Trophic Levels in an Ecosystem

(Biology only — 4.7.4.1) Every organism in an ecosystem fits into a trophic level — its feeding position in a food chain. The word comes from the Greek trophe, meaning 'nourishment'.

Level 1 — Producers. Plants and algae that make their own food by photosynthesis. They convert light energy into glucose, which is stored in their tissues. Examples in the UK: grass, oak trees, phytoplankton in ponds and seas.

Level 2 — Primary consumers (herbivores). Animals that eat producers. Examples: rabbits, sheep, caterpillars, aphids, zooplankton.

Level 3 — Secondary consumers (carnivores). Animals that eat primary consumers. Examples: foxes (eating rabbits), ladybirds (eating aphids), small fish (eating zooplankton).

Level 4 — Tertiary consumers / apex predators. Carnivores that eat other carnivores. Often the top of the food chain with no natural predators. Examples: golden eagles (eating foxes), pike (eating smaller fish), sparrowhawks.

Some ecosystems have a level 5 (e.g. orcas eating sharks eating tuna).

Decomposers. Bacteria and fungi that break down dead organisms (and animal waste) at every level. They aren't usually given a 'level' number, but they're vital — they release nutrients back into the soil so producers can use them again.

Woodland food chain:

oak leaves → caterpillar → blue tit → sparrowhawk

Trophic level	Organism	Role
1	Oak leaves	Producer (photosynthesis)
2	Caterpillar	Primary consumer (herbivore)
3	Blue tit	Secondary consumer (insectivore = carnivore)
4	Sparrowhawk	Tertiary consumer / apex predator

Farmland food chain:

grass → rabbit → fox → (no natural predator in UK)

Pond food chain:

algae → water flea (Daphnia) → small fish (stickleback) → pike

Marine food chain:

phytoplankton → zooplankton → herring → mackerel → tuna → orca

Key skills you need:

1. Identify trophic level from a food chain. Count from the producer. The producer is always level 1, regardless of how many other organisms are in the chain.

2. Describe the role of each level. Producers make food via photosynthesis; consumers obtain energy by eating others; decomposers release nutrients from dead matter.

3. Read food chain arrows correctly. Arrows go from food → eater. They show energy flow direction. NEVER draw an arrow backwards.

4. Recognise omnivores. Animals that eat both plants and animals (humans, badgers, foxes) can occupy more than one trophic level — they'll appear at level 2 in some chains and level 3 in others.

Decomposers are organisms — mostly bacteria and fungi — that break down dead bodies, leaves, and animal waste. They are essential because:

1. They release nutrients. Locked-up minerals (nitrogen, phosphorus, potassium) inside dead organisms are released back into the soil. Without decomposition, all the nutrients in an ecosystem would eventually end up locked in dead matter, and producers couldn't grow.

2. They feed on every level. Decomposers don't care whether the dead organism was a plant, herbivore or carnivore. Dead oak leaves, dead rabbits, fox droppings — they're all food.

3. They speed up nutrient cycling. Some decomposers are very fast (e.g. soil bacteria can decompose a fallen leaf in a few weeks); others slower (a fallen tree trunk takes years). The speed depends on temperature, moisture and oxygen availability.

Examples of UK decomposers:

• Bacteria — most decomposition is done by bacteria in soil and water.
• Fungi — visible 'fruiting bodies' (mushrooms, toadstools, bracket fungi) are decomposers.
• Detritivores — animals like earthworms, woodlice and dung beetles physically chop up dead material, speeding up bacterial action. (Detritivores aren't strictly decomposers but help them.)

Cross-link to topic 4.7.2: decomposition is part of the carbon cycle and nitrogen cycle — without decomposers, neither would work.

(Biology only — 4.7.4.2) Biomass is the mass of living material in an organism, population, or trophic level. Usually expressed as:

• Dry mass (with water removed) — because water content varies between organisms and from day to day.
• Units of grams per square metre (g/m²), kilograms per hectare (kg/ha), or tonnes per square kilometre.

Why biomass beats counting individuals. Consider an oak tree with thousands of caterpillars eating its leaves. If you draw a pyramid of NUMBERS, you'd get:

| Numbers | Level 2 (caterpillars) | 10,000 | | | Level 1 (oak tree) | 1 |

— which is an INVERTED pyramid (narrow base, wide top). It looks weird but it's true.

If you draw a pyramid of BIOMASS for the same system, you get:

| Biomass | Level 2 (caterpillars) | 0.5 kg | | | Level 1 (oak tree) | 2,000 kg |

— a normal pyramid with a wide base. Biomass shows the real energy story.

Why dry mass? Living tissue contains 60–90% water. A lettuce leaf weighs much more wet than dry, but the actual biomass (the carbon-based living tissue) is the dry mass. Drying organisms in a low-temperature oven removes water without burning the tissue.

For exam questions — you don't usually need to dry organisms yourself; data tables give you biomass already. You just need to plot the bars to scale.

Step-by-step:

1. Identify the trophic levels. From the food chain, list each organism and its trophic level (1 = producer, 2 = primary consumer, etc.).

2. Note the biomass values. From the data table, find the biomass (usually in g/m²) at each level.

3. Choose a scale. E.g. 1 cm = 1000 g/m². This determines bar widths.

4. Draw bars to scale.

• Start with the producer at the bottom.
• Each bar is centred on the same vertical axis.
• Bar width is proportional to biomass.
• Bar height is the same for all levels (or roughly constant).

