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Biology · 19 Organisms and their environment

19.2 Food chains and food webs

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Before you start

  • From 19.1: the Sun is the principal source of energy input, and energy is transferred to the environment as heat released in respiration.
  • Photosynthesis makes glucose using light energy, carbon dioxide and water.
  • Respiration releases energy from glucose in every living cell, all the time.
  • That food is digested, and that some of what an animal eats is never absorbed and leaves in the faeces.
  • Working out a percentage of a quantity with a calculator.
Syllabus objectives covered (19)
  • Describe a food chain as showing the transfer of energy from one organism to the next, beginning with a producer
  • Construct and interpret simple food chains
  • Describe a food web as a network of interconnected food chains and interpret food webs
  • Describe a producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
  • Describe a consumer as an organism that gets its energy by feeding on other organisms
  • State that consumers may be classed as primary, secondary, tertiary and quaternary according to their position in a food chain
  • Describe a herbivore as an animal that gets its energy by eating plants
  • Describe a carnivore as an animal that gets its energy by eating other animals
  • Describe a decomposer as an organism that gets its energy from dead or waste organic material
  • Use food chains and food webs to describe the impact humans have through overharvesting of food species and through introducing foreign species to a habitat
  • Draw, describe and interpret pyramids of numbers and pyramids of biomass
  • Discuss the advantages of using a pyramid of biomass rather than a pyramid of numbers to represent a food chain
  • Describe a trophic level as the position of an organism in a food chain, food web or ecological pyramid
  • Identify the following as the trophic levels in food webs, food chains and ecological pyramids: producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers
  • Draw, describe and interpret pyramids of energy
  • Discuss the advantages of using a pyramid of energy rather than pyramids of numbers or biomass to represent a food chain
  • Explain why the transfer of energy from one trophic level to another is often not efficient
  • Explain, in terms of energy loss, why food chains usually have fewer than five trophic levels
  • Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants

Part 1 of 8

A food chain is a picture of energy moving

  1. A food chain shows the transfer of energy from one organism to the next. That is the definition the syllabus asks for, and it is worth noticing that it is a definition about energy, not about eating. Eating is just the mechanism.
  2. Every food chain begins with a producer. It has to: the producer is the only organism that can bring new energy into the chain, by capturing light in photosynthesis. A chain that starts with an animal has nowhere for its energy to have come from.
  3. Here is a complete one, written the way you should write it: grass → rabbit → fox.
  4. Now the single most expensive detail in this whole topic. The arrow means "is eaten by", so it points from the organism that is eaten towards the organism that eats it. Grass is eaten by the rabbit, so the arrow goes from grass to rabbit.
  5. The reason the arrow points that way is that it is showing you the direction the energy travels. The energy is in the grass, and feeding moves it into the rabbit. Draw the arrow backwards and you have drawn the rabbit's energy flowing into the grass, which is why it costs the mark every time.
    correctgrassrabbitfoxenergy flows this waywronggrassrabbitfoxthis says the fox is eaten firstthe arrow means "is eaten by"it points the way the energy flows — never the other way
    Read a chain aloud with 'is eaten by' in place of each arrow, and a reversed one gives itself away.
  6. If you are ever unsure which way round to draw it, say the chain out loud with "is eaten by" in place of each arrow. "Grass is eaten by rabbit is eaten by fox" makes sense; the reverse does not.
  7. To construct a food chain from a description, find the producer first and put it on the left. Then ask repeatedly: what eats that? Add each organism in turn and only then put the arrows in, all pointing the same way.
  8. Do not add extra organisms that are not in the information you were given, and do not put the Sun in the chain. The Sun is the source of the energy, but it is not an organism and it is not a trophic level.

