CDS (II) 2021 General Knowledge · Question 13 of 120

Correct — C, Decrease in the available energy at higher trophic levels. Energy enters a food chain when green plants capture roughly one per cent of the sunlight falling on their leaves, and only about ten per cent of what is present at any step is passed on to the next. The rest is spent on the organism's own life processes or lost as heat, and because that loss happens again at every transfer, the amount left shrinks by a factor of about ten with each level. After three or four steps there is so little usable energy left that no further population can be supported, which is why food chains generally consist of only three or four steps. The direction of flow matters too: energy moves one way through the chain and never returns to a previous level, so what is lost cannot be recovered. That steady decline, and nothing about the supply of food or the state of the air, is the ceiling on the number of trophic levels.

• (a)Deficient food supply — Turns the argument round. Food is not scarce in absolute terms at the base of a chain; what runs out is the energy that survives repeated transfers, and that shortage is the consequence of the ten per cent rule rather than an independent cause.
• (b)Polluted air — Pollution can damage particular species and shorten particular chains, but the three-to-four-step limit holds in clean ecosystems too. It is set by energetics, not by air quality.
• (d)Parasitic organisms — Parasites are themselves consumers within an ecosystem and can lengthen a chain rather than cap it. They do not explain why the cap sits at three or four levels everywhere.

Each step of a food chain is a trophic level: producers first, then herbivores as primary consumers, small carnivores as secondary consumers and larger carnivores as tertiary consumers. Energy passes upward from one level to the next, but the transfer is very inefficient — about ten per cent of the organic matter at a level reaches the next. Energy flow through an ecosystem is unidirectional and diminishes at every step, while nutrients, by contrast, cycle and return to the soil through decomposers.

All four options describe things that can harm an ecosystem, so the item is really asking which one sets a universal limit rather than a local one. The test to apply is whether the factor would still operate in a pristine ecosystem with abundant vegetation and clean air; only the energy loss does. Getting the number right also helps: roughly ten per cent per transfer means a thousandth of the producers' energy has survived by the fourth level, which is why the fifth is rarely worth having. The same arithmetic explains two related observations that examiners like — that there are generally more individuals at the lower trophic levels, with producers the most numerous, and that persistent chemicals such as certain pesticides become more concentrated at each step even as the energy thins out.

• Green plants in a terrestrial ecosystem capture about 1% of the sunlight falling on their leaves and convert it into food energy.
• About 10% of the organic matter present at each step reaches the next level of consumers; the rest is used up or lost as heat.
• Because so little energy survives each transfer, food chains generally consist of only three or four steps.
• Energy flow through an ecosystem is unidirectional and never returns to a previous trophic level, while nutrients cycle back through decomposers.
• There are generally more individuals at the lower trophic levels, and the greatest number is of the producers.

• Reading 'deficient food supply' as the same thing as declining energy; the energy loss is the cause and the shortage is its effect.
• Assuming the ten per cent figure is exact; it is an average across ecosystems, not a constant.
• Confusing the fall in energy with the rise in concentration of persistent pollutants, which moves the opposite way up the chain.

As a cause question like this one, as a statements item on the direction of energy and nutrient flow, or as an application question on biomagnification and which organism carries the highest pesticide load.

• practice — not a real PYQ

  If the producers in a grassland ecosystem contain 10,000 kJ of energy, roughly how much would be available to the secondary consumers?

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

  Answer(b) 100 kJ — about 10% carries over at each transfer, so two transfers leave about one per cent of the original energy.

• practice — not a real PYQ

  In the food chain grass → grasshopper → frog → snake → hawk, the hawk occupies which trophic level?

  • (a)Second
  • (b)Third
  • (c)Fourth
  • (d)Fifth

  Answer(d) Fifth — grass is the first level, and each consumer in turn occupies the next.