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Energy Flow Through
Ecosystems

Explore how energy enters, moves through trophic levels, and is lost at each step: the foundation of all ecological dynamics.

AP Biology

What is Energy Flow?

Energy flow describes the one-way transfer of energy through an ecosystem, from autotrophs (producers) to heterotrophs (consumers) and ultimately lost as heat. Unlike matter, energy is not recycled: it enters the system primarily as sunlight and exits as thermal energy.

Understanding Trophic Levels

Producers
Autotrophs (plants, algae) that convert sunlight into chemical energy via photosynthesis. Trophic Level 1.
Primary Consumers
Herbivores that eat producers. Trophic Level 2. Examples: rabbits, grasshoppers, zooplankton.
Secondary Consumers
Carnivores or omnivores that eat primary consumers. Trophic Level 3. Examples: frogs, small fish.
Tertiary Consumers
Top predators that eat secondary consumers. Trophic Level 4. Examples: hawks, sharks, wolves.

The Sun as the Ultimate Source

Sun Producers Photosynthesis GPP Gross Primary Productivity Cellular Respiration Heat lost NPP Net Primary Productivity
\( \text{NPP} = \text{GPP} - \text{Respiration} \)

The 10% Rule of Energy Transfer

On average, only about \( 10\% \) of the energy at one trophic level is transferred to the next. The rest is lost primarily as heat through metabolic processes (cellular respiration), as well as through incomplete digestion and excretion.

10,000 kcal/m²/yr Producers 1,000 Primary Consumers 10% 100 Secondary Consumers 10% 10 Tertiary Consumers 10% ~90% lost as heat at each transfer

The Ecological Energy Pyramid

Producers 10,000 kcal/m²/yr Primary Consumers 1,000 kcal/m²/yr Secondary Consumers 100 kcal/m²/yr Tertiary 10 kcal/m²/yr Heat Heat Heat ★ Solar Energy

An ocean food web has low standing stock but high producer turnover

A sunlit ocean cutaway contains enlarged phytoplankton and copepods, a school of fish, a larger fish and organic material above benthic detritivores. Microscopic plankton are enlarged for visibility; organism sizes are not comparable.Standing biomass is a stock; production is a rate. Rapid producer turnover can support a larger consumer stock even as energy production decreases between trophic levels.

A stock is not a production rate

A stock is not a production rate. Standing biomass is a stock; production is a rate. Rapid producer turnover can support a larger consumer stock even as energy production decreases between trophic levels.Standing biomass is a stock; production is a rate. Rapid producer turnover can support a larger consumer stock even as energy production decreases between trophic levels.

Food Chains vs Food Webs

Food Chain Food Web
Structure Single linear pathway Interconnected network
Complexity Simple, easy to trace Complex, realistic
Resilience Fragile: one species removed collapses chain More stable: alternate pathways exist
Energy Tracking Easy to calculate transfer Harder: energy splits among pathways
Example Grass → Rabbit → Fox Multiple producers feeding multiple consumers

GPP vs NPP: A Key Distinction

GPP
Gross Primary Productivity: the total amount of energy fixed by photosynthesis per unit area per unit time. Includes energy used by producers for respiration.
NPP
Net Primary Productivity: the energy remaining after producers use some for their own respiration. This is the energy available to consumers.

Assimilation Efficiency

Not all energy an organism eats (ingested) is actually absorbed. Assimilation efficiency is the fraction of ingested energy that is absorbed across the gut wall. The rest is lost as feces.

Ingested Energy (I) Assimilated Energy (A) Feces Respiration Heat lost Production Growth + Repro

How to Solve Energy Transfer Problems

1
Identify the trophic levels. Label each organism in the food chain and its trophic level number (producers = 1).
2
Determine energy at the base. Find the starting energy available at the producer level (NPP, not GPP).
3
Apply the transfer efficiency. Multiply by \( 10\% \) (or the given efficiency) for each trophic level transition.
4
Use the formula: \( E_n = E_1 \times (0.10)^{n-1} \) where \( n \) is the trophic level.
5
Check units and context. Report in kcal/m²/yr or J. State what percentage of the original energy remains.

