Biology
Ecosystems & food webs — quick study summary
An ecosystem is a community of organisms plus their non-living environment, linked by energy flow and nutrient cycling. Producers (plants, algae) fix sunlight into chemical energy; consumers eat producers or each other; decomposers break dead matter back into nutrients. Energy flows one way and is lost as heat at every trophic level (~10% transfer efficiency), while nutrients like carbon, nitrogen and water cycle indefinitely between living things and the environment.
Key points
- Trophic levels: producer → primary consumer → secondary consumer → tertiary consumer
- Only ~10% of energy passes between trophic levels (Lindeman's rule)
- Nutrients cycle (carbon, nitrogen, water); energy flows one way
- Food chain = linear path of energy; food web = interconnected chains
- Keystone species disproportionately shape their ecosystem despite low biomass
Practice quiz
Click each question to reveal the answer.
1. Approximately what percentage of energy is transferred from one trophic level to the next?
- 1%
- 10%
- 50%
- 90%
Answer: 10%
Lindeman's 10% rule — most energy is lost as heat through respiration, with about a tenth available to the next level.
2. Which group breaks dead organic matter back into nutrients?
Answer: Decomposers (fungi and bacteria)
Decomposers return carbon, nitrogen, phosphorus and other nutrients to the soil and atmosphere for re-use by producers.
3. Why do food chains rarely have more than 4–5 trophic levels?
Answer: Energy loss between levels means there isn't enough left to support a higher predator
After several 90% losses, the available energy is too small to sustain a viable population at the top.
Last reviewed: May 2026