Biology · Ecosystems, Energy Flow & Biogeochemical Cycles

Trophic energy transfer, efficiencies and dissipation

A consumer ingests one hundred energy units. Forty leave in unassimilated material; sixty are assimilated. Of those sixty, forty-five support respiratory expenditure, five leave in excreted material and ten become new biomass. There is more than one meaningful efficiency.

Begin with Organisms, Environments & Ecological Niches for ecological boundaries and scales, and Energy, Enzymes & Metabolism for chemical energy and respiration. Communities, Species Interactions & Food Webs explains why feeding relations form a web. That chapter is a reading preparation path once available, rather than a claim of an already published lesson. The budgets here introduce every symbol and require ordinary addition, subtraction and ratios. Advanced carbon-cycle measurement methods and chemical reaction mechanisms are not required.

Back to the Biology map

By the end, you can

Choose the correct energy denominator for a consumer or trophic transfer, and distinguish energy dissipation from material recycling.

Follow the energy through a consumer

Feeding moves both materials and associated chemical energy. Ingestion is food taken in. Assimilation is the portion absorbed from that intake rather than remaining in the unassimilated outgoing material. In this energy account, assimilation is counted before respiration; it must not be defined by first subtracting the respiratory amount.

The original inventory closes two stages. Intake one hundred equals forty unassimilated plus sixty assimilated. Assimilated sixty equals forty-five allocated to respiration, five exported with excreted material and ten retained as new biomass. The assigned respiration amount ultimately becomes dissipated energy outside the retained chemical-energy production account; no included energy is created by the subtraction.

Assimilation efficiency is 60/100 = 0.6. Production divided by assimilation is 10/60, about 0.167; production divided by ingestion is 10/100 = 0.1. These ratios have different denominators. Unassimilated and excreted material can carry energy into detrital pathways, so treating all outgoing material as already dissipated heat would misclassify it.

The same ten-unit production has different denominators

Material carrying chemical energy can remain usable to other organisms; the unassimilated and excreted branches are not all heat.

Read the diagram in words

Intake one hundred splits into forty unassimilated plus sixty assimilated. Sixty splits into forty-five respiration, five excretion and ten production. Assimilation per intake is 0.6; production per assimilation is one-sixth; production per intake is 0.1.

Energy units during one interval
Ingested energy / Energy leaving unassimilated / Assimilated energy / Respiratory allocation, ultimately dissipated / Energy exported with excreted material / Energy retained in new biomass	Energy units during one interval
Ingested energy	100
Energy leaving unassimilated	40
Assimilated energy	60
Respiratory allocation, ultimately dissipated	45
Energy exported with excreted material	5
Energy retained in new biomass	10

The same ten-unit production has different denominators
Ingested energy / Energy leaving unassimilated / Assimilated energy / Respiratory allocation, ultimately dissipated / Energy exported with excreted material / Energy retained in new biomass	Energy leaving unassimilated / Assimilated energy / Respiratory allocation, ultimately dissipated / Energy exported with excreted material / Energy retained in new biomass	Energy units during one interval
Ingested energy → Energy leaving unassimilated	Energy leaving unassimilated	40
Ingested energy → Assimilated energy	Assimilated energy	60
Assimilated energy → Respiratory allocation, ultimately dissipated	Respiratory allocation, ultimately dissipated	45
Assimilated energy → Energy exported with excreted material	Energy exported with excreted material	5
Assimilated energy → Energy retained in new biomass	Energy retained in new biomass	10

The same ten-unit production has different denominators
Assimilation / ingestion	Production / assimilation	Production / ingestion
0.6	0.16666666666666666	0.1

Complete original consumer energy budget for one interval, arbitrary compatible energy units. No additional body-reserve drawdown, input or stock change; categories refer to energy rather than identical amounts of carbon or nitrogen.

Compare compatible production accounts

A trophic transfer efficiency compares production at a receiving feeding level with a specified production supply at a lower level. It requires compatible energy measures, area, period and source boundary. In the original closed linear example, lower-level production one hundred supports consumer production twenty, which supports next-level production three.

The two assigned ratios are 20/100 = 0.2 and 3/20 = 0.15. Across the two links, three is 0.03 of the starting hundred. No step is compelled to equal ten percent. Real food webs include diet mixtures, omnivory, imported food and stock use, making the choice of denominator more complex than dividing two bars on a page.

A second explicit account has local lower-level production forty and imported compatible food supply sixty. The receiver produces twenty from the complete supply of one hundred. Relative to local production alone the ratio is one-half; relative to the supplied local-plus-imported production it is one-fifth. Naming the denominator explains the difference. Such accounts must specify whether imports represent production from the corresponding interval or another supplied energy transfer.

