Syllabus Edition
First teaching 2015
Last exams 2025
Trophic Levels
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Trophic Levels & Feeding Relationships
What are trophic levels?
The trophic level is the position that an organism occupies in a food chain, or a group of organisms in a community that occupy the same positions in a food chain
Trophic Levels
Trophic Level | Name of Trophic Level | Description of Organisms in Trophic Level |
1 | Producers | Plants and algae – produce their own biomass using energy from sunlight |
2 | Primary consumers | Herbivores – feed on producers |
3 | Secondary consumers | Predators – feed on primary consumers |
4 | Tertiary consumers | Predators – feed on secondary consumers |
5 | Quaternary consumers | Predators - feed on tertiary consumers |
Producers (autotrophs) are typically plants or algae and produce their own food using photosynthesis and form the first trophic level in a food chain
Exceptions include chemosynthetic organisms that produce food without sunlight
For example, bacteria living inside the giant tube worm (Riftia pachyptila), found near hydrothermal vents on the ocean floor (where there in no light), use hydrogen sulfide from the vents to produce organic compounds through chemosynthesis, providing a source of energy for the tube worm
The chemical energy stored in producers is then transferred to primary consumers as they consume (eat) producers
The chemical energy is then transferred from one consumer to the next as they eat one another
Apex predators are at the very top of the food chain – they are carnivores with no predators
The chemical energy stored within apex predators can be passed on to decomposers when apex predators die and are decomposed
Feeding Relationships
Feeding relationships involve producers, consumers and decomposers
These can be modelled using food chains, food webs and ecological pyramids
Trophic levels for a simple food chain - the blue arrows show how the chemical energy originally produced by the primary producer (grass) is transferred to other organisms in the community
Trophic levels for a simple food web – note that some organisms can belong to more than one trophic level (such as the squirrel, fox and eagle in this food web)
Decomposers
The net primary production (i.e. the chemical energy) of producers is also available to another group of organisms known as decomposers
The two main groups of decomposers are bacteria and fungi
Decomposers carry out a very important function in ecosystems - they break down dead plant and animal material (in the process gaining the chemical energy still stored in the dead matter)
They do this by:
Secreting digestive enzymes onto the surface of the dead organism
These enzymes break down the dead matter into small soluble food molecules
These molecules are then absorbed by the decomposers
This process of decomposition also helps to release organic nutrients back into the environment (e.g. the soil), which are essential for the growth of plants and other producers
Examiner Tips and Tricks
Don’t forget - animals (known as consumers) can be at different levels within the same food web as they could be omnivores (animals that can eat both plants and animals) or could be predators that eat both primary, secondary and/or tertiary consumers!
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