3.8.4Additional lessons
Trophic Levels — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 94 of 142
Part 1 of 4The Trophic Levels
A trophic level is the position of an organism in a food chain, counted by the number of feeding steps from the producer.
Producer
- Occupies the first trophic level and converts light or chemical energy into carbon compounds that support the whole chain.
Primary consumer
- A herbivore on the second trophic level that feeds directly on producers such as grasses, algae or leaves.
Secondary consumer
- A carnivore or omnivore on the third level that feeds on primary consumers like insects or rodents.
Tertiary consumer
- Feeds on secondary consumers, occupying the fourth trophic level in a chain of that length.
Apex predator
- A top carnivore that has no natural predators, so energy leaves its level mainly through death and decomposition.
Part 2 of 4Food Chains and Food Webs
Food Chains and Food Webs
Food chain
- A single linear sequence in which arrows show the direction of energy transfer from the organism eaten to the one that eats it.
Food web
- A network of linked chains that shows the many feeding relationships actually present in an ecosystem.
Arrow meaning
- Arrows point from prey to predator because they represent the flow of chemical energy, not simply who eats whom.
Decomposers
- Detritivores and saprotrophs act on dead material from every level and are usually drawn alongside rather than inside the chain.
Part 3 of 4Species at Several Levels
1
In a food web a species with a varied diet may feed on organisms from different trophic levels.
2
An omnivore eating both plants and herbivores acts as a primary consumer and also as a secondary consumer.
3
In one web, a bird that eats aphid-eating beetles sits at level four, but when it eats caterpillars it sits at level three.
4
The trophic level is therefore a description of a role in a particular chain, not a fixed label attached to a species.
5
To classify a species, count the steps from the producer along the chain you are asked about.
| Chain | Organisms | Level of final consumer |
|---|---|---|
| Short | Grass → mouse → hawk | 3 |
| Longer | Tree → aphid → beetle → robin → hawk | 5 |
Part 4 of 4Exam Checklist & Tips
3.8.4 – Key Points for the Exam
- Trophic level The feeding position of an organism in a food chain, counted from the producer as level one.
- Producer An organism that makes its own carbon compounds and forms the first trophic level.
- Food web A diagram of interconnected food chains showing the feeding relationships in a community.
- Apex predator A consumer at the top of a food chain that has no natural predators.
- Omnivore A consumer that feeds on both plant and animal material and so can occupy more than one trophic level.
Common Mistakes
- Drawing arrows from predator to prey; arrows must show the direction of energy flow, from food to feeder.
- Counting the producer as zero; producers are the first trophic level.
- Placing decomposers as a normal level in the pyramid; they act on dead material from all levels.
- Assuming each species has just one trophic level; species with mixed diets occupy several.
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