Example 1: Simple Chain

A grassland ecosystem has a net primary productivity of \( 20{,}000 \) kcal/m²/yr. How much energy is available to a tertiary consumer (trophic level 4)?

Producers 20,000
\(\times 0.10\)
 Primary 2,000
\(\times 0.10\)
 Secondary 200
\(\times 0.10\)
 Tertiary 20 All values in kcal/m²/yr

Example 2: Working Backwards

A hawk (tertiary consumer) requires \( 25 \) kcal/day to survive. Assuming \( 10\% \) transfer efficiency at each level, how much energy must be produced by plants to sustain this hawk?

1
Hawk is trophic level 4. We need energy at level 1 (producers).
2
Rearrange: \( E_1 = \frac{E_4}{(0.10)^3} = \frac{25}{0.001} \)
3
Calculate: \( E_1 = \) \( 25{,}000 \) kcal/day must be produced by plants.

Example 3: Variable Efficiency

In an aquatic ecosystem, phytoplankton fix \( 50{,}000 \) kcal/m²/yr (GPP). Producer respiration consumes \( 60\% \). Zooplankton (level 2) have \( 15\% \) trophic efficiency. Small fish (level 3) have \( 10\% \). How much energy reaches small fish?

1
Find NPP: \( \text{NPP} = 50{,}000 \times 0.40 = \) \( 20{,}000 \) kcal/m²/yr
2
Zooplankton (level 2): \( 20{,}000 \times 0.15 = \) \( 3{,}000 \) kcal/m²/yr
3
Small fish (level 3): \( 3{,}000 \times 0.10 = \) \( 300 \) kcal/m²/yr

Biomagnification and Energy Flow

While energy decreases at higher trophic levels, toxin concentration increases. Persistent chemicals (DDT, mercury) are not metabolized and accumulate in tissues. Since predators eat many prey organisms, toxins concentrate at each level.

Producers: 0.01 ppm Primary: 0.1 ppm Secondary: 1.0 ppm Tertiary: 10.0 ppm High toxin Low toxin Toxin

Common Mistakes

✕ Confusing GPP and NPP
Consumers access NPP, not GPP. Always subtract producer respiration before calculating energy transfer to the next level.
✕ Saying Energy is "Recycled"
Energy flows one way and exits as heat. Only matter (nutrients like C, N, P) is recycled through biogeochemical cycles.
✕ Applying 10% as Exact
The \( 10\% \) rule is an approximation. Actual trophic efficiency varies from \( 5-20\% \). Use the specific value if one is given.

Summary of Key Concepts

Concept Definition Key Detail
GPP Total energy fixed by producers Includes respiration
NPP GPP minus producer respiration Energy available to consumers
10% Rule ~10% energy transfers per level Approximate, varies \( 5-20\% \)
Energy Pyramid Visual of energy at each level Always upright
Biomagnification Toxin concentration increases upward Opposite pattern to energy
Decomposers Break down dead matter at all levels Recycle matter, not energy

Question 1 of 5

If producers in an ecosystem fix \( 50{,}000 \) kcal/m²/yr (GPP) and use \( 55\% \) for respiration, what is the NPP available to consumers?

Question 2 of 5

A food chain has 4 trophic levels. If \( 10{,}000 \) kcal/m²/yr is available at the producer level with \( 10\% \) efficiency at each transfer, how much energy is available to the tertiary consumer?

Question 3 of 5

Which statement best explains why ecological energy pyramids are always upright?

Question 4 of 5

An endotherm at trophic level 3 assimilates \( 500 \) kcal/day and has a production efficiency of \( 2\% \). How much energy is available for growth and reproduction?

Question 5 of 5

A scientist measures that a secondary consumer population contains \( 150 \) kcal/m²/yr of energy. If the trophic efficiency from primary to secondary consumers is \( 12\% \), how much energy was available at the primary consumer level?