UPPSC 2019 Prelims GS Paper-I · Question 99 of 150

Correct — B, Decreases. (The booklet prints 'tropic level'; the standard ecological term is trophic level — a feeding step in a food chain. The meaning is unaffected.) Energy enters an ecosystem as sunlight, is fixed by green plants and then flows in one direction only — producer to herbivore to carnivore — losing a large fraction at every step. The losses are unavoidable and of three kinds: most of the energy in any trophic level has already been burnt in that level's own respiration and released as heat; a large part of the available biomass is never eaten at all, or dies and passes to decomposers; and of what is eaten, some is not assimilated and leaves as faeces. Raymond Lindeman's classic study of 1942 summed this up as the ten per cent law — roughly a tenth of the energy at one trophic level is passed on to the next, the remaining ninety per cent being lost, mainly as heat. The second law of thermodynamics guarantees that no transfer can be complete, so the direction is fixed: energy always decreases upwards. Two familiar consequences follow — the pyramid of energy is always upright, never inverted, and food chains rarely run beyond four or five links, because by then there is too little energy left to support another level.

• (a)Increases — Energy can never increase up a food chain — that would create energy from nothing. What does increase along the chain is the concentration of persistent fat-soluble pollutants such as DDT, through biomagnification; students often carry that picture across to energy by mistake.
• (c)Remains constant — A constant transfer would mean a perfectly efficient one, with no respiration, no heat loss, nothing uneaten and nothing undigested. The second law of thermodynamics rules this out; every biological energy transfer degrades part of the energy to heat.
• (d)May increase or decrease — The direction is not variable. Transfer efficiencies do vary between ecosystems — often quoted between about five and twenty per cent — but they are always well below a hundred per cent, so the amount of energy always falls from one trophic level to the next.

An ecosystem runs on a one-way flow of energy and a cycling of matter. Nutrients such as carbon, nitrogen and phosphorus go round and round between organisms and the environment, but energy does not: it enters as sunlight, is captured by producers as gross primary productivity, and thereafter only degrades — each organism spends most of what it takes in on respiration, and that heat leaves the ecosystem for good. What is left after a level's own respiration is its net productivity, and only a fraction of that ever reaches the level above. That is why ecology speaks of energy flow but of nutrient cycles.

Even without the ten per cent law, elimination settles this. 'Remains constant' would demand perfect efficiency; 'increases' would demand energy from nowhere; 'may increase or decrease' would make the direction arbitrary when in fact it is fixed by thermodynamics. Only 'decreases' survives. Keep clear which quantity you are being asked about: energy always falls upwards, numbers and biomass usually fall but can invert (a single large tree supporting many insects, or the fast-turnover phytoplankton of the sea supporting a greater standing biomass of zooplankton), and pollutant concentration rises.

• Lindeman's ten per cent law (1942): about 10 per cent of the energy at one trophic level is transferred to the next; the rest is lost, chiefly as respiratory heat.
• Energy flow in an ecosystem is unidirectional and non-cyclic, unlike nutrients, which are recycled through biogeochemical cycles.
• Gross primary productivity minus the producers' own respiration gives net primary productivity — the energy actually available to consumers.
• The pyramid of energy is always upright; pyramids of number and of biomass can be inverted in some ecosystems.
• Because energy dwindles at each step, food chains are usually limited to about four or five trophic levels.

• Confusing energy transfer (always decreases) with pollutant concentration (increases up the chain).
• Believing the pyramid of biomass is always upright — in some aquatic ecosystems it is inverted, though the energy pyramid never is.
• Treating energy like nutrients and assuming it is recycled within the ecosystem; energy flow is one-way.

UPPSC asks this directly as a one-line recall of the ten per cent principle; UPSC prefers applied versions — which organism accumulates most pesticide, what a food chain does and does not illustrate, or what happens if a trophic level is removed.

• practice — not a real PYQ

  If the producers of an ecosystem fix 10,000 kJ of energy, roughly how much of it would be available to the secondary consumers according to the ten per cent law?

  • (a)1,000 kJ
  • (b)100 kJ
  • (c)10 kJ
  • (d)5,000 kJ

  Answer(b) 100 kJ — herbivores at the second trophic level receive about 1,000 kJ, and the secondary consumers at the third level about a tenth of that.

• practice — not a real PYQ

  Which one of the following 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 lost at every transfer, so a higher trophic level can never hold more energy than the one below it, whereas pyramids of number and biomass can be inverted.