Questions: Trophic Efficiency and Energy Loss Between Levels

Question 1

Primary producers in an ecosystem fix 100,000 kcal of energy. Applying the 10% rule, how much energy is available to secondary carnivores (eating primary carnivores that ate herbivores)?

• A. 10,000 kcal
• B. 1,000 kcal
• C. 100 kcal
• D. 10 kcal

Answer: (C)

The 10% rule compounds multiplicatively at each transfer: producers (100,000 kcal) → herbivores (10,000 kcal) → primary carnivores (1,000 kcal) → secondary carnivores (100 kcal). At each step, 90% is lost as heat through cellular respiration. By the third trophic transfer, only 0.1% of the original energy remains. This exponential — not linear — decline is the fundamental reason food chains cannot extend indefinitely.

Question 2

An aquatic ecosystem is dominated by insects rather than birds and mammals. All else being equal, which prediction follows from trophic efficiency principles?

• A. Food chains will be shorter because insects are smaller than mammals
• B. This ecosystem can support longer food chains because ectotherms have higher trophic efficiency — they do not burn energy maintaining body temperature
• C. Trophic efficiency will be lower because insects reproduce faster
• D. Trophic efficiency is the same regardless of whether organisms are ectotherms or endotherms

Answer: (B)

Endotherms (birds, mammals) allocate a large fraction of ingested energy to thermogenesis (maintaining constant body temperature), leaving less for growth and reproduction — lower trophic efficiency. Ectotherms (insects, fish) lack this overhead, so more of their ingested energy converts to biomass. A given amount of primary productivity can therefore support more trophic levels in an insect-dominated ecosystem than in a mammal-dominated one. Body size (option A) is not the relevant variable.

Question 3

Energy loss between trophic levels is additive, so a four-level food chain loses primarily about twice as much energy as a two-level chain.

• T. True
• F. False

Answer: False

Energy loss is multiplicative, not additive. At 10% efficiency per level, a two-level chain retains 10% of producer energy; a four-level chain retains 10% × 10% × 10% = 0.1% — 100× less, not 2× less. This compounding is why every additional trophic level doesn't just add a fixed cost — it multiplies the accumulated loss by another factor of ~10. Treating the loss as additive dramatically underestimates how constraining trophic inefficiency is.

Question 4

The 5–20% trophic efficiency at each level explains why food chains are generally limited to 4–5 trophic links — there is simply not enough energy left to sustain a viable top-predator population at greater depths.

• T. True
• F. False

Answer: True

With ~10% efficiency, each level has roughly 1/10 the energy of the level below. By the 4th or 5th trophic level, the residual energy is insufficient to support a breeding population of top predators. This is a hard energetic constraint — not a behavioral or evolutionary preference — that shapes the structure of virtually all ecosystems. The rarity and small population sizes of apex predators are a direct consequence.

Question 5

Why does a human population eating grain directly require far less agricultural land than one obtaining its food energy by eating cattle raised on grain, and which ecological principle explains this?

Model answer: Grain → human is a single trophic transfer (~10% efficiency). Grain → cattle → human is two transfers (~1% efficiency). To obtain the same food energy, the cattle-based diet requires roughly 10× more grain to be grown, and thus far more land. Trophic efficiency loss is multiplicative: each additional trophic step loses ~90% of the energy, so inserting an extra level multiplies the land requirement by approximately an order of magnitude.

This is one of the most policy-relevant applications of trophic ecology. 'Eating lower on the food chain' is not merely a preference — it reflects a fundamental energetic reality about how much primary productivity is needed to sustain a population. The efficiency gain is not incremental but roughly 10× per trophic level removed from the diet.