📖 In-Depth Theory
What Are Trophic Levels?
A TROPHIC LEVEL describes the position of an organism in a food chain — how far it is from the original energy source (the Sun).
TROPHIC LEVEL 1 — PRODUCERS:
Plants and algae. Make their own food by photosynthesis. All energy in the ecosystem enters through producers.
Examples: grass, oak trees, phytoplankton, seaweed.
TROPHIC LEVEL 2 — PRIMARY CONSUMERS:
Herbivores — eat plants/algae.
Examples: rabbits, caterpillars, cows, zooplankton.
TROPHIC LEVEL 3 — SECONDARY CONSUMERS:
Carnivores that eat primary consumers.
Examples: foxes (eating rabbits), frogs (eating insects), small fish (eating zooplankton).
TROPHIC LEVEL 4 — TERTIARY CONSUMERS:
Carnivores that eat secondary consumers.
Examples: eagles, large sharks, humans (in some food chains).
APEX PREDATORS: at the top of the food chain — not eaten by any other organism in that ecosystem.
Decomposers
DECOMPOSERS occupy a special role — they break down dead organisms and waste from all trophic levels.
Decomposers include: bacteria and fungi.
They digest complex organic molecules → release nutrients back into the soil.
Essential for NUTRIENT CYCLING — without them, nutrients would be locked in dead matter forever.
Decomposers are not usually assigned a numbered trophic level but are fundamental to ecosystem function.
FOOD CHAINS and FOOD WEBS:
Food chain: linear sequence showing feeding relationships. Arrow = energy transfer direction.
Grass → Rabbit → Fox → Eagle
Food web: multiple interconnected food chains — more realistic picture of an ecosystem.
Most organisms eat more than one thing and are eaten by more than one predator.
KEYPOINT:
In food chains and food webs, ARROWS show the direction of ENERGY FLOW — from prey to predator. The arrow means 'is eaten by' or 'energy flows to'.
Energy Flow Through Trophic Levels
Energy enters ecosystems via PHOTOSYNTHESIS at trophic level 1.
At each trophic level, energy is LOST:
Respiration — organisms use energy for life processes → released as heat.
Movement, excretion, undigested material.
Only a fraction of energy is incorporated into the organism's BIOMASS.
Typically only about 10% of energy is transferred from one trophic level to the next.
(The exact value varies — 10% is a common approximation used at GCSE.)
CONSEQUENCE:
Higher trophic levels have LESS energy available → can support fewer organisms.
This is why food chains rarely have more than 4–5 levels — too little energy to sustain another level.
EXAMPLE:
1000 kJ enters at trophic level 1 (producers).
~100 kJ available at level 2 (primary consumers).
~10 kJ available at level 3 (secondary consumers).
~1 kJ available at level 4 (tertiary consumers).
This pattern of decreasing energy is shown in PYRAMIDS OF BIOMASS.
⚠️ Common Mistake
Arrows in food chains show the direction of ENERGY FLOW — from prey TO predator. They do NOT show 'what eats what' in the backward direction. Arrow = 'is eaten by' or 'energy flows to'. Also: only ~10% of energy passes to the next level — most is lost in respiration.