AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/1/1-9)
Unit 1 · Topic 1.9
1.9 Trophic Levels
Trophic levels describe an organism's feeding position, from producers to top consumers. Energy enters ecosystems from the sun and flows one way through these levels, while decomposers return matter to be used again.
Key terms
- producer
- primary consumer
- secondary consumer
- tertiary consumer
- decomposer
The trophic levels
Producers are also called autotrophs, meaning ‘self-feeders’. All consumers are heterotrophs, meaning they get energy by eating other organisms. An omnivore, like a bear or a human, eats at more than one level, so it can be a primary consumer at one meal and a secondary consumer at the next.
| Level | Role | Example (grassland) |
|---|---|---|
| Producers (1st level) | Make their own food from sunlight | Grasses |
| Primary consumers (2nd) | Herbivores that eat producers | Grasshoppers, mice |
| Secondary consumers (3rd) | Eat primary consumers | Snakes, shrews |
| Tertiary consumers (4th) | Eat secondary consumers | Hawks |
| Decomposers and detritivores | Break down dead matter and wastes from every level | Fungi, bacteria, earthworms |
Energy in, energy out
Every ecosystem needs a constant input of energy. In almost all ecosystems, that energy comes from the sun: producers capture sunlight in photosynthesis and store it in sugar. Consumers get energy by eating, then use it through cellular respiration to move, grow and stay warm.
Each time energy is used, some is converted into heat that escapes the ecosystem and can't be reused by living things. That's why energy flows one way through an ecosystem and must be constantly replaced by the sun. If sunlight stopped, the food web would run out of energy.
A few ecosystems, like deep-sea hydrothermal vents, have no sunlight. There, bacteria make food by chemosynthesis, using energy from chemicals like hydrogen sulfide that seep out of the seafloor. They play the producer role for the vent community.
Matter cycles, energy doesn't
Matter, like carbon, nitrogen and phosphorus atoms, isn't used up. Decomposers (bacteria and fungi) break down dead organisms and waste and release nutrients back to the soil, water and air, where producers can use them again. Detritivores, like earthworms, millipedes and dung beetles, eat dead material and break it into smaller pieces, which speeds up decomposition.
So an ecosystem recycles its matter but needs a continuous supply of new energy. Remembering this one contrast will help you across this unit.
Without decomposers, dead leaves, bodies and wastes would pile up and the nutrients locked inside them would never return to the soil. Plant growth would slowly stop for lack of nutrients, even with plenty of sunlight.
Respiration and decomposition without oxygen
Most organisms use aerobic respiration, which requires oxygen. Some microbes break down food without oxygen (anaerobic respiration or fermentation), which releases less energy. In waterlogged soils, landfills and the stomachs of cows, microbes that work without oxygen produce methane (CH₄), a powerful greenhouse gas. This is why wetlands, rice paddies, landfills and cattle are all methane sources.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Assigning trophic levels
In a pond, algae are eaten by zooplankton, zooplankton are eaten by small fish, small fish are eaten by a heron, and the heron also sometimes eats tadpoles that eat algae. Give the trophic level of each organism.
Show the solutionHide the solution
- Step 1: Start at the bottom. Algae make their own food: producers.
- Step 2: Zooplankton and tadpoles eat algae: primary consumers.
- Step 3: Small fish eat zooplankton: secondary consumers.
- Step 4: The heron eats small fish (secondary consumers), making it a tertiary consumer there. It also eats tadpoles (primary consumers), making it a secondary consumer there. So it feeds at more than one level.
Answer: Algae: producer; zooplankton and tadpoles: primary consumers; small fish: secondary consumer; heron: tertiary consumer when eating fish and secondary consumer when eating tadpoles.
- Example 2
Why ecosystems need the sun
Explain why an ecosystem needs a continuous input of solar energy but doesn't need a continuous input of new carbon.
Show the solutionHide the solution
- Step 1: Energy: every time organisms use energy, some becomes heat that leaves the ecosystem. It can't be recycled, so new energy must keep arriving.
- Step 2: Matter: carbon atoms aren't destroyed. Respiration and decomposition return carbon to the air and soil, where producers reuse it.
- Step 3: Conclude: energy flows through in one direction, while matter cycles.
Answer: Energy is lost as heat at each step and can't be recycled, but carbon is recycled by respiration and decomposers, so only energy must be constantly replaced.
Common mistakes
- Saying energy cycles. Matter cycles; energy flows one way and is lost as heat.
- Counting producers as the first consumer level. Producers are level 1, primary consumers are level 2.
- Leaving decomposers out of the picture. They connect to every level by breaking down dead matter and waste.
- Assuming every ecosystem runs on sunlight. Hydrothermal vent communities rely on chemosynthesis.
On the exam
- Be ready to label organisms in a described food chain or web with their trophic level, including omnivores at more than one level.
- If asked about the role of decomposers, say they return nutrients (matter) to the ecosystem for producers to reuse.
Connected topics
Videos
Check yourself
4 questions on 1.9 Trophic Levels. Pick an answer to see if you got it, and why.
In the food chain grass → grasshopper → frog → snake → hawk, which organism is a tertiary consumer at the fourth trophic level?
A grizzly bear eats berries. It also eats salmon, which eat insects, which eat algae. Which statement correctly describes the bear's position in this food web?
Suppose all the decomposers and detritivores in a forest were suddenly removed. Which change would most likely happen over time?
Which statement best explains why an ecosystem needs a constant input of sunlight but not a constant input of new carbon and nitrogen?
0 of 4 answered