Part 2 of 8

The job titles: producer, consumer, herbivore, carnivore, decomposer

  1. These five words have precise definitions and each is worth a mark of its own. Learn them as sentences, not as vague ideas, because a loose version of a definition earns nothing.
  2. A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. Note what the definition does not say: it does not say "a plant". Algae and some bacteria are producers too, so answer with the definition rather than the example.
  3. A consumer is an organism that gets its energy by feeding on other organisms. That is the whole definition — the word covers every animal in the chain, whatever it eats.
  4. A herbivore is an animal that gets its energy by eating plants. A carnivore is an animal that gets its energy by eating other animals. These describe diet.
  5. Keep the two labelling systems apart in your head. Producer, consumer and decomposer describe where an organism sits in the flow of energy. Herbivore and carnivore describe what an animal eats. A rabbit is a primary consumer and a herbivore — both labels, from two different systems, and a question may ask for either.
  6. A decomposer is an organism that gets its energy from dead or waste organic material. Bacteria and fungi are the examples to have ready.
  7. Decomposers matter because most of the material in a food chain is never eaten by the next level — plants die uneaten, animals die of old age, and waste is produced constantly. Decomposers get the energy out of all of it, and in doing so they release the nutrients back into the soil, which is where 19.3 picks the story up.

Part 3 of 8

Counting positions: trophic levels

  1. A trophic level is the position of an organism in a food chain, food web or ecological pyramid. It is a position, not a type of organism — that phrasing is the definition mark.
  2. The levels, in order, are: producers, then primary consumers, then secondary consumers, then tertiary consumers, then quaternary consumers. Consumers are classed as primary, secondary, tertiary or quaternary according to their position in the food chain.
  3. Apply it to grass → rabbit → fox. Grass is the producer. The rabbit eats the producer, so it is the primary consumer. The fox eats the primary consumer, so it is the secondary consumer.
  4. The off-by-one trap: the rabbit is the primary consumer but it sits at the second trophic level, because the producer occupies the first. Primary consumer and first trophic level are not the same thing. If you name the organisms rather than count numbers, you cannot go wrong.
  5. The counting rule is simply how many feeding steps the organism is away from the producer. One step, primary consumer. Two steps, secondary consumer. Three steps, tertiary. Four steps, quaternary.
  6. In a food web an organism can occupy more than one trophic level, because it eats at more than one level. If a question asks for the trophic level of such an organism, name the chain you are using: "in the chain lettuce → slug → thrush, the thrush is a secondary consumer".
  7. Decomposers are usually shown separately rather than being given a number, because they feed on dead material from every level at once.

Part 4 of 8

Food webs: many chains, joined up

  1. A food web is a network of interconnected food chains. It exists because real animals do not eat one thing only, and are not eaten by one thing only.
  2. The distinction examiners test is exactly this: a food chain shows one single route for energy, a food web shows many routes joined together, with organisms appearing in more than one chain. If you are asked why a diagram is a web and not a chain, say that it contains several interconnected chains, and that at least one organism appears in more than one of them.
  3. Here is one, described in words first, because that is how a question will usually hand it to you. Lettuce is eaten by slugs and by caterpillars. Slugs are eaten by hedgehogs and by thrushes. Caterpillars are eaten by thrushes. Thrushes are eaten by sparrowhawks.
    level 4tertiary consumerlevel 3secondary consumerlevel 2primary consumerlevel 1producerlettuceslugscaterpillarshedgehogsthrushessparrowhawkseach arrow means "is eaten by" — it points the way the energy flows
    Several chains sharing organisms — which is exactly what makes it a web and not a chain.
  4. To pull a single food chain out of a web, pick one starting producer and follow one route through, ignoring the branches: lettuce → slug → thrush → sparrowhawk. Every chain you extract must still start at a producer and have all its arrows the same way round.
  5. The standard interpreting question is: what happens to organism B if organism A is removed? Answer it in three moves — what A was eaten by, what A ate, and what alternative food now matters.
  6. Work an example. Remove the slugs. The thrushes have lost one of their two foods, so they eat more caterpillars, and the caterpillar population falls. The hedgehogs have lost their only food shown here, so their numbers fall.
  7. A knock-on effect can run in either direction, so justify whichever you choose. If you argue instead that losing slugs makes the thrush population fall, and that fewer thrushes then eat fewer caterpillars so caterpillars increase, that is also creditworthy — as long as the reasoning is stated and consistent. What loses marks is a bare "they would decrease" with no route.

