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Energy Flow & the
10% Rule

Trace how energy moves through ecosystems, why only \(\approx 10\%\) transfers between trophic levels, and what this means for food webs and ecological pyramids.

AP Environmental Science

What is Energy Flow?

Energy flow describes the one-way transfer of energy through an ecosystem: from the sun to producers, then to consumers, and finally lost as heat at every step. Unlike nutrients, energy does not cycle: it enters, passes through, and exits.

Understanding Trophic Levels

Producers
Trophic Level 1: Plants, algae, and cyanobacteria that convert sunlight into glucose via photosynthesis.
Primary Consumers
Trophic Level 2: Herbivores that eat producers directly: rabbits, grasshoppers, zooplankton.
Secondary Consumers
Trophic Level 3: Carnivores that eat herbivores: frogs, small fish, snakes.
Tertiary Consumers
Trophic Level 4: Top predators that eat other carnivores: hawks, sharks, wolves.

Why is Energy Lost at Each Level?

At every trophic level, organisms use most of the energy they consume for life processes. Only a fraction gets stored in biomass that the next level can eat. The rest is lost as:

1
Cellular respiration: Organisms break down glucose to power movement, growth, and reproduction, releasing heat energy.
2
Metabolic heat: The second law of thermodynamics dictates that every energy conversion produces waste heat.
3
Incomplete consumption: Not all parts of an organism are eaten: bones, roots, and shells remain.
4
Waste products: Feces and urine contain energy that passes to decomposers rather than to the next consumer.

A bite branches into growth, waste and heat

A caterpillar bites a leaf and a transparent side window shows one gut leading to feces. The exact budget partitions1000kcal into330feces,570respiration and100growth.Suppose a consumer ingests 1,000 kcal: 330 kcal leaves in feces, 570 kcal supports respiration, and 100 kcal becomes new biomass. The 10% ratio of production to ingestion is gross growth efficiency; whole-trophic transfer efficiency uses a different system boundary.

Consumer energy partition

Consumer energy partition. Suppose a consumer ingests 1,000 kcal: 330 kcal leaves in feces, 570 kcal supports respiration, and 100 kcal becomes new biomass. The 10% ratio of production to ingestion is gross growth efficiency; whole-trophic transfer efficiency uses a different system boundary.Suppose a consumer ingests 1,000 kcal: 330 kcal leaves in feces, 570 kcal supports respiration, and 100 kcal becomes new biomass. The 10% ratio of production to ingestion is gross growth efficiency; whole-trophic transfer efficiency uses a different system boundary.

The 10% Rule: Core Principle

Mathematically, if a trophic level has energy \(E\), the next level receives approximately:

The Energy Pyramid

Producers
\(10,000\) kcal
 Primary Consumers
\(1,000\) kcal
 Secondary Consumers
\(100\) kcal
 Tertiary
\(10\) kcal
\(90\%\) lost as heat
\(90\%\) lost as heat
\(90\%\) lost as heat
\(10\%\)
 Sun

GPP vs. NPP: Key Distinction

Gross Primary Productivity (GPP)
The total amount of energy captured by producers through photosynthesis per unit area per unit time.
Net Primary Productivity (NPP)
The energy remaining after producers use some for their own respiration. This is energy available to consumers.

Tracing Energy Flow Through a Food Chain

Sun Producers
\(10,000\) kcal
\(10\%\)
 Primary Consumers
\(1,000\) kcal
\(10\%\)
 Secondary Consumers
\(100\) kcal
\(10\%\)
 Tertiary
\(10\) kcal
\(9,000\) kcal
 lost as heat
\(900\) kcal
 lost as heat
\(90\) kcal
 lost as heat Decomposers: recycle nutrients (not energy) back to soil

How to Apply the 10% Rule

1
Identify the starting energy. Find the energy available at the producer level (or whichever level you start from).
2
Count the trophic transfers. Determine how many levels you must move up in the food chain.
3
Multiply by \(0.10\) for each transfer. Use: \(E_{\text{final}} = E_{\text{start}} \times (0.10)^n\), where \(n\) is the number of transfers.
4
Calculate energy lost. Subtract: \(E_{\text{lost}} = E_{\text{start}} - E_{\text{final}}\). This is the cumulative heat loss.