5. Label each level (level 1, 2, 3...) and the organism's name.

6. Add the scale (e.g. '1 cm = 100 g/m²').

Example data:

Trophic level	Organism	Biomass (g/m²)
1	Grass	1000
2	Rabbits	80
3	Foxes	8

Using scale 1 cm = 100 g/m²:

• Grass bar: 10 cm wide.
• Rabbit bar: 0.8 cm wide.
• Fox bar: 0.08 cm wide (almost a line).

Pyramids of biomass almost always narrow upwards because biomass is lost between trophic levels. AQA wants you to know the reasons (covered in detail in 4.7.4.3):

1. Not all biomass is eaten. Animals don't eat every part of their food (rabbits don't eat grass roots; foxes don't eat rabbit bones). Some biomass dies and is decomposed without ever being eaten.

2. Not all eaten biomass is absorbed. Some material passes through the gut and is egested as faeces. This goes to decomposers, not to the next trophic level.

3. Energy is used in respiration. The absorbed biomass releases energy through respiration — used for movement, body heat (in mammals/birds), reproduction. This energy escapes the food chain.

4. Some biomass is excreted. Waste products from metabolism (urea in urine, CO₂ from respiration) are removed and not passed up the chain.

Net effect: typically only about 10% of the biomass at one level becomes biomass at the next level. That's why:

• Level 1 → Level 2 loses ~90%.
• Level 2 → Level 3 loses another ~90%.
• Level 3 → Level 4 loses another ~90%.

After 4–5 levels, there's almost no biomass left — that's why food chains rarely have more than 5 levels.

Real-world implication. Eating plants directly is far more efficient than eating animals that eat plants. This is why a vegetarian or vegan diet has a smaller environmental footprint per calorie.

(Biology only — 4.7.4.3) When biomass moves through a food chain, most of it is lost at each step. As a rule of thumb, only about 10% of biomass at one trophic level becomes biomass at the next level.

An example:

Trophic level	Organism	Biomass (g/m²)
1	Wheat	10,000
2	Cows	1,000
3	Humans (eating beef)	100

In this chain:

• 10,000 g/m² of wheat → 1,000 g/m² of cow biomass (10% efficient).
• 1,000 g/m² of cow → 100 g/m² of human biomass (10% efficient).
• Overall: only 1% of wheat biomass reaches the human level.

The formula:

$\text{efficiency (\%)} = \frac{\text{biomass at higher level}}{\text{biomass at lower level}} \times 100$

Why 10% is only approximate. The 10% figure is an average. In reality:

• Some efficient food chains: 15–20% (e.g. cold-blooded fish in cool water — they need less energy for body heat).
• Some inefficient food chains: 5% or lower (e.g. warm-blooded mammals in cold climates — huge heat losses).

For exams, use 10% as your default unless the question gives different data.

AQA's spec lists four specific losses. You need to be able to describe each.

1. Not all material is eaten. Predators leave bones, hooves, fur, beaks. Herbivores often don't eat roots, bark or tough leaves. The uneaten biomass dies and is decomposed — it goes to decomposers, not the next trophic level.

2. Faeces (egestion). Some eaten material passes straight through the digestive system without being absorbed — particularly cellulose in plant cell walls. This is egested as faeces, which decomposers break down.

(Don't confuse egestion with excretion — see below.)

3. Respiration losses (the biggest loss in animals). Once biomass is absorbed by the animal, the glucose etc. is broken down in respiration to release energy. That energy is used for:

• Movement (muscles, blood pumping).
• Body heat — especially big in endotherms (mammals, birds) which maintain a constant body temperature.
• Cell processes (active transport, protein synthesis).

The energy released eventually leaves the animal as heat — wasted to the environment, not available for the next level. This is the single biggest loss in mammal/bird food chains.

4. Excretion. Metabolic waste products (e.g. urea in urine, CO₂ from respiration) are removed from the body. These contain some chemical energy that's also lost from the chain.

Why food chains are short.

Because so much is lost at each step:

• After 1 transfer: 10% remains.
• After 2 transfers: 1%.
• After 3 transfers: 0.1%.
• After 4 transfers: 0.01% — usually not enough to support a 5th level.

That's why ecosystems rarely have more than 4–5 trophic levels.

Calculation practice.

Example 1. Grass = 12,000 g/m². Sheep = 800 g/m². Calculate transfer efficiency.

$\text{efficiency} = \frac{800}{12000} \times 100 = 6.7\%$

(Lower than the 10% average — sheep are warm-blooded so lose lots to body heat in UK winters.)

Example 2. Phytoplankton = 50 g/m². Small fish = 8 g/m². Calculate efficiency.

$\text{efficiency} = \frac{8}{50} \times 100 = 16\%$

(Higher than average — fish are cold-blooded so save energy on body heat.)

Implications for food security. The 10% rule has huge implications:

• Plant-based diets are far more efficient land use. 1 hectare of wheat feeds many more people than 1 hectare of beef cattle.
• Beef has a very high environmental footprint per calorie because of the 10% step.
• Insects as protein are efficient — fast-growing, cold-blooded — and could feed many more people per unit of feed.

This is highly relevant to food security (covered in 4.7.5.1) and farming techniques (4.7.5.2).

Practice question: A field can grow 5,000 kg of cabbage OR support 50 sheep (giving 4,000 kg sheep biomass total). Which feeds more people if humans eat directly?

• Cabbage: 5,000 kg of food.
• Sheep meat: ~400 kg available to humans (sheep need to grow first; only ~10% of grass biomass becomes useable meat).

Plant-eating beats meat-eating by ~10× for the same land area.