Part 5 of 8

What humans do to food webs

  1. The syllabus asks you to use food chains and webs to describe two specific human impacts. Both are answered the same way: find the organism affected, then follow the arrows outwards in both directions.
  2. Overharvesting means removing a species faster than it can reproduce and replace itself, as in overfishing. The population of that species falls.
  3. Follow the arrows forwards from it. The predators that fed on it lose a food source, so they must eat alternative prey or their numbers fall too.
  4. Follow the arrows backwards from it. Whatever it fed on is now eaten less, so that population increases. Naming both directions is usually worth two of the marks.
  5. Introducing a foreign species means bringing in a species that did not live in that habitat. It may have no natural predators there, so its population grows rapidly and unchecked.
  6. Its effects on the web are then: it competes with native species for food or space; it may prey on native species that have no defences against it; and it may introduce disease. Native populations fall as a result, and any species that depended on them is affected in turn.
  7. Answer these questions with named organisms from the web you have been given, not in general terms. "The population of X falls because Y now eats it" scores; "the ecosystem is damaged" does not.

Part 6 of 8

Pyramids of numbers and pyramids of biomass

  1. An ecological pyramid is a set of horizontal bars stacked up, one per trophic level, with the producer at the bottom and each level above it in order. The width of each bar is drawn in proportion to the quantity it represents, and the bars are drawn symmetrically about a central vertical line.
  2. Two rules when you draw one: label every bar with the name of the organism and the trophic level, and keep the widths to a sensible scale. Marks are given for the correct order of levels and for the relative widths, so a neat freehand sketch is enough.
  3. A pyramid of numbers shows the number of organisms at each trophic level. It is easy to collect — you just count — but it produces some strange shapes.
  4. Take one oak tree supporting 5000 caterpillars, eaten by 200 blue tits, eaten by 2 sparrowhawks. The bottom bar has a width of one. The pyramid has a tiny base and a huge second bar, so it is not pyramid-shaped at all.
    numbers of organisms in an oak woodbar widths show the order of size, not the exact ratiosparrowhawks2blue tits200caterpillars5000oak tree1one oak feeds thousands of caterpillars, so the base is the narrowest bar
    Not a pyramid at all — counting takes no account of the size of an oak tree.
  5. The reason is that a number takes no account of size. One oak tree and one caterpillar count the same, even though the tree could feed caterpillars for years.
  6. A pyramid of biomass fixes this by showing the mass of the organisms at each trophic level, rather than how many there are. Biomass means the mass of living material, usually measured as dry mass per unit area.
  7. Redraw the oak wood as biomass and it becomes a proper pyramid: the mass of one oak tree is far greater than the total mass of all the caterpillars, which in turn is greater than the mass of the blue tits.
    biomass at each level, g per m²bar widths show the order of size, not the exact ratiosparrowhawks3blue tits30caterpillars300oak tree5000weighing the level instead of counting it puts the huge oak at the base
    The same wood and the same organisms — only the quantity being measured has changed.
  8. So the advantage to state for core marks is that a pyramid of biomass takes the size of the organisms into account, so it is nearly always a true pyramid shape, and it lets you compare trophic levels fairly.
  9. Biomass has its own drawbacks, which are worth knowing because the next section builds on them. Measuring dry mass means killing the organisms, and a pyramid of biomass is only a snapshot of one moment — it says nothing about how quickly the organisms are replaced.

