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KS3 Science · Year 8Chapter 5 · Ecosystems and interdependence
Year 8 · Chapter 5

Ecosystems and interdependence

By the end of this chapter you will be able to

  • Use the words habitat, population, community and ecosystem correctly
  • Name producers, consumers, predators and prey, and read a food web
  • Describe competition and predator-prey relationships using data
  • Explain the role of decomposers and describe the carbon cycle in simple terms
  • Estimate a population from quadrat samples, and say how to make a sample fair

You should already know

  • Respiration and photosynthesis (Year 8 Chapter 3)
  • Mean and simple division (Year 8 Chapter 1)

Where this leads

  • Organisms and their environment in later biology work

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8B7 organisms in ecosystems depend on each other: food chains and webs, predator-prey and competition; biotic and abiotic factors; decomposers and the carbon cycle (basic); sampling with quadrats and transects.

Exercise P  Do you remember?

1
FWhat is one fifth of 40? Write the number.
2
FWhat is 7 multiplied by 6? Write the number.
3
FWhich process do plants use to make glucose from carbon dioxide and water? Write the word.
4
FWhat is 100 minus 37? Write the number.
5
FWhich gas do animals breathe in for respiration? Write the word.
5.1

Habitats, populations and communities

Year 8
8B7: organisms in ecosystems; biotic and abiotic factors; habitat, population, community and ecosystem
Video lesson: Habitats, populations and communitiesComing soon
Story. Sam visits a pond with his class. There are frogs, pond snails, weed, and insects skating on the surface. “Are these all one group?” he asks. His teacher says, “Each type of living thing is a population. All the populations in one place are a community, and the community with its non-living surroundings is an ecosystem.”

A habitat is the place where an organism lives. A population is all the organisms of one species living in one area. A community is all the populations living together in a habitat. An ecosystem is the community together with its non-living, or abiotic, surroundings.

Biotic factors are living things that affect other organisms, such as food supply, predators, disease and competitors. Abiotic factors are non-living, such as light, temperature, water, oxygen and soil pH.

The table sorts some factors into biotic and abiotic.

Biotic (living) factorsAbiotic (non-living) factors
food supplylight intensity
predatorstemperature
diseasewater or moisture
competitorsoxygen level
soil pH

Key ideas

  • A habitat is where an organism lives.
  • A population is one species in one area; a community is all the populations together.
  • An ecosystem is the community plus its non-living environment.
  • Biotic factors are living; abiotic factors are non-living.
Worked example 5.1.1
What do we call all the populations living together in a habitat? Write the word.

Method. Recall the word.

  1. Recall. All the populations living together in a habitat form a community.
    community
  2. Check. Plants and animals in one habitat are all in the same community.
    same community
  3. Write the answer.
    community

Check. The community plus its environment is an ecosystem. ✓

Worked example 5.1.2
Is temperature a biotic or an abiotic factor? Write the word.

Method. Decide whether it is living.

  1. Think. Temperature is not a living thing; it is a non-living part of the environment.
    non-living
  2. Decide. So temperature is an abiotic factor.
    abiotic
  3. Write the answer.
    abiotic

Check. Biotic means living. ✓

Worked example 5.1.3
Is a predator a biotic or an abiotic factor? Write the word.

Method. Decide whether it is living.

  1. Think. A predator is a living animal that affects the population of its prey.
    living
  2. Decide. So a predator is a biotic factor.
    biotic
  3. Write the answer.
    biotic

Check. Living things that affect others are biotic. ✓

Watch out

Watch out: a population is one species only. A group of different species is a community, not a population.

Exam tip

Exam tip. Name the level: population (one species), community (all species), ecosystem (community plus its environment).

Try it (with your teacher, or a grown-up at home). With a grown-up, look at a pond or a park from the side of the path. Name one population you can see, and two abiotic factors that affect the place. Do not touch the water or any wild animal.

Exercise 5A  Habitats, populations and communities

1
FWhat is the place where an organism lives called? Write the word.
2
FWhat is a group of one species living in one area called? Write the word.
3
FWhat is the word for all the populations living together in a habitat? Write the word.
4
FWhat is the name for a community together with its non-living surroundings? Write the word.
5
FIs temperature biotic or abiotic? Write the word.
6
FIs competition for food biotic or abiotic? Write the word.
7
FIs light intensity biotic or abiotic? Write the word.
8
FName one biotic factor that affects a rabbit population. Accept any one: food supply, predators, or disease.
9
FName one abiotic factor that affects plants. Accept any one: light, water, or temperature.
10
FIs a pond an example of a habitat? Write yes or no.
11
FIs a population the same thing as a community? Write yes or no.
12
PSAli says the number of worms in a field is a biotic factor. Is Ali right? Write yes or no.
13
RExplain why a change in temperature can affect a whole ecosystem. Write one sentence.
5.2

Food chains and food webs

Year 8
8B7: organisms depend on each other through food chains and food webs; producers, consumers, predators and prey
Video lesson: Food chains and food websComing soon
Story. Omar watches a robin catch a worm on the lawn. “Where does the energy in the worm come from?” he asks. His teacher draws an arrow from the grass to the worm, and from the worm to the robin. “Follow the arrows back, and you find the energy began in the sun.”

