The amount of energy during transfer from one tropic level to another in an ecosystem

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2019 Q99 — Environment & Ecology PYQ with Answer

State PCS 2019 Q99 — Environment & Ecology PYQ with Answer