Part 7 of 8

Pyramids of energy, and why so little gets through

  1. A pyramid of energy shows the energy transferred to each trophic level over a period of time, in a given area — the units are kilojoules per square metre per year. Because it measures a flow rather than a snapshot, it is always a true pyramid: no level can receive more energy than the level below it passed on.
    energy at each level, kJ per m² per yearbar widths show the order of size, not the exact ratiosparrowhawks9blue tits120caterpillars1500oak leaves20 0007.5% transferred8% transferred7.5% transferredroughly the same small fraction gets through at every step
    No level can receive more than the level below it passed on, so this one can never come out inverted.
  2. That is the advantage to state for supplement marks. A pyramid of energy is always pyramid-shaped, it accounts for the rate at which organisms are replaced rather than freezing one moment, and it lets you compare how efficiently energy is transferred between levels.
  3. Now the central Extended idea: only a small fraction of the energy at one trophic level ends up in the next. You must be able to explain why, and vague answers are worth nothing here, so learn the losses as a list of separate, nameable routes.
  4. Route one: not all of the organism is eaten. A herbivore leaves roots, woody stems and bark; a carnivore leaves bones, fur and feathers. The energy in those parts is never transferred to the next level, though decomposers get it.
  5. Route two: not everything eaten is digested and absorbed. The energy in undigested material passes out of the body in faeces.
  6. Route three: energy is lost in excretory products such as urine, formed from the excess amino acids the animal cannot store.
  7. Route four, and the biggest one: most of the energy that is absorbed is released in respiration and used for movement, active transport and — in mammals and birds — maintaining a constant body temperature. It is transferred to the surroundings as heat.
  8. Only what is left over is used for growth, and only that becomes new body tissue for the next trophic level to eat. That is the sentence that ties the list together.
    energy in the food eaten1000 kJit splits four waysfaeces 350 kJ (35%)the parts not eaten or not digestedurine 50 kJ (5%)excretory productsrespiration 520 kJ (52%)released to the surroundings as heatgrowth 80 kJ (8%)the only part the next level can eatonly a small fraction becomes growth — that is what the next level can eat
    Name the routes and where each one goes. That is what an inefficiency answer is marked on.
  9. So when a question asks why the transfer is inefficient, give at least two named routes with their destinations. "Energy is lost" is worth nothing; "energy is transferred to the surroundings as heat from respiration, and energy in undigested material is lost in faeces" is worth two marks.
  10. To compare two levels numerically you use percentage energy transfer. There is no formula sheet in Biology 0610 — nothing is printed for you in any paper — so you must recall this relation and be able to use it in either direction.
    percentage energy transfer=energy transferred to the next trophic levelenergy in the trophic level before it×100
  11. Always divide by the energy at the earlier level, because you are asking what fraction of what was there got through. If your answer comes out above 100 you have divided the wrong way round, since a level cannot gain more energy than it received.
  12. You may have met a rough figure of about 10% transferred between levels. Treat it as a rough guide only. The paper gives you the actual energy values, and the marks are for using those numbers, so never quote a percentage you were not given.

Part 8 of 8

Two consequences: short chains, and eating the crop yourself

  1. If only a small percentage of energy gets through each transfer, then the energy available shrinks at every step. Follow it: a large amount at the producer, a much smaller amount at the primary consumer, smaller still at the secondary consumer.
  2. After four or five levels there is so little energy left that it cannot support another trophic level — there is not enough to sustain a population of a further consumer. That is why food chains usually have fewer than five trophic levels.
  3. Say it in terms of energy, because the objective demands it. "The chain is too long" is not an explanation; "too much energy is transferred to the surroundings as heat at each stage, so too little remains to support another level" is.
  4. This is also why quaternary consumers are rare. You still have to be able to name one — it is the fourth consumer along, at the fifth trophic level — but the word "usually" in "usually fewer than five trophic levels" is doing real work. Long chains exist; they are just uncommon, because of the energy losses.
  5. The second consequence is about feeding people. Compare two ways of using one field: grow a crop and let people eat it, or grow the crop, feed it to cattle, and let people eat the cattle.
  6. In the first, humans are primary consumers and the chain has two levels. In the second, humans are secondary consumers and the chain has three, so there is an extra transfer.
  7. At that extra transfer most of the crop's energy never reaches the humans: the cattle release it in respiration as heat, and lose more in faeces, in urine and in the parts of the plant they do not eat.
  8. So eating crop plants directly is more energy efficient than eating livestock fed on those crops: fewer transfers means less energy transferred to the surroundings, so more of the crop's energy is available to people, and a given area of land can feed more of them.

Worked examples

Example 1. In a pond, algae are eaten by water fleas. Water fleas are eaten by small fish, and small fish are eaten by herons. Construct a food chain for these organisms and identify the trophic level of each.

Show the working
  1. Find the producer first, because a food chain must begin with one. The algae make their own organic nutrients by photosynthesis, so they go on the left.
  2. Now ask "what eats that?" repeatedly and write the organisms in that order: algae, water fleas, small fish, herons.
  3. Put the arrows in last, all pointing the same way, from the organism eaten towards the organism that eats it: algae → water fleas → small fish → herons.
  4. Check the arrows by reading it aloud as "is eaten by". Algae is eaten by water fleas is eaten by small fish is eaten by herons — correct.
  5. Now label the positions, counting feeding steps from the producer. Water fleas are one step away, so they are the primary consumer; small fish are two steps, so secondary; herons are three steps, so tertiary.
  6. Do not write that the water fleas are the first trophic level. The algae hold that position, so the water fleas are at the second trophic level even though they are the primary consumer.