Example 1: Simple Food Chain

A grassland ecosystem starts with 20,000 kcal of energy at the producer level. How much energy is available to a secondary consumer?

1
Starting energy: \(E_0 = 20{,}000\) kcal (producers)
2
Transfers needed: Producers → Primary → Secondary = \(n = 2\) transfers
3
Apply formula: \(E_2 = 20{,}000 \times (0.10)^2 = 20{,}000 \times 0.01 = \) \(200\) kcal

Example 2: Reverse Calculation

A hawk (tertiary consumer) needs 50 kcal of energy per day. How much energy must be available at the producer level to sustain it?

1
End energy needed: \(E_3 = 50\) kcal at the tertiary consumer level
2
Transfers: Producers → Primary → Secondary → Tertiary = \(n = 3\)
3
Rearrange: \(E_0 = \dfrac{E_3}{(0.10)^3} = \dfrac{50}{0.001} = \) \(50{,}000\) kcal

Example 3: Ecological Efficiency Varies

Real ecosystems do not always transfer exactly \(10\%\). Suppose a marine food chain has an ecological efficiency of \(15\%\) between levels. With 8,000 kcal at producers, what reaches a secondary consumer?

1
Efficiency: \(e = 0.15\) instead of 0.10
2
Apply: \(E_2 = 8{,}000 \times (0.15)^2 = 8{,}000 \times 0.0225 = \) \(180\) kcal
3
Compare: With the \(10\%\) rule: \(8{,}000 \times (0.10)^2 = 80\) kcal. The \(15\%\) efficiency yields more than double the energy at the top.

Types of Ecological Pyramids

Energy Pyramid
Always upright. Energy decreases at each trophic level due to the 10% rule. Cannot be inverted.
Biomass Pyramid
Usually upright, but can be inverted in aquatic ecosystems where phytoplankton reproduce rapidly but have low standing biomass.
Numbers Pyramid
Can be inverted. One large tree (producer) may support millions of insects. Numbers do not reflect energy content.

Common Mistakes

✕ Confusing Energy with Nutrients
Energy flows one way and exits as heat. Nutrients cycle through the ecosystem repeatedly. They are fundamentally different.
✕ Counting Levels Wrong
A common error is counting the starting level as a "transfer." Remember: transfers = number of arrows in the chain, not the number of organisms.
✕ Treating 10% as Exact
The \(10\%\) rule is an approximation. Real efficiencies vary (\(5\%\)–\(20\%\)). On the AP exam, use \(10\%\) unless told otherwise.

Summary Comparison

Energy Flow Nutrient Cycling
Direction One-way (sun → heat) Cyclical (recycled)
Source Solar radiation Earth's crust, atmosphere
Lost at each level? Yes: \(\approx 90\%\) as heat No: recycled by decomposers
Pyramid shape Always upright N/A (no pyramid)
Measured in kcal or kJ per area per time grams or moles per area
Key rule 10% Rule Conservation of matter

Question 1 of 5

If producers in an ecosystem fix 50,000 kcal of energy, approximately how much energy is available to tertiary consumers?

Question 2 of 5

Which of the following best explains why top predators are rare in most ecosystems?

Question 3 of 5

A wolf needs 200 kcal/day. If the wolf is a tertiary consumer, how much producer energy is needed to sustain it daily?

Question 4 of 5

Which ecological pyramid can never be inverted?

Question 5 of 5

If a grassland's GPP is 12,000 kcal and producers use 4,000 kcal for respiration, what is the NPP available to primary consumers?