Home › Learning Hub › IGCSE Biology › 19a Energy, food chains, pyramids
Topic 19 · 19.1–19.2

Energy flow, food chains and ecological pyramids

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • State that the Sun is the principal energy source and describe energy flow through organisms to the environment.
  • Construct and interpret food chains and webs; define producer, consumer, herbivore, carnivore, decomposer and trophic level.
  • Describe human impacts through overharvesting and introduced species; draw and interpret pyramids of numbers and biomass.
  • Draw pyramids of energy; explain inefficient energy transfer, short food chains and why eating crops is more efficient EXTENDED.

📚The biology

Energy flow

The Sun is the principal source of energy input to biological systems. Light energy is converted by producers into chemical energy in organic molecules; this chemical energy passes along food chains, and is eventually transferred to the environment as heat. Energy flows through an ecosystem — it is not recycled — while nutrients cycle (19b).

Food chains and food webs

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. A food web is a network of interconnected food chains.

A food web from a rice field. Rice plants are eaten by grasshoppers, rats and caterpillars. Grasshoppers are eaten by frogs and sparrows; caterpillars by sparrows; frogs and rats by snakes; rats, snakes and sparrows by eagles.
One chain in this web: rice plants → grasshopper → frog → snake → eagle.
producer: makes its own organic nutrients, usually by photosynthesis (rice plants)
consumer: gets its energy by feeding on other organisms — primary, secondary, tertiary and quaternary consumers by position in the chain
herbivore: an animal that gets its energy by eating plants (grasshopper)
carnivore: an animal that gets its energy by eating other animals (snake)
decomposer: gets its energy from dead or waste organic material (bacteria and fungi)
trophic level: the position of an organism in a food chain, food web or pyramid

Human impacts. Overharvesting a food species (such as overfishing) reduces the food for the next level, whose numbers fall, while the species it ate may increase. An introduced foreign species may have no predators and outcompete or eat native species (for example cane toads in Australia).

Ecological pyramids

Three pyramids for the same chain. Pyramid of numbers: one tree at the bottom, a very wide bar of 5000 insects, then 40 birds, so it is not pyramid-shaped. Pyramid of biomass: a wide bar for the tree, narrower for insects, narrowest for birds. Pyramid of energy: producers, primary, secondary and tertiary consumers, each much narrower than the one below.
Each bar is a trophic level. Numbers can give odd shapes (one big tree feeds thousands of insects); biomass and energy usually cannot.

A pyramid of biomass is often more useful than a pyramid of numbers because it takes account of the size of the organisms. EXTENDED A pyramid of energy is better still: it shows the energy actually transferred over time, is always pyramid-shaped, and allows fair comparisons between ecosystems.

Why energy transfer is inefficient EXTENDED

Energy flows from the Sun to producers (10 000 kilojoules), primary consumers (1000), secondary consumers (100) and tertiary consumers (10). At each level about 90 percent is lost as heat from respiration, movement, excretion, and parts not eaten or not digested.
EXTENDED About 10% passes on at each step, so the energy left falls tenfold per level. (Illustrative figures.)
  • EXTENDED Energy is lost at each level: not all of an organism is eaten (roots, bones); not all that is eaten is digested (lost in faeces); much is used in respiration and lost as heat; some is lost in excretion.
  • Food chains usually have fewer than five trophic levels because too little energy is left to support another level.
  • It is more energy-efficient for humans to eat crop plants than livestock fed on crops: eating the crop directly removes one trophic level and its roughly 90% loss, so the same land feeds many more people.

✏️Worked example

Use the food web above. (a) Name one secondary consumer and one herbivore. [2] (b) Farmers kill most of the snakes. Suggest two effects on the web. [2] (c) EXTENDED Producers in a field capture 25 000 kJ; rats receive 2250 kJ and eagles 180 kJ of it. Calculate the efficiency of transfer from producers to rats. [2]

(a) Secondary consumer: frog, snake or sparrow (any that eats a primary consumer). Herbivore: grasshopper, rat or caterpillar.

(b) Rats and frogs may increase, because fewer are eaten; more rats may eat more rice; eagles may eat more rats and sparrows instead of snakes.

(c) \( \tfrac{2250}{25\,000} \times 100 = 9\% \).

Check it. About 10% per step is typical, so 9% is sensible; an answer above 100% means the fraction is upside down.
Arrows show energy, not “eats”. The arrow points to the eater: rice → rat, never rat → rice.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) What is the principal source of energy for most food chains? A: the soil. B: the Sun. C: producers. D: decomposers.
B.
2. (Theory.) Define a decomposer and give one example. [2]
An organism that gets its energy from dead or waste organic material. E.g. bacteria or fungi (mushrooms).
3. (Theory.) In the chain seaweed → sea urchin → sea otter → shark, give the trophic level of the sea otter. [1]
Secondary consumer (third trophic level).
4. (Theory.) Explain why a pyramid of numbers for oak tree → caterpillars → birds is not pyramid-shaped. [2]
One oak tree is very large and supports thousands of caterpillars, so the producer bar is narrower than the primary consumer bar.
5. (Theory.) Give one advantage of a pyramid of biomass over a pyramid of numbers. [1]
It takes account of the size (mass) of the organisms, so a single large producer does not give a misleading shape.
6. (Theory.) EXTENDED Give three ways energy is lost between one trophic level and the next. [3]
Any three: heat from respiration; not all of the organism is eaten; not all is digested (lost in faeces); lost in excretion (urine).
7. (Theory.) EXTENDED Explain why food chains rarely have more than five trophic levels. [2]
About 90% of the energy is lost at each level, so by the fifth level too little energy remains to support a population.
8. (Theory.) EXTENDED Explain why more people can be fed from a field of soya eaten directly than from the same field used to feed cattle. [3]
Feeding cattle adds a trophic level; about 90% of the energy in the soya is lost by the cattle (respiration, heat, faeces), so far less energy reaches humans.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • HHMI BioInteractive — food web and trophic cascade animations
  • BBC Bitesize — food chains, webs and pyramids