Answer: algae → water fleas → small fish → herons. Algae: producer. Water fleas: primary consumer. Small fish: secondary consumer. Herons: tertiary consumer.

Example 2. In one year, the caterpillars in a wood take in 1500 kJ per square metre of energy from the leaves they eat. The blue tits that feed on them gain 120 kJ per square metre in the same year. Calculate the percentage of the caterpillars' energy that is transferred to the blue tits, and explain why the figure is so low.

Show the working
  1. Decide which number goes on the bottom of the fraction. You are asked what fraction of the caterpillars' energy got through, so the caterpillars' figure is the one you divide by.
  2. Write the relation out before substituting. Nothing is printed for you in a Biology paper, so this has to come from memory.
    percentage energy transfer=energy in the blue titsenergy in the caterpillars×100
  3. Substitute the two values.
    percentage energy transfer=1201500×100
  4. Work it out: 120÷1500=0.08, and 0.08×100=8, so 8%. Check it is below 100, which it must be.
  5. Now the explanation, and this is where the marks are. Name the routes and where the energy goes, one at a time — never write the bare word "lost".
  6. Not all of each caterpillar is eaten or digested, so energy passes out of the blue tits in faeces, and further energy leaves in excretory products such as urine.
  7. Most of the energy the blue tits do absorb is released in respiration and used for movement and for keeping their body temperature constant, and is transferred to the surroundings as heat.
  8. Finish with what that leaves: only the small remainder is used for growth, and only that becomes body tissue available to the next trophic level.

Answer: 8%. The transfer is low because energy in the parts not eaten and in undigested material passes out in faeces, some is lost in excretory products such as urine, and most of the absorbed energy is released in respiration and transferred to the surroundings as heat, especially in maintaining body temperature. Only the remainder is used for growth and is available to the blue tits' predators.

Example 3. In a lake food web, water plants are eaten by snails and by mayfly larvae. Snails and mayfly larvae are both eaten by native perch. Perch are eaten by otters. A foreign fish species that eats mayfly larvae and snails is introduced to the lake, and it has no predators there. Suggest the effects of this introduction on the food web.

Show the working
  1. Start by placing the new species in the web. It eats mayfly larvae and snails, which are exactly what the native perch eat, so it occupies the same trophic level as the perch — it is a secondary consumer and a competitor.
  2. Say what happens to the introduced species itself, and why. With no predators in the lake, nothing eats it, so its population grows rapidly and is not controlled.
  3. Follow the arrows backwards, to its food. Snails and mayfly larvae are now eaten by two predators instead of one, so their populations fall.
  4. Follow the competition sideways. The perch now compete with the introduced fish for the same prey, and with less food available the perch population falls.
  5. Follow the arrows forwards from the perch. Otters feed on perch, so with fewer perch the otters have less food and their numbers fall too — unless they can eat the introduced fish instead.
  6. Notice how every statement names organisms and a route. That is what "use food chains and food webs to describe the impact" is asking for; a general sentence about damage to the ecosystem earns nothing.

Answer: The introduced fish has no predators in the lake, so its population grows unchecked. It competes with the native perch for snails and mayfly larvae, so those prey populations fall and the perch have less food, reducing the perch population. Fewer perch means less food for otters, so otter numbers fall as well, unless they begin to feed on the introduced fish.

Example 4. A food chain is oak tree → caterpillars → blue tits → sparrowhawks. In one wood there is 1 oak tree, 5000 caterpillars, 200 blue tits and 2 sparrowhawks. Describe the shape of the pyramid of numbers for this food chain, and explain why a pyramid of biomass would be a better representation.

