Biology • Year 11 • Module 4 • Lesson 4
Trophic Levels and Energy Transfer — The 10% Rule
Lock in the core vocabulary and energy-flow concepts: trophic levels, trophic efficiency, the three loss pathways, and the key distinction between energy flow and matter cycling.
1. Term–definition match
The eight definitions below are shuffled. In the right-hand column, write the matching term from this list: trophic level, trophic efficiency, 10% rule, ecological pyramid, biomass, energy transfer, respiration, egestion. 8 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The total mass of living material in a given area, usually measured in g m−2 or kg. | |
| 1.2 | The feeding position of an organism in a food chain; producers are at T1. | |
| 1.3 | A graphical model showing the relationship between organisms at different trophic levels — can represent numbers, biomass or energy. | |
| 1.4 | An approximation stating that only about 10% of the energy stored in biomass at one trophic level is converted into biomass at the next. | |
| 1.5 | The percentage of energy transferred from one trophic level to the next; typically ~10% but ranging from 5–20%. | |
| 1.6 | The flow of chemical energy stored in organic compounds from one organism to another when one is consumed. | |
| 1.7 | The metabolic process by which organisms break down glucose to release ATP, producing heat as a by-product — the single largest energy loss at each trophic level. | |
| 1.8 | The elimination of undigested food material in faeces; cellulose, chitin and bone pass through the gut and are lost from the consumer's energy budget. |
2. Label the trophic pyramid and energy values
The Australian grassland pyramid below has five rows. Using the worked example in Card 2 (producers = 20,000 kJ m−2 yr−1, trophic efficiency = 10%), write the correct energy value in each row and the name of the trophic group. 8 marks (1 per row label + 1 per energy value)
Fill in the trophic group name and energy value for each level. Assume 10% trophic efficiency and T1 = 20,000 kJ m−2 yr−1.
3. Energy loss pathways — complete the table
A rabbit ingests 2,000 kJ of grass energy. The table below lists the three pathways through which energy is lost before it can become new biomass. Complete the missing information. 9 marks (3 per row)
| Loss pathway | Mechanism (what happens to the energy?) | Approximate % of ingested energy lost this way |
|---|---|---|
| Respiration | ||
| Egestion | ||
| Excretion |
3.4 How much energy (in kJ) is available to become new rabbit biomass if 2,000 kJ was ingested and overall production efficiency is 10%? Show your working.
4. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 8 marks (1 for T/F, 1 for correction where needed)
4.1 Energy pyramids can be inverted (wider at the top than the bottom) in some ecosystems. T / F
4.2 Decomposers recycle both matter (nutrients) and energy back to the producer level. T / F
4.3 The 10% rule is an average — actual trophic efficiency can range from about 5% to 20% depending on the organism and ecosystem. T / F
4.4 Matter (carbon, nitrogen, phosphorus) flows one-way through ecosystems, just like energy. T / F
5. Build a concept map
Draw labelled arrows between the six terms below. Each arrow must carry a linking phrase (e.g. "transfers", "is lost via", "limits the length of"). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: solar energy · producers (T1) · consumers · respiration · trophic efficiency (~10%) · food chain length.
Q1 — Term–definition matches
1.1 biomass • 1.2 trophic level • 1.3 ecological pyramid • 1.4 10% rule • 1.5 trophic efficiency • 1.6 energy transfer • 1.7 respiration • 1.8 egestion.
Q2 — Trophic pyramid labels and energy values
T1 — Producers: 20,000 kJ m−2 yr−1.
T2 — Primary consumers: 2,000 kJ m−2 yr−1 (20,000 × 0.10).
T3 — Secondary consumers: 200 kJ m−2 yr−1 (2,000 × 0.10).
T4 — Tertiary consumers: 20 kJ m−2 yr−1 (200 × 0.10).
T5 — Quaternary consumers / apex predators: 2 kJ m−2 yr−1 (20 × 0.10).
Award 1 mark per correct trophic group name and 1 mark per correct energy value. Accept reasonable alternatives (e.g. "herbivores" for T2, "apex predators" for T5).
Q3 — Energy loss pathways
Respiration: Glucose is broken down via cellular respiration to release ATP for movement, growth and maintenance; the majority of chemical energy is released as heat and lost to the environment. Approximately 60–90% of ingested energy.
Egestion: Undigested material (cellulose in plant cell walls, chitin in insect exoskeletons, bone) passes through the gut and is eliminated in faeces. Approximately 10–30% of ingested energy.
Excretion: Excess nitrogen from protein metabolism is converted to urea or uric acid and lost in urine; these compounds contain chemical energy the consumer cannot use. Approximately 2–5% of ingested energy.
3.4: 2,000 kJ × 0.10 = 200 kJ available for new rabbit biomass (production).
Q4 — True / false with correction
4.1 False. Correction: energy pyramids are always upright because energy is lost at each trophic level (mostly as heat via respiration), so total energy at each successive level is always less than the level below. Numbers and biomass pyramids can be inverted, but energy pyramids cannot.
4.2 False. Correction: decomposers recycle matter (nutrients such as nitrogen and phosphorus compounds are returned to the soil as inorganic ions that producers can reabsorb), but they do not recycle energy. The energy in dead organisms is released as heat during decomposition and cellular respiration, and cannot be returned to the producer level.
4.3 True.
4.4 False. Correction: matter (atoms of carbon, nitrogen, phosphorus, etc.) is cycled between biotic and abiotic components of ecosystems. Decomposers break down dead organisms and release inorganic nutrients that producers can re-absorb. Unlike energy, matter is not lost to the ecosystem — it is continuously recycled in a closed loop.
Q5 — Sample concept map
A correct map should include arrows such as:
- solar energy — is captured by → producers (T1)
- producers (T1) — is transferred to → consumers
- consumers — lose ~90% via → respiration
- trophic efficiency (~10%) — determines how much energy passes to each → consumers
- trophic efficiency (~10%) — limits → food chain length
- respiration — reduces available energy, constraining → food chain length
Award 1 mark per correctly labelled arrow that respects causal direction (max 6). Accept any biologically valid linking phrases.