In an ecosystem, flow of energy declines as it passes from lower to higher trophic level. This is explained by the following:
- (a)First law of thermodynamics
- (b)Second law of thermodynamics
- (c)Newton's second law
- (d)Newton's third law
Correct — B, Second law of thermodynamics. Energy flow through an ecosystem is unidirectional, and no energy transformation is ever 100% efficient: at every trophic transfer, a large share of the energy is lost as heat, chiefly through respiration, rather than being passed on as usable biomass. This progressive loss of usable energy at each step — the reason flow declines from lower to higher trophic levels — is a direct consequence of the second law of thermodynamics (entropy increases; some energy always dissipates as unusable heat).
- (a)First law of thermodynamics — The first law (conservation of energy) only says total energy is neither created nor destroyed — it explains the overall energy balance, not why the USABLE energy specifically shrinks at every trophic transfer; that decline is a second-law (entropy/heat-loss) phenomenon.
- (c)Newton's second law — Newton's second law (F = ma) relates force, mass and acceleration in mechanics — it has no bearing on energy transfer efficiency in an ecosystem.
- (d)Newton's third law — Newton's third law describes equal-and-opposite paired forces between interacting bodies — irrelevant to trophic energy flow.
Energy flows through an ecosystem only in one direction — from producers to successive consumer levels — and is never fully passed on: at every trophic transfer a large fraction (commonly cited as roughly 80-90%) is lost as heat, mainly through respiration, so only a small share becomes new biomass at the next level. This step-by-step loss of usable energy is a direct application of the second law of thermodynamics: no energy conversion is completely efficient, and some energy is always dissipated as heat rather than remaining available to do biological work.
The trap is picking the first law because it 'sounds like' the general law about energy. The first law only guarantees total energy is conserved somewhere (as heat, biomass, etc.); it says nothing about the DECLINE in usable energy at each step — that specific decline is what the second law explains.
- Energy flow through trophic levels is unidirectional, from producers upward, and never fully efficient
- A large share of energy is lost as heat (chiefly via respiration) at each trophic transfer
- The decline in usable energy at each step follows the second law of thermodynamics (entropy)
- This underlies the ecological efficiency concept often summarised as the 'ten per cent law'
At every transfer, energy is lost as heat rather than passed on — an application of the second law of thermodynamics.
- Confusing the first law (conservation of energy) with the second law (why usable energy declines)
- Assuming any 'law' option involving energy must be correct without checking which law explains the decline specifically
MPPSC/UPSC often test the thermodynamic basis of energy-flow decline in food chains, or ask students to identify the 'ten per cent law' / ecological efficiency directly.
No directly related past PYQ was found.
- practice — not a real PYQ
The 'ten per cent law' in ecology, which states that only about ten per cent of energy is transferred from one trophic level to the next, is consistent with which physical law?
- (a)Newton's first law
- (b)Second law of thermodynamics
- (c)Law of conservation of mass
- (d)Hardy-Weinberg law
Answer(b) Second law of thermodynamics — most energy is lost as heat at each transfer, leaving only a small fraction usable at the next level.
- practice — not a real PYQ
In a food chain, the major share of energy lost between trophic levels is dissipated mainly as:
- (a)Light
- (b)Heat (through respiration)
- (c)Sound
- (d)Kinetic energy of movement
Answer(b) Heat, through respiration — this respiratory loss is why usable energy declines at each higher trophic level.