A food chain shows what eats what. The arrows point from the food to the eater, showing the direction of energy transfer. The first organism is a producer, which makes its own food by photosynthesis. Animals that eat other organisms are consumers. A predator hunts and eats another animal, called its prey.

A food web links many food chains together. Energy is lost at each step, so food chains are usually short.

grasscloverrabbitgrasshopperfoxfroghawkArrows point from the food to the eater.
A food web with arrows pointing from the food to the eater.

Key ideas

  • A producer makes its own food by photosynthesis.
  • A consumer eats other organisms; a predator hunts its prey.
  • Arrows point from the food to the eater.
  • Energy is lost at each step, so food chains are short.
Worked example 5.2.1
In the food chain grass → rabbit → fox, what is the producer? Write the word.

Method. Find the organism that makes its own food.

  1. Look. Grass makes its own food by photosynthesis.
    photosynthesis
  2. Name it. So the producer is grass.
    grass
  3. Write the answer.
    grass

Check. The rabbit and the fox must eat other organisms. ✓

Worked example 5.2.2
In the food chain grass → rabbit → fox, which animal is the predator? Write the word.

Method. Find the animal that hunts another.

  1. Look. The fox hunts and eats the rabbit.
    hunts
  2. Name it. So the predator is the fox.
    fox
  3. Write the answer.
    fox

Check. The fox is the top predator in this chain. ✓

Worked example 5.2.3
In the same chain, which animal is the prey? Write the word.

Method. Find the animal that is eaten.

  1. Look. The rabbit is eaten by the fox.
    is eaten
  2. Name it. So the prey is the rabbit.
    rabbit
  3. Write the answer.
    rabbit

Check. Prey is eaten by a predator. ✓

Watch out

Watch out: the arrows point to the eater, not the food. An arrow from the fox to the rabbit would be wrong.

Exam tip

Exam tip. Label each organism in a chain: producer, primary consumer, secondary consumer, and so on, starting from the producer.

Try it (with your teacher, or a grown-up at home). Draw a food chain of at least three organisms from a habitat you know, such as a garden or a park. Start with a producer and draw the arrows from food to eater. Do not handle any wild animal.

Exercise 5B  Food chains and food webs

1
FWhat is an organism that makes its own food called? Write the word.
2
FWhat is an organism that eats other organisms called? Write the word.
3
FIn the chain grass → rabbit → fox, which is the primary consumer? Write the word.
4
FIn the chain grass → rabbit → fox, which is the secondary consumer? Write the word.
5
FWhat is the name for an animal that is eaten by another animal? Write the word.
6
FWhat is the name for the animal that hunts and eats another? Write the word.
7
FIn a food web, which way do the arrows point? Write the words: from food to eater, or from eater to food.
8
FIn the food web on the picture, how many arrows point to the rabbit? Write the number.
9
FIn the food web, how many organisms are producers? Write the number.
10
FIn the food web, what does the hawk eat? Write the word.
11
PSRuby says the fox is a producer because it eats rabbits. Is Ruby right? Write yes or no.
12
PSEnergy is lost at each level of a food chain. Does this explain why food chains are usually short? Write yes or no.
13
RExplain why most of the energy in the grass does not reach the fox. Write one sentence.
5.3

Competition and predator-prey relationships

Year 8
8B7: organisms compete for resources; predator and prey populations change together, with data over time
Video lesson: Competition and predator-prey relationshipsComing soon
Story. Two birds fight over the same seeds at a bird table. “They are competing,” says the teacher. “They want the same resource.” Later, the class looks at a graph of rabbits and foxes in a wood, and sees that the numbers rise and fall together.

Competition happens when organisms need the same limited resource, such as food, water, light, space or mates. The stronger competitor usually survives better.

In a predator-prey relationship the numbers are linked. When there is plenty of prey, the predators have more food and their numbers rise. As the predators eat more prey, the prey numbers fall, and then the predator numbers fall too.

The table gives example data for rabbits (prey) and foxes (predators) over five years.

YearRabbits (prey)