Production ratios depend on the supplied sources

Ratios are scoped quantities, not an invariant ten-percent rule. Flow arrows do not imply every consumed unit is turned into new biomass.

Read the diagram in words

The sequence is one hundred, twenty and three, with adjacent ratios 0.2 and 0.15 and overall ratio 0.03. In the open case, forty local plus sixty imported supply one hundred; receiver twenty is 0.5 of local production but 0.2 of complete supply.

Compatible production units during one interval
Lower-level production / Consumer production / Next-level production	Compatible production units during one interval
Lower-level production	100
Consumer production	20
Next-level production	3

Receiving production / supplied lower production
Lower-level production / Consumer production / Next-level production	Receiving production / supplied lower production
Lower-level production → Consumer production	0.2
Consumer production → Next-level production	0.15

Next-level production / starting production
Next-level production / starting production
0.03

Complete assigned production supply
Local lower-level production	Imported corresponding food production	Complete assigned production supply	Receiver production
40	60	100	20

Relative to all supplied production
Relative to local production alone	Relative to all supplied production
0.5	0.2

Invented compatible energy-production amounts for the same area and interval. Linear case has no imported support or reserve drawdown. Open case supplies imported corresponding food production explicitly; neither is an empirical universal food web.

An inverted stock is not an energy creation claim

The original standing-stock comparison assigns producers ten carbon-mass units and consumers twenty at one time. During the common interval, the supplied new production amounts are one hundred and twenty in compatible production units. A larger consumer standing stock is therefore compatible with lower consumer production. Fast replacement of producer material can support this kind of contrast.

The producer and consumer production-to-reference-stock ratios are ten and one when the stock and production amounts use the same carbon measure. They describe the supplied comparison, without proving those stocks were constant all interval or that every individual has a lifespan equal to the reciprocal. A standing biomass diagram and a production diagram must label their different measures.

Energy remains conserved in the full physical account, but metabolism dissipates part of useful chemical energy as heat. Element atoms can be reused in new molecules; dissipated heat is not returned to the same food-production pathway as an equivalent fresh supply of usable energy. Sustained ecosystem activity therefore needs energy input as well as appropriate material transfers.

Standing biomass and production have different shapes

The standing-stock comparison is inverted while the producer has greater interval production. Stocks and production use their own stated measures and scales.

Read the diagram in words

Standing stocks are ten for producers and twenty for consumers. New production during the common interval is one hundred versus twenty. The standing comparison is inverted while production is larger for producers.

Carbon-mass units
Producers / Consumers	Standing carbon biomass at the stated time	New carbon production during the interval	Interval production / reference standing stock
Producers	10	100	10
Consumers	20	20	1

Original carbon-mass stock and new-production amounts for one boundary and interval, with sufficient supplied transfers to make the comparison possible. Endpoint stocks do not impose constant stocks or individual lifespan.

Worked example

Ask what the percentage divides

For the consumer, sixty of the hundred ingested units are assimilated, giving sixty percent. Ten of those sixty become production, giving one-sixth. Ten of the ingested hundred become production, giving ten percent. Multiplying 0.6 by 1/6 gives 0.1 for this account.

For the linear feeding-production sequence, multiply 0.2 by 0.15 to obtain 0.03, matching 3/100. This multiplication works because the intermediate twenty is the same compatible denominator and numerator at the adjacent links.

In the open example, dividing twenty by only forty omits the supplied sixty-unit import from the denominator. It answers a local-relative comparison rather than the ratio to the complete assigned supply. It does not show fifty-percent conversion of the full hundred.

Check your understanding

Why are the consumer ratios 10/60 and 10/100 different?
Show reasoning
The first divides production by assimilated energy; the second divides by ingested energy, which also includes forty unassimilated units. The same numerator has two distinct denominators.
How much of the lower-level hundred appears as next-level production three in the linear model?
Show reasoning
Three percent. The adjacent ratios are twenty percent and fifteen percent; their product is 0.03 under the shared compatible account.
Does producer standing biomass ten and consumer biomass twenty prove energy increased through feeding?
Show reasoning
No. Those are stocks at a time. The separately supplied production amounts are one hundred and twenty during a common interval. Stock size does not itself identify energy-transfer efficiency.

What to carry forward

Keep intake, assimilation, production, standing stock and dissipation separate; an efficiency has meaning only with its denominator.

References

Resources supporting this lesson’s explanations and examples.