🐟 Apex predator
An apex predator is a carnivore that has no predators. It sits at the top of its food chain. The number of its trophic level depends on the chain, so it may be level 3, level 4 or higher.
Trophic Levels in an Ecosystem » Trophic Levels and Pyramids of Biomass
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.7.4.1, 4.7.4.2 (Biology only)
- How to number the trophic levels in a food chain
- What apex predators and decomposers do
- What a pyramid of biomass shows
- How to construct an accurate pyramid of biomass from data
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Unlock This CourseTrophic levels
You already know that every food chain shows who eats whom. Ecologists also give each feeding step a number. A trophic level is the position of an organism along a food chain, counted from the start of the chain.
Key terms:
- Trophic level: the feeding level of an organism in a food chain, shown as a number starting at level 1.
- Apex predator: a carnivore with no predators.
- Decomposer: a microorganism that breaks down dead plant and animal matter.
The numbering always starts at level 1 with plants and algae. Each organism is then numbered by how far along the food chain it is.
- Level 1: plants and algae. They make their own food, so they are the producers.
- Level 2: herbivores. They eat plants or algae, so they are the primary consumers.
- Level 3: carnivores that eat herbivores. These are the secondary consumers.
- Level 4: carnivores that eat other carnivores. These are the tertiary consumers.
A pond example
Here is a food chain from a pond, with each organism's trophic level.
Pond food chain
Algae (level 1, producer) → water fleas (level 2, primary consumer) → minnows (level 3, secondary consumer) → pike (level 4, tertiary consumer)
Pike eat minnows, which are carnivores, so pike are tertiary consumers at level 4. Nothing in this pond hunts the pike, so the pike is an apex predator.
🌿 Not an apex predator
A minnow is a carnivore, but pike hunt it. It has a predator, so it is not an apex predator.
Decomposers
Dead plants and animals do not just pile up. Decomposers break down dead plant and animal matter. They do it in two steps:
- Decomposers secrete enzymes into the environment around them. The enzymes break the dead matter down.
- Small soluble food molecules are made, and these diffuse into the microorganism.
So decomposers feed on dead matter from every level of a food chain, not on one level only.
Pyramids of biomass
A food chain tells you who eats whom, but not how much living material there is at each step. A pyramid of biomass shows the relative amount of biomass in each level of a food chain. Remember, biomass is the mass of living material.
- Trophic level 1 is at the bottom of the pyramid.
- Each higher trophic level is drawn above it, in order.
- The length of each bar shows the amount of biomass in that level.
Most pyramids get narrower towards the top, which is why we call them pyramids. Why this happens is explained in the next lesson, Transfer of Biomass.
Constructing an accurate pyramid
You may be asked to draw a pyramid from a table of data. Follow these steps.
- Put the organisms in food chain order. The producer is level 1.
- Choose one scale for the whole pyramid, such as 1 cm for every 20 g. Use the same scale for every bar.
- Work out the length of each bar by dividing its biomass by the scale.
- Draw level 1 at the bottom. Draw each bar as a rectangle above the one before, with the bars centred.
- Label each bar with the name of the organism, and give the pyramid a title and the units.
Worked example
The table shows the biomass in one square metre of the pond.
Algae 160 g, water fleas 80 g, minnows 40 g, pike 10 g.
Scale: 1 cm = 20 g. Bar lengths: algae 160 ÷ 20 = 8 cm. Water fleas 80 ÷ 20 = 4 cm. Minnows 40 ÷ 20 = 2 cm. Pike 10 ÷ 20 = 0.5 cm.
Draw algae at the bottom (8 cm), then water fleas (4 cm), minnows (2 cm) and pike (0.5 cm) at the top.
📏 Accuracy
Bars must be drawn to scale. If the biomass is half as big, the bar must be half as long. A rough sketch with equal bars does not score the marks.
📝 Check it
Level 1 is at the bottom. The organisms are in the order of the food chain. Each bar is labelled with its organism.
Common mistakes
1. Putting the producer at the top. Level 1 is always at the bottom.
2. Using a different scale for each bar. One scale must be used for the whole pyramid.
3. Calling every carnivore an apex predator. Only a carnivore with no predators is one.
4. Saying decomposers feed on living plants. They break down dead matter.
Exam-style question
A food chain in a woodland is: oak leaves → caterpillars → blue tits → sparrowhawks. The biomass in one square metre is: oak leaves 100 g, caterpillars 40 g, blue tits 10 g, sparrowhawks 2 g.
(a) Give the trophic level of the blue tits. [1 mark]
(b) Which organism is the apex predator in this food chain? Give a reason. [2 marks]
(c) You are drawing a pyramid of biomass using 1 cm for every 10 g. Give the length of the bar for the caterpillars. [1 mark]
(d) Describe how decomposers break down a dead blue tit. [3 marks]
Model answer
(a) Level 3 (1). Blue tits eat caterpillars, which are level 2.
(b) The sparrowhawk (1) because it is a carnivore (1) and nothing in this food chain eats it, so it has no predators here (1).
(c) 40 ÷ 10 = 4 cm (1).
(d) Decomposers secrete enzymes (1) into the environment, which break down the dead matter (1). The small soluble food molecules then diffuse into the microorganism (1).
Exam tip
In part (d) the word secrete and the word diffuse each earn a mark, so write both. Say that the enzymes are released outside the decomposer first.