9.7B Explain how some energy is transferred to less useful forms at each…

The idea

At every trophic level, energy taken in as food is not all passed on. Much is lost as heat from respiration, and more leaves in uneaten parts, faeces and urine, so only a small fraction becomes new biomass available to the next consumer. Because each transfer is inefficient, the energy reaching higher levels shrinks rapidly. That shrinking supply supports fewer organisms at each successive level, so a food chain rarely runs beyond four or five links. It also shapes a pyramid of biomass: wide at the producer base and narrowing upwards, because biomass, like energy, diminishes along the chain. A pyramid that is narrow at the base signals an unusual ecosystem, not the normal pattern.

What earns the marks

• Energy is lost at each trophic level as heat released by respiration, and in uneaten material, faeces and urine, so not all energy is transferred to the next level.
• Only a small proportion of energy becomes biomass available to the next consumer, so the energy reaching each higher trophic level is reduced.
• The reduced energy supply supports fewer organisms at each successive trophic level.
• The progressive loss of energy limits the length of a food chain, typically to about four or five trophic levels.
• The same loss determines the shape of a pyramid of biomass, which is broadest at the producer level and narrows towards the top.
• Less useful forms include heat transferred to the surroundings, which cannot be reused by organisms in the chain.

Common mistakes and how to put them right

• Thinking energy is destroyed at each level.

  Put it right: Energy is conserved but transferred to less useful forms such as heat, not lost from existence.

• Assuming every organism at a level is eaten by the next.

  Put it right: Much biomass is uneaten, or lost in faeces and urine, and never enters the next trophic level.

• Believing a pyramid of biomass is always widest at the top.

  Put it right: Normal pyramids are widest at the producer base and narrow upwards because biomass decreases.

• Claiming food chains can be indefinitely long.

  Put it right: Energy loss limits chain length to a few trophic levels.

Exam tips

• Name the specific less useful forms (heat from respiration, uneaten material, faeces, urine) rather than saying vaguely that energy is 'lost'.
• Link each consequence explicitly: energy loss → fewer organisms → shorter chain → pyramid shape, so each clause of the question is answered.
• When describing a pyramid, state that the base represents producers and that width decreases towards higher trophic levels.