Which of the following is the best definition of ecological efficiency?

Correct — B. Ecological efficiency asks how much of the energy entering one trophic level is passed on to the next, so it has to be a ratio of energy to energy — the flow at one point of the chain measured against the flow at another.

That is the comparison the keyed wording makes.

The transfer is small: only about 10 per cent of the energy at a level reaches the level above, the rest being spent in respiration and lost as heat → option (b).

Why the others are wrong

• (a)
  The ratio between the mass and the energy flow at different trophic levels in a food — This wording compares mass with energy flow — two different quantities. A ratio between unlike things is not an efficiency of energy transfer, and this option belongs to the pyramid of biomass instead.
• (c)
  The amount of energy stored at different trophic levels in a food chain — Energy stored at a level is a standing quantity, closer to standing crop. An amount only becomes an efficiency once it is divided by the amount it came from.
• (d)
  The amount of energy utilised at different trophic levels in a food chain — Energy utilised at each level is again an absolute amount, and most of it is respired away. Efficiency asks what fraction moved on, not how much was consumed.

Energy enters an ecosystem through producers and moves in one direction along the trophic levels, dwindling at each step.

Lindeman's ten per cent law states that roughly a tenth of the energy at one trophic level is available at the next. The remainder goes into respiration, movement and heat, or is never eaten.

Two consequences follow that examiners like: the pyramid of energy is always upright, and food chains stay short, usually three or four links, because too little energy survives to support another level.

Key facts

• Ecological efficiency compares the energy flow at one trophic level with the flow at the level below it.
• Lindeman's ten per cent law puts the transfer between successive trophic levels at about 10 per cent.
• Energy flow through an ecosystem is unidirectional, which is why the pyramid of energy is always upright.
• Most food chains stop at three or four trophic levels because so little energy survives each transfer.

Common traps

• Choosing an option that names an amount of energy when the word efficiency demands a ratio.
• Letting the pyramid of biomass into the definition, which is what the mass-and-energy wording invites.
• Reading efficiency as how much energy is used rather than what fraction is passed on.

No directly related past PYQ was found.