A FOOD CHAIN shows the FEEDING RELATIONSHIPS between organisms — who eats whom — and the direction of energy flow.
Arrows in a food chain show the direction of ENERGY TRANSFER (from food to the organism eating it).
Roles in a food chain:
PRODUCER — an organism that MAKES its own food using sunlight energy via PHOTOSYNTHESIS (green plants and algae). All food chains start with a producer.
PRIMARY CONSUMER — eats the producer (usually a herbivore).
SECONDARY CONSUMER — eats the primary consumer.
TERTIARY CONSUMER — eats the secondary consumer (often a top predator).
Example:
Grass → Rabbit → Fox → Eagle
(Producer) → (Primary) → (Secondary) → (Tertiary)
Each feeding level is called a TROPHIC LEVEL.
Energy Transfer and Loss
Energy enters food chains as SUNLIGHT — absorbed by producers during photosynthesis and stored in glucose.
At each trophic level, ENERGY IS LOST — only a fraction is transferred to the next level.
Energy is lost because:
ORGANISMS USE ENERGY for movement, keeping warm, growth and reproduction — energy used in RESPIRATION is released as HEAT and lost to the environment.
NOT ALL OF THE ORGANISM IS EATEN — bones, roots, shells and other parts may not be consumed.
SOME MATERIAL is excreted as faeces and not digested.
Typically only about 10% of energy at one trophic level passes to the next level.
This means:
Producers have the most energy and biomass.
Each level up has less energy and biomass — and fewer organisms.
Food chains are rarely more than 4–5 levels long — by the top level, so little energy remains that few organisms can survive.
Food Webs
A FOOD WEB shows many INTERCONNECTED food chains within an ecosystem — a more realistic picture of feeding relationships.
In reality, most organisms eat more than one thing and are eaten by more than one thing.
Food webs show:
Which organisms are predators and which are prey.
Which organisms are most at risk if a species is removed.
How changes in one population ripple through the ecosystem.
WHAT HAPPENS WHEN A SPECIES IS REMOVED:
If a predator is removed → its prey increase → the things the prey eat may decrease.
If a prey species is removed → predators that depended on it may decline → other prey species may increase.
KEYSTONE SPECIES are species that have a disproportionately large effect on the ecosystem relative to their numbers — removing them causes major ecosystem changes (e.g. wolves in Yellowstone, sea otters in kelp forests).
⚠️ Common Mistake
Arrows in food chains show the direction of ENERGY FLOW — from what is eaten to what eats it. So the arrow goes FROM prey TO predator. Students often draw arrows the wrong way, showing what the predator eats rather than which way energy flows. Also: PRODUCERS are PLANTS (photosynthetic) — not all plants in a food chain are 'prey', they are the PRODUCERS.
📌 Key Note
Food chain: producer → primary → secondary → tertiary consumer. Arrows show direction of energy flow. Only ~10% of energy passes to each next level — rest lost as heat, waste, movement. Food web = many interconnected chains. More realistic.
🎯 Matching Activity — Match the Trophic Level
Match each organism to its role in a food chain. — drag the symbols on the right to match the component names on the left.
Producer
Drop here
Primary consumer
Drop here
Secondary consumer
Drop here
Tertiary consumer
Drop here
Decomposer
Drop here
Grass — makes its own food by photosynthesis
Fox — eats the primary consumer (rabbit)
Bacteria and fungi — break down dead organisms at all levels
Rabbit — eats the producer (grass)
Eagle — eats the secondary consumer (fox)
🎯 Test Yourself
Question 1 of 2
1. Why do food chains rarely have more than 4–5 trophic levels?
2. In a food web, the rabbit population suddenly increases due to a disease killing foxes. What happens to the grass?
⭐ How Well Do You Understand This Topic?
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