3.9.4Additional lessons
Trophic Levels — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 22 of 101
Part 1 of 5Food Chains
A food chain is a linear sequence showing who feeds on whom.
Trophic level
- This is the position of an organism in a food chain, determined by the number of energy transfers from the producer.
Producers
- Organisms at the first trophic level that make organic compounds by photosynthesis or chemosynthesis.
Primary consumers
- Herbivores that feed on producers occupy the second trophic level.
Secondary and tertiary consumers
- Carnivores that eat herbivores, and carnivores that eat other carnivores, occupy higher trophic levels.
Arrows
- Arrows in a food chain point from the organism that is eaten to the organism that eats it, showing the direction of energy flow.
Part 2 of 5Food Webs
Food Webs
Complex relationships
- Most consumers eat more than one food, so many chains are linked in a food web.
Omnivores
- An omnivore can occupy different trophic levels in different chains, for example as a primary and secondary consumer.
Stability
- A web with many links can usually cope with the loss of one species better than a simple chain.
Decomposers
- Decomposers feed on all trophic levels, releasing nutrients from dead organisms and waste.
Part 3 of 5Limits to Chain Length
1
Producers capture energy from the Sun and store some of it in organic compounds.
2
Only a part of the energy at each level passes to the next, because much is lost as heat and in uneaten material.
3
By the fourth or fifth trophic level the energy remaining is too small to support a viable population.
4
This is why food chains seldom have more than five links.
| Trophic level | Name | Example |
|---|---|---|
| 1 | Producer | Phytoplankton |
| 2 | Primary consumer | Zooplankton |
| 3 | Secondary consumer | Small fish |
| 4 | Tertiary consumer | Seal |
Part 4 of 5Worked Classification
1
Take a food chain: grass, grasshopper, frog, snake, hawk, and identify the producer first.
2
Grass is the producer and occupies trophic level 1, because it makes its own organic compounds.
3
The grasshopper feeds on grass, so it is a primary consumer at level 2, and the frog that eats it is a secondary consumer at level 3.
4
The snake is a tertiary consumer at level 4, and the hawk is a quaternary consumer at level 5.
5
If the hawk also ate grasshoppers, it would occupy more than one level, which only a food web can show.
Part 5 of 5Exam Checklist & Tips
3.9.4 – Key Points for the Exam
- Trophic level Feeding position of an organism in a food chain.
- Food web Network of interconnected food chains.
- Primary consumer An organism that feeds on producers.
- Omnivore An organism that eats both plant and animal matter.
- Decomposer An organism that feeds on dead matter at any level.
Common Mistakes
- Drawing arrows from predator to prey instead of from food to feeder.
- Leaving decomposers out of a food web, though they feed on every trophic level.
- Counting the Sun as a trophic level.
- Saying all herbivores are secondary consumers.
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