NDA (I) 2018 General Ability Test · Question 91 of 150

Correct — B, Grass, goat and human. A food chain is a sequence in which each organism is eaten by the next, so energy passes step by step from a producer upwards. Grass is the producer, making its own food by photosynthesis; the goat is the primary consumer, a herbivore that eats grass; and the human is the secondary consumer that eats the goat. Every link in that sequence is a genuine feeding relationship, and the chain begins where every food chain must, with a green plant. None of the other three groups meets that test.

• (a)Grass, human and fish — The order is broken. Grass is a producer and a human could eat it, but the fish does not eat the human — the arrow at the last step points the wrong way, and in fact humans eat fish rather than the reverse.
• (c)Tree, tree cutter and tiger — A woodcutter cuts a tree; he does not feed on it, and cutting is not a trophic relationship at all. The tiger then does not eat the woodcutter as a matter of its ordinary diet either. Neither link transfers food energy in the way a food chain requires.
• (d)Goat, cow and human — It has no producer at the base, and cows do not eat goats. Two herbivores placed side by side are at the same trophic level; they are not a chain but merely a list.

Every food chain starts with a producer, which captures solar energy, and rises through consumers arranged by trophic level — herbivores first, then carnivores. Energy is lost at each transfer, largely as heat and in respiration, so only a small fraction passes upward at each step; the familiar rule of thumb is that roughly a tenth carries forward. That is why chains are short, usually three or four links, and why the mass of organisms shrinks as one goes up. In nature these chains are interconnected rather than isolated, forming a food web, and decomposers such as bacteria and fungi return the nutrients from every level to the soil.

Two questions settle nearly every item of this type. Does the group begin with a producer? And does each organism really eat the one before it? Applying the first question alone knocks out option (d), which starts with a goat, and applying the second knocks out (a) and (c). Option (c) also carries a broader lesson worth noticing: a food chain records who eats whom, not merely who interacts with whom. A woodcutter and a tree have a relationship, but no energy passes from the tree into the woodcutter, so there is no trophic link. The unavoidable conclusion is that a human in a food chain has to be there as an eater of something living — grass, goat or fish — and only option (b) arranges that correctly.

• A food chain runs producer to herbivore to carnivore, with energy moving up one step at a time.
• Green plants are producers because they capture solar energy through photosynthesis.
• Only about ten per cent of the energy at one level is passed to the next, which keeps chains short.
• Organisms at the same trophic level, such as a goat and a cow, do not form a chain with each other.
• In nature, interlinked food chains form a food web, and decomposers recycle nutrients from every level.

• Accepting a group as a chain because the organisms are all familiar, without checking who eats whom.
• Starting a chain with an animal instead of a producer.
• Treating any interaction between organisms as a trophic link; only feeding counts.

NDA and UPSC both offer four sequences and ask which is a correct food chain, planting errors in the direction of feeding or in the absence of a producer.

• practice — not a real PYQ

  In a food chain, green plants are called

  • (a)primary consumers
  • (b)producers
  • (c)decomposers
  • (d)secondary consumers

  Answer(b) producers — they capture solar energy by photosynthesis and start every food chain.

• practice — not a real PYQ

  Roughly what fraction of the energy at one trophic level is passed on to the next?

  • (a)one hundredth
  • (b)one tenth
  • (c)one half
  • (d)nine tenths

  Answer(b) one tenth — the familiar ten per cent rule, which is why food chains rarely exceed four or five links.