Cambridge O Level Biology · Syllabus 5090 · Relationships of Organisms with One Another and with the Environment
Trophic Level
What is Trophic Level?
The feeding position an organism occupies in a food chain, food web or ecological pyramid. Producers form the first trophic level, the herbivores that eat them form the second as primary consumers, the carnivores that eat those herbivores form the third as secondary consumers, and so on. An organism's trophic level depends on what it is eating in the chain being considered, so an omnivore can occupy different trophic levels in different chains of the same food web.
This definition is part of the Relationships of Organisms with One Another and with the Environment chapter in Cambridge O Level Biology.
Common mistakes with Trophic Level
- “Decomposers are the highest trophic level.” Why it is wrong: they receive dead material and waste from producers and from every consumer level, so no single level can be assigned to them. Exam-safe: draw decomposers to the side of the chain with arrows coming into them from several levels.
- “Respiration destroys energy.” Why it is wrong: respiration releases energy from glucose. The energy is not destroyed; it is transferred, largely to the environment as thermal energy. Exam-safe: energy is released in respiration and transferred to the environment, so less remains available for the next trophic level.
- “Biomass means the mass of one organism.” Why it is wrong: a pyramid of biomass compares whole trophic levels, not individuals. Exam-safe: biomass is the total mass of living material at that trophic level, preferably as dry mass, for a stated area at a stated time.
- “All pesticides build up through food chains.” Why it is wrong: persistence varies enormously, and many pesticides break down within days. Exam-safe: pesticides that are not broken down may accumulate within organisms and become more concentrated at successive trophic levels.
Examiner tips on Trophic Level
- Units carry marks in this topic. Energy at a trophic level is a rate per unit area: \(\mathrm{kJ/m^2/year}\). It has to be per area, because a bigger field holds more energy, and per time, because energy keeps arriving. If a question supplies a unit, use that exact unit in your answer; if it asks you to comment on a pyramid of energy, the time component is part of why the pyramid is meaningful.
Questions students ask about Trophic Level
How much energy is transferred between trophic levels?
Only a small fraction, often around ten per cent, though the exact figure varies. Use the value a question gives you rather than quoting a general figure, and be ready to explain why so little is transferred: uneaten parts, egested undigested material, respiration and thermal transfer to the environment, movement and other life processes, excretory losses, and organisms dying without being eaten.