumers are herbivores and depend wholly and directly on producers

• Quoting the inverted pyramid of numbers (single tree, parasites) as if it were the general rule
• Assuming the largest animal must sit at the top of the food chain
• Confusing the pyramid of numbers with the pyramid of energy — only the energy pyramid is always upright

UPPSC asks it as a blunt one-liner about what is true of an ecosystem or of trophic levels; UPSC prefers statement-based questions on food chains, producers and energy flow. Anchor every such question to the ten per cent law.

• practice — not a real PYQ

  Which one of the ecological pyramids is always upright?

  • (a)Pyramid of numbers
  • (b)Pyramid of biomass
  • (c)Pyramid of energy
  • (d)All three are always upright

  Answer(c) Pyramid of energy — energy is always lost at each transfer, so it can never be inverted, unlike the pyramids of numbers and biomass.

• practice — not a real PYQ

  In the food chain grass → grasshopper → frog → snake, if the grass fixes 10,000 kJ of energy, the energy available to the snake would be approximately

  • (a)1,000 kJ
  • (b)100 kJ
  • (c)10 kJ
  • (d)1 kJ

  Answer(c) 10 kJ — applying the ten per cent law three times: 10,000 → 1,000 → 100 → 10 kJ.