Show the working
  1. Set the bars out in order first, producer at the bottom: oak tree, then caterpillars, then blue tits, then sparrowhawks, each bar drawn to a width proportional to its number.
  2. Compare the bottom two widths. The producer bar represents one organism and the caterpillar bar represents 5000, so the base is extremely narrow and the bar above it is very wide.
  3. Describe the resulting shape honestly: it is not a true pyramid. It is inverted at the bottom, then narrows normally from the caterpillars upwards through the blue tits to the sparrowhawks.
  4. Explain the cause, because that is the mark: a pyramid of numbers counts organisms without regard to their size, and one oak tree is enormous compared with one caterpillar.
  5. Now switch to biomass. Biomass is the mass of living material at each level, so the single oak tree contributes a very large mass — far greater than the total mass of the 5000 caterpillars.
  6. State the resulting shape and the advantage together: the biomass pyramid comes out as a true pyramid, because it takes the size of the organisms into account, and that allows the trophic levels to be compared fairly.

Answer: The pyramid of numbers has a very narrow base, because there is only one oak tree, and a very wide bar above it for the 5000 caterpillars, so it is inverted at the bottom rather than being a true pyramid; above the caterpillars it narrows to the blue tits and the sparrowhawks. A pyramid of biomass is better because it shows the mass of living material at each level rather than the number of organisms, so the large mass of the single oak tree gives the widest bar at the base and the pyramid takes the correct shape, allowing fair comparison between the levels.

Where people go wrong

Mistake: Drawing the food-chain arrows backwards, from the predator to its prey.

Instead: The arrow means "is eaten by" and points from the organism that is eaten to the organism that eats it: grass → rabbit → fox.

The arrow shows the direction in which energy is transferred, and the energy travels from the food into the feeder. Reversed arrows are marked wrong even when the organisms are in the right order, and it is the most common lost mark in the whole topic.

Mistake: Calling a branching diagram a food chain, or writing out a whole web when asked for a chain.

Instead: A food chain is one single route with one organism at each level; a food web is a network of interconnected food chains.

If you are asked to give a food chain from a web, you must pick one route and follow it through — extra branches make the answer a web and it will not be credited as a chain.

Mistake: Writing "energy is lost between the levels" as an explanation of inefficient transfer.

Instead: Name the route and the destination: energy is transferred to the surroundings as heat released in respiration, energy in undigested material passes out in faeces, energy is lost in excretory products such as urine, and not all of the organism is eaten.

The examiner is marking for specific, named losses. "Lost" says nothing about where the energy went, so it earns nothing; each named route earns its own mark.

Mistake: Assuming a pyramid of numbers must always be pyramid-shaped.

Instead: A pyramid of numbers can be a strange shape — a single large producer such as one tree gives a very narrow base.

Numbers take no account of the size of the organisms. Recognising when a pyramid is not pyramid-shaped, and saying why, is exactly what the interpretation questions are testing.

Mistake: Saying that the primary consumer is at the first trophic level.

Instead: The producer occupies the first trophic level, so the primary consumer sits at the second.

The counting starts at the producer, since that is where energy enters. Naming the organisms as producer, primary consumer and secondary consumer avoids the off-by-one error entirely.

Check yourself

  1. Give the definitions of a producer and a consumer.
    AnswerA producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. A consumer is an organism that gets its energy by feeding on other organisms.
  2. What is the difference between a herbivore and a carnivore, and why is "herbivore" not the same label as "primary consumer"?
    AnswerA herbivore is an animal that gets its energy by eating plants; a carnivore is an animal that gets its energy by eating other animals. Herbivore and carnivore describe an animal's diet, while primary and secondary consumer describe its position in the food chain — one organism carries both kinds of label.
  3. Describe what a decomposer is, and name two types of organism that are decomposers.
    AnswerA decomposer is an organism that gets its energy from dead or waste organic material. Bacteria and fungi.
  4. In the food chain wheat → aphid → ladybird → blackbird → hawk, identify the trophic level of each organism, and state what a trophic level is.
    AnswerWheat is the producer, the aphid the primary consumer, the ladybird the secondary consumer, the blackbird the tertiary consumer and the hawk the quaternary consumer. A trophic level is the position of an organism in a food chain, food web or ecological pyramid.
  5. In one sentence, what makes a diagram a food web rather than a food chain?
    AnswerA food web is a network of interconnected food chains, with organisms appearing in more than one chain, whereas a food chain shows only a single route of energy transfer.
  6. A fish species is overharvested until very few remain. Give one effect on the animals that eat it and one effect on the animals it used to eat.
    AnswerThe animals that ate it have lost a food source, so they must eat alternative prey or their populations fall. The organisms it fed on are eaten less, so their populations increase.
  7. State one advantage of a pyramid of biomass over a pyramid of numbers, and one advantage of a pyramid of energy over a pyramid of biomass.
    AnswerA pyramid of biomass takes the size of the organisms into account rather than just counting them, so it is nearly always a true pyramid shape and the levels can be compared fairly. A pyramid of energy shows the energy transferred over a period of time rather than a snapshot of mass, so it is always a true pyramid and it shows how efficiently energy is transferred between levels.
  8. Explain, in terms of energy, why food chains rarely have more than four or five trophic levels, and why more people can be fed from a field of wheat than from cattle fed on that wheat.
    AnswerOnly a small percentage of energy is transferred at each level, because most is released in respiration and transferred to the surroundings as heat and more is lost in faeces and urine, so after four or five levels too little energy remains to support another trophic level. Eating the wheat directly removes a trophic level, so there is one fewer transfer and much more of the crop's energy is available to people.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. In a garden, lettuce plants are eaten by slugs and by caterpillars. Slugs are eaten by hedgehogs and by thrushes. Caterpillars are also eaten by thrushes. Thrushes are eaten by sparrowhawks.

(a) Give a food chain from this garden that contains four organisms. 2 marks No calculator

Mark scheme
  • B1 correct sequence of four organisms beginning with the producer, e.g. lettuce, slug, thrush, sparrowhawk or lettuce, caterpillar, thrush, sparrowhawk
  • B1 all arrows pointing from the organism eaten towards the organism that eats it (a reversed arrow scores zero for this mark)

(b) Identify the trophic level of the thrush in this garden. 1 mark No calculator

Mark scheme
  • B1 secondary consumer (accept third trophic level)

(c) Explain why the feeding relationships in this garden are described as a food web rather than as a food chain. 2 marks No calculator

Mark scheme
  • B1 there are several food chains present and they are interconnected
  • B1 at least one organism appears in more than one chain, e.g. the thrush eats both slugs and caterpillars, whereas a food chain shows only one route, ORA

(d) Suggest what would happen to the number of caterpillars in this garden if all the slugs were removed. 3 marks No calculator

Mark scheme
  • B1 the thrushes have lost one of their two food sources / have less alternative prey
  • B1 so the thrushes eat more caterpillars
  • B1 so the caterpillar population falls; alternatively credit a consistent opposite route — less food overall for thrushes, so the thrush population falls and fewer caterpillars are eaten, so caterpillar numbers rise, ORA

Question 2. A field of grass is grazed by cattle. In one year the grass transfers 12 000 kJ per square metre of energy into its organic nutrients, and the cattle gain 1080 kJ per square metre.

(a) Calculate the percentage of the energy in the grass that is transferred to the cattle. 2 marks

Mark scheme
  • C1 1080 divided by 12 000, multiplied by 100
  • A1 9%

(b) Explain why the percentage of energy transferred from the grass to the cattle is so low. 3 marks No calculator

Mark scheme
  • B1 energy is released in respiration and transferred to the surroundings as heat, e.g. in movement or in maintaining body temperature
  • B1 energy in undigested material passes out of the cattle in faeces
  • B1 energy is lost in excretory products such as urine, or not all of the grass is eaten (roots remain), so its energy is not transferred ('energy is lost' unqualified is insufficient)

(c) Explain why more people could be fed from this field if it were used to grow a crop for people to eat, rather than to raise cattle. 2 marks No calculator

Mark scheme
  • B1 eating the crop directly removes a trophic level, so humans are primary rather than secondary consumers and there is one fewer energy transfer
  • B1 only a small percentage of the energy passes to the cattle, so more energy from the same area is available to humans

Question 3. In a wood, one oak tree supports 5000 caterpillars. The caterpillars are eaten by 200 blue tits, which are eaten by 2 sparrowhawks.

(a) Sketch the pyramid of numbers for this food chain and label each bar. 2 marks No calculator

Mark scheme
  • B1 four horizontal bars stacked in the correct order, oak tree at the bottom then caterpillars, blue tits and sparrowhawks, each labelled
  • B1 the oak tree bar drawn much narrower than the caterpillar bar above it, so the shape is not a true pyramid

(b) Explain why a pyramid of biomass for this food chain would be a true pyramid shape. 2 marks No calculator

Mark scheme
  • B1 a pyramid of biomass shows the mass of living material at each level rather than the number of organisms
  • B1 the mass of the single oak tree is far greater than the total mass of the caterpillars, so the producer bar is the widest

(c) State one advantage of using a pyramid of energy rather than a pyramid of biomass to represent this food chain. 1 mark No calculator

Mark scheme
  • B1 it shows the energy transferred per unit area per unit time rather than a snapshot of mass, so it is always a true pyramid / it allows the efficiency of energy transfer between levels to be compared

How this is examined

  • Why so little energy gets from one trophic level to the next: A three- or four-mark Explain, almost always attached to a pyramid or a table of energy values: "Explain why only a small percentage of the energy in the grass is transferred to the cattle." One mark per named loss, each with its destination: "energy is released in respiration and transferred to the surroundings as heat"; "energy in undigested material is lost in faeces"; "energy is lost in excretory products such as urine"; "not all of the organism is eaten (roots, bones)". A closing mark often comes from "only the energy used for growth becomes available to the next level". The words "energy is lost", with nothing after them, score zero.
  • Calculating the percentage of energy transferred between two trophic levels: A two- or three-mark Calculate, with the energy values given in a table or on a pyramid of energy, sometimes followed by "Explain your answer". Show the division before the arithmetic: energy at the higher level divided by energy at the lower level, multiplied by 100. The method mark is for that expression, the answer mark for the correct value with a % sign. A value above 100 is automatically wrong — you have inverted the fraction.
  • Predicting the knock-on effects of removing or adding a species in a food web: A three-mark Suggest following a food web: "Suggest what would happen to the number of X if all the Y were removed." A route, not a verdict. Name the organism affected, state the change to its food supply or its predators, then state the change in its population — for example "the thrushes have fewer slugs to eat, so they eat more caterpillars, so the caterpillar population falls". Either direction of argument can be credited if the chain of reasoning is stated and consistent.
  • Constructing a food chain and naming the trophic levels: A two- or three-mark opening part: a short description of who eats whom, then "Give a food chain for these organisms" and "Identify the secondary consumer". Start with the producer, include every organism in the right order, and draw every arrow from the organism eaten towards the organism that eats it — a reversed arrow loses the mark even when the order is right. For the levels, name them (producer, primary consumer, secondary consumer) rather than numbering them.
  • Choosing between a pyramid of numbers, biomass and energy: A two- or three-mark Explain after a pyramid with an odd shape: "Explain why the pyramid of numbers for this food chain is not pyramid-shaped" or "Give the advantages of using a pyramid of biomass instead". For the odd shape: "there is only one producer and it is very large, and a pyramid of numbers does not take the size of the organisms into account". For biomass: "it shows the mass of living material, so it accounts for the size of the organisms and is nearly always a true pyramid". For energy: "it shows energy transferred per unit area per unit time, so it is always a true pyramid and shows the efficiency of transfer".
  • Why eating crops feeds more people than eating livestock fed on those crops: A three-mark Explain at the end of an energy-transfer question, often about land use or feeding a growing population. Three linked statements: "eating the crop directly makes humans primary consumers, removing a trophic level"; "at each transfer most energy is transferred to the surroundings as heat in respiration, with more lost in faeces and urine"; "so more of the crop's energy is available to humans and the same area of land feeds more people". Marks are for the energy reasoning, not for saying crops are cheaper.

Words to know

Food chain
A diagram showing the transfer of energy from one organism to the next, beginning with a producer. The arrows mean "is eaten by" and point in the direction the energy travels.
Food web
A network of interconnected food chains, in which organisms appear in more than one chain.
Producer
An organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.
Consumer
An organism that gets its energy by feeding on other organisms. Classed as primary, secondary, tertiary or quaternary according to its position in the food chain.
Decomposer
An organism that gets its energy from dead or waste organic material, such as bacteria and fungi.
Trophic level
The position of an organism in a food chain, food web or ecological pyramid.
Biomass
The mass of living material present at a trophic level, usually measured as dry mass per unit area.
Pyramid of energy
A diagram showing the energy transferred to each trophic level per unit area per unit time, which is always a true pyramid shape.
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