SCIENCE · INVESTIGATE AND EXPLAIN

Keep food-web energy and nutrient accounts separate

Trace production through two feeding transfers and interpret the untransferred balance.

Learning outcome: Calculate successive energy transfers without treating efficiency as universal or energy as a cycling nutrient.

Starting recap

A trophic level groups organisms by feeding role. Production energy over an interval differs from standing biomass, organism count and total energy eaten.

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Try

With producer production of 1000 kJ and efficiency 10% at each transfer, primary production is 100 kJ and secondary production is 10 kJ.

Open the experiment

Predict

Will doubling efficiency from 10% to 20% double secondary-consumer production or change it by a larger factor?

Your prediction and reason

Experiment

Capture efficiencies 10% and 20% with input 1000 kJ. Account for both untransferred balances. Check that secondary production plus the two balances equals the original input.

Trace energy passed to the next trophic level

Worked teaching model · synthetic values · A linear producer → primary consumer → secondary consumer pathway applies one selected efficiency at both transfers. It tracks production energy over one common interval, not organism counts or standing biomass. The untransferred balance combines respiration, uneaten material and waste; not all is immediate heat.

Producer production over the interval: 1000 kJ Transfer efficiency per level: 10 %

Reset keeps your prediction, captures and practice history.

Follow one energy input through the pathway

Producers: 1000 kJ
To primary production: 100 kJ. Other pathways: 900 kJ.
Primary consumers: 100 kJ
To secondary production: 10 kJ. Other pathways: 90 kJ.
Secondary consumers: 10 kJ
The model stops here; further consumption and respiration are outside its boundary.

Whole-pathway check: 900 + 90 + 10 = 1000 kJ. Intermediate production is not counted again.

Calculated values on the same fixed scale
Row	Quantity	Value (kJ)
1	Producer production	1000
2	Primary-consumer production	100
3	Secondary-consumer production	10
4	First-transfer untransferred balance	900
5	Second-transfer untransferred balance	90

Secondary-consumer production: 10.00 kJ from 1000 kJ producer production.

Each transfer multiplies by efficiency/100. First-level input = primary production + first untransferred balance; primary production = secondary production + second untransferred balance. Do not add intermediate production again to the final whole-pathway ledger.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Multiply successive transfers

At 1000 kJ producer production and efficiency 10%, primary-consumer production is 100 kJ. Applying 10% again gives 10 kJ secondary-consumer production. At 20% efficiency, the two results are 200 and 40 kJ. Doubling each efficiency multiplies the final production by four.

The efficiencies are selected assumptions, not a universal “10% law.” Real efficiencies depend on organisms, food quality and conditions. The lab tracks production over a common interval; it does not directly predict numbers of individuals or standing biomass.

Close the energy account once

For the 10% example, 900 kJ is not transferred into primary-consumer production and another 90 kJ is not transferred from primary to secondary production. The whole-pathway ledger is 900 + 90 + 10 = 1000 kJ.

Do not also add the intermediate 100 kJ: its fate is already partitioned into 90 and 10. Nor should the entire untransferred balance be called immediate heat. Uneaten material and waste can remain as chemical energy available to decomposers and detrital consumers; respiration contributes to eventual energy dissipation.

Separate material cycles from energy flow

Nutrient atoms can move through organisms, waste, decomposers and abiotic pools and be used again. Energy transformations require continuing input and disperse usable energy as heat. Food webs include multiple feeding pathways rather than a single chain. Background: OpenStax, ecosystem energy flow.

Return to the carbon-ledger lesson: carbon atoms remain in material compounds while energy is transferred and dissipated. A nutrient diagram should include the relevant environmental reservoir and decomposers. An energy diagram should label amounts, time interval and transfer assumptions.

Use your captures to explain the fourfold change, then draw one omitted detrital pathway in your reflection. State why adding that pathway requires additional flux information rather than treating the untransferred balance as destroyed energy.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Explain where uneaten material and waste can enter a detrital food web. Contrast the recycling of nutrient atoms with the continual input and dissipation of usable energy.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Challenge an unlimited-growth prediction Next: Measure abundance without overlooking diversity

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