In an ecosystem why is the number of trophic levels of a food chain usually limited?

Correct — B, Because of the loss of energy at each trophic level. Energy enters an ecosystem as sunlight, is fixed by producers, and then passes up the chain — but only a small fraction survives each transfer. Most of what an organism assimilates is spent on respiration and lost as heat, and more is lost as undigested material and in excretion. Lindeman's ten per cent rule captures the usual figure: roughly a tenth of the energy at one level becomes energy at the next. Starting from 10,000 units at the producers, the fourth consumer level would be left with about 10, which is too little to support a further population. That is why food chains almost always stop at four or five links.

• (a)Lower trophic level has lesser energy than the higher trophic level — This reverses the true relation. Energy decreases upward, so a lower trophic level holds more energy than the one above it, not less. Read as printed, the statement contradicts the pyramid of energy.
• (c)Higher trophic level individual can control the lower ones more efficiently — Efficiency of control by predators is not what limits the chain. Predation regulates population sizes within a chain; the ceiling on the number of levels is set by the energy budget.
• (d)Limited trophic level makes a food chain less vulnerable to parasites — Vulnerability to parasites has nothing to do with the number of trophic levels. Parasites occur at every level and do not set the limit.

A trophic level is a feeding position: producers, then primary consumers, then secondary and tertiary consumers, with decomposers acting across the whole system. Energy flow through the chain is unidirectional and diminishing, unlike nutrients, which cycle. The pyramid of energy is therefore always upright. The pyramid of numbers and the pyramid of biomass can be inverted in special cases — a single tree supporting many insects inverts numbers, and an ocean where fast-growing phytoplankton support a larger standing mass of zooplankton inverts biomass — but energy never can be.

Three of the four options describe real ecological phenomena and only one is the limiting factor, so the item is a cause test rather than a fact test. The check is quantitative: with about ninety per cent of the energy lost at each step, the arithmetic runs out after four or five transfers regardless of how the species behave. This also explains why a population eating lower on the chain can be supported on much less land than one eating higher up, and why top predators are always few.

• Lindeman's ten per cent rule: roughly ten per cent of the energy at one trophic level reaches the next.
• Most energy is lost as heat in respiration, with further loss as undigested and excreted material.
• Energy flow through an ecosystem is unidirectional; nutrients, by contrast, cycle.
• Food chains typically have four or five trophic levels because of this energy loss.
• The pyramid of energy is always upright, though pyramids of number and biomass can be inverted.

• Choosing predator control, which regulates populations but does not cap the number of levels.
• Reversing the direction of the energy pyramid.
• Confusing energy flow, which is one-way, with nutrient cycling, which is not.

A cause item where three plausible ecological statements compete with the one that actually sets the limit.

• practice — not a real PYQ

  Roughly what proportion of energy is transferred from one trophic level to the next?

  • (a)About 1 per cent
  • (b)About 10 per cent
  • (c)About 50 per cent
  • (d)About 90 per cent

  Answer(b) About 10 per cent — the figure known as Lindeman's ten per cent rule.

• practice — not a real PYQ

  Which ecological pyramid can never be inverted?

  • (a)Pyramid of numbers
  • (b)Pyramid of biomass
  • (c)Pyramid of energy
  • (d)All three can be inverted

  Answer(c) Pyramid of energy — energy always decreases as it moves up the chain.