Syllabus Edition

First teaching 2024

First exams 2026

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Trophic Levels & Food Chains (DP IB Environmental Systems & Societies (ESS))

Revision Note

Trophic Levels & Food Chains

What are trophic levels?

  • The trophic level is the position that an organism occupies in a food chain (or food web)

    • If multiple organisms occupy the same position in a food chain, they are in the same trophic level

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

  • Producers are typically plants or algae and produce their own food using photosynthesis

    • They form the first trophic level in a food chain

  • 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

  • Consumers have diverse strategies for obtaining energy-containing carbon compounds

Consumer Strategies

Type of Consumer

Description

Examples

Herbivores

Feed primarily on plants and plant-derived material

Deer: graze on grasses, leaves, and shrubs

Rabbits: consume grasses, herbs, and vegetables

Detritivores

Consume decomposing organic matter (detritus) and help break it down further

Earthworms: feed on decaying plant material and enhance soil structure

Dung beetles: consume animal dung, aiding in nutrient recycling

Predators

Hunt and consume other organisms (prey) for food

Lions: prey on various herbivores such as gazelles and zebras

Wolves: hunt animals like deer and elk in packs

Parasites

Depend on a host organism for survival, often harming but not immediately killing it

Tapeworms: live in the intestines of mammals, absorbing nutrients from the host's food

Mosquitoes: feed on the blood of animals, including humans, for nourishment

Saprotrophs and decomposers

Saprotrophs: decompose dead organic matter externally and absorb nutrients

Decomposers: break down organic matter into simpler substances, playing a vital role in nutrient recycling

Fungi: break down dead plant material, such as fallen leaves and wood, into simpler compounds

Bacteria: decompose organic matter, releasing nutrients for plant uptake

Scavengers

Consume dead animal carcasses, helping to clean up ecosystems

Vultures: feed on the remains of dead animals, scavenging carrion

Hyenas: opportunistic scavengers known to consume a wide range of animal remains

Food chains

  • Feeding relationships in ecosystems can be modelled using food chains

    • Because producers in ecosystems make their own carbon compounds by photosynthesis, they are at the start of food chains

    • Consumers obtain carbon compounds from producers or other consumers, so are placed in the higher trophic levels

    • In a food chain, carbon compounds and the energy they contain are passed from primary producers to primary consumers to secondary consumers, and so on

    • Apex predators are at the very top of the food chain—they are carnivores or omnivores with no predators

      • The chemical energy stored within apex predators can be passed on to decomposers when apex predators die and are decomposed

      • Traditionally, decomposers are not included in food chains as they gain carbon compounds from a variety of trophic levels

Diagram showing an example of a food chain
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

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

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