Key ideas
| Level | Name | Gets energy from | Example |
|---|---|---|---|
| 1 | Producers (autotrophs) | Sunlight, by photosynthesis | Grass, algae |
| 2 | Primary consumers (herbivores) | Producers | Grasshopper, zooplankton |
| 3 | Secondary consumers | Primary consumers | Frog, small fish |
| 4 | Tertiary consumers | Secondary consumers | Snake, larger fish |
| 5 | Quaternary consumers | Tertiary consumers | Hawk |
Decomposers such as fungi and bacteria break down dead matter from every level and return nutrients to the soil. Arrows in a food web point the way energy flows: from the food to the eater.
OpenStax measures how much energy passes between levels with the trophic level transfer efficiency (TLTE):
- Production is the energy built into new living tissue at a level, often in kJ or kcal per square meter per year.
- Real values vary a lot. In the Silver Springs ecosystem in OpenStax, TLTE between the first two levels was about 14.5%. Many classes use 10% as a round rule of thumb.
- The rest of the energy is used for life processes (cellular respiration) and leaves as heat, or is never eaten. So after four to six transfers there is too little energy left for another level.
| Pyramid | Measures | Can it be upside down? |
|---|---|---|
| Numbers | How many organisms at each level | Yes (one tree can feed thousands of insects) |
| Biomass | Mass of living tissue at each level | Yes (in the English Channel, phytoplankton biomass is smaller than zooplankton biomass) |
| Energy | Energy flow through each level | No, energy pyramids are always upright |
Worked examples
Example 1: the 10% rule
Problem Producers in a meadow capture 20,000 kJ/m² of energy in a year. Using the 10% rule, how much reaches the primary, secondary and tertiary consumers?
- Primary consumers get 10% of the producers' energy.
- Secondary consumers get 10% of that.
- Tertiary consumers get 10% again.
Answer 2,000, 200 and 20 kJ/m² per year
Example 2: calculate a real transfer efficiency
Problem In a pond, producers make 12,000 kcal/m² of new tissue in a year, and the primary consumers make 1,800 kcal/m². What is the TLTE?
- Use the OpenStax formula.
- That is a bit above the 10% rule of thumb, which is normal: real values vary from ecosystem to ecosystem.
Answer 15%
Example 3: name the trophic level
Problem In the food chain grass → grasshopper → frog → snake → hawk, what trophic level is the hawk, and what kind of consumer is it?
- Count from the producer: grass 1, grasshopper 2, frog 3, snake 4, hawk 5.
- Level 2 is the primary consumer, so level 5 is the fourth consumer in line.
Answer Trophic level 5, a quaternary consumer
Common mistakes and how to fix them
- Drawing arrows from eater to food. Arrows show energy flow, so they point from the food to the eater: grass → grasshopper.
- Saying the lost energy disappears. Energy is not destroyed. It is used for life processes and leaves as heat, which the next level cannot use.
- Thinking matter is lost like energy. Nutrients such as carbon and nitrogen are recycled by decomposers. Energy flows one way; matter cycles.
- Assuming every animal has one level. Omnivores can feed at several levels. A bear eating berries is a primary consumer, and eating fish it is a secondary or tertiary consumer.
Practice questions
Producers in a lake store 50,000 kcal. Using the 10% rule, how much energy reaches the secondary consumers?
- 5,000 kcal
- 500 kcal
- 50 kcal
- 45,000 kcal
Show answer
Answer: 500 kcal
Two transfers: kcal for primary consumers, then kcal for secondary consumers.
An animal eats only herbivores. What trophic level is it on?
- Level 1, producer
- Level 2, primary consumer
- Level 3, secondary consumer
- Level 4, tertiary consumer
Show answer
Answer: Level 3, secondary consumer
Herbivores are level 2, so anything that eats them is level 3.
Which ecological pyramid can never be upside down?
- Pyramid of numbers
- Pyramid of biomass
- Pyramid of energy
Show answer
Answer: Pyramid of energy
Each level can only pass on part of the energy it receives, so energy always shrinks going up.
Primary consumers make 5,000 kJ/m² of new tissue in a year, and secondary consumers make 400 kJ/m². What is the TLTE between them?
Show answer
Answer: 8%
.
Where does most of the energy at one trophic level go, instead of to the next level?
- It is destroyed
- It is used for life processes and lost as heat
- It turns into nutrients in the soil
- It goes back to the producers
Show answer
Answer: It is used for life processes and lost as heat
Organisms use most of their energy for cellular respiration, movement and staying warm, and that energy leaves as heat.
Frequently asked questions
What is the difference between a food chain and a food web?
A food chain is a single path of energy, such as grass to rabbit to fox. A food web shows all the feeding links in an ecosystem at once, with many overlapping chains. Webs are more realistic, because most animals eat more than one thing.
Why are there so few top predators?
So much energy is lost at each step that only a small share of the producers' energy reaches the top. A hawk at level 5 gets roughly 0.01% of the energy the plants captured, if each step passes 10%. That can only support a few hawks over a large area.
What is biomagnification?
Some toxins, such as mercury or the pesticide DDT, are stored in body tissue instead of being broken down. Each level eats many organisms from the level below, so the toxin becomes more concentrated going up the food chain. Top predators carry the highest levels.
Sources
- OpenStax Biology 2e, 46.1 Ecology of Ecosystems, accessed October 1, 2026
- OpenStax Biology 2e, 46.2 Energy Flow through Ecosystems, accessed October 1, 2026
- Lindeman, The Trophic-Dynamic Aspect of Ecology, Ecology 1942 (origin of the 10% rule of thumb), accessed October 1, 2026
