IGCSE · Practice questions · Food chains, webs and pyramids

Arrows point to the eater.

Six original Cambridge-style questions on constructing and interpreting food chains and webs, ecological pyramids, the effects of overharvesting and introduced species, and the inefficiency of energy transfer.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Level by level.

Arrows show energy flow: from the organism eaten to the organism that eats it.
Trophic levels: producer, primary, secondary, tertiary, quaternary consumer.
Biomass beats numbers because it allows for size; (Extended) energy beats both because it allows for time.
Extended: about 10% passes on; the rest is lost as heat from respiration, movement, faeces, urine and uneaten parts.

Recall

[4 marks]

Define each term: (a) producer, (b) consumer, (c) herbivore, (d) decomposer.

(a) An organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. ✓
(b) An organism that gets its energy by feeding on other organisms. ✓
(c) An animal that gets its energy by eating plants. ✓
(d) An organism that gets its energy from dead or waste organic material. ✓

Analysis

[4 marks]

In a pond: algae are eaten by water fleas and by snails; water fleas are eaten by small fish; snails are eaten by ducks; small fish are eaten by pike and by ducks. (a) Draw the food web. (b) Write the longest food chain. (c) Name the trophic level of the duck in each chain it belongs to.

(a) algae → water flea, algae → snail, water flea → small fish, snail → duck, small fish → pike, small fish → duck (arrows towards the eater). ✓
(b) algae → water flea → small fish → pike (or → duck): four trophic levels. ✓
(c) In algae → snail → duck the duck is a secondary consumer. ✓
In algae → water flea → small fish → duck it is a tertiary consumer. ✓

Explain

[4 marks]

A fish species that eats water fleas is introduced into the pond in the previous question. It has no predators there. Predict and explain the effects on the water fleas, the algae and the pike.

Water fleas decrease, because they are eaten by the new fish as well as the small fish. ✓
Algae may increase, because fewer water fleas are eating them. ✓
Small fish have less food (more competition for water fleas), so their numbers may fall. ✓
Pike, which eat small fish, may then decrease too. ✓

Data

[3 marks]

A rose bush supports 2000 aphids, which are eaten by 40 ladybirds. Sketch the pyramid of numbers and the pyramid of biomass for this chain and explain why they have different shapes.

Pyramid of numbers: a narrow bar for 1 rose bush, a wide bar for 2000 aphids and a medium bar for 40 ladybirds (not a pyramid shape). ✓
Pyramid of biomass: a wide bar for the rose bush, narrower for aphids and narrowest for ladybirds (a true pyramid). ✓
Numbers ignore size: one large bush supports many small aphids; biomass takes the mass of each level into account. ✓

Calculation

[3 marks]

Extended. In a grassland, producers fix 20 000 kJ per m² per year, primary consumers receive 2000 kJ and secondary consumers 180 kJ. (a) Calculate the percentage efficiency of energy transfer from primary to secondary consumers. (b) Give two ways energy is lost between these levels.

(a) 180 ÷ 2000 × 100 = 9%. ✓
(b) Heat released by respiration (including energy used for movement) in the primary consumers. ✓
Energy in faeces and urine, or in parts such as bones and fur that are not eaten (any two). ✓

Explain

[4 marks]

Extended. Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants, and give one advantage of a pyramid of energy for showing this.

Feeding crops to livestock adds an extra trophic level to the food chain. ✓
At each transfer only about 10% of the energy becomes biomass in the next level; most is lost as heat from respiration, movement, faeces and urine. ✓
So eating the crop directly gives humans about ten times as much energy from the same area of land. ✓
A pyramid of energy shows energy flow per m² per year, is always pyramid shaped and allows a fair comparison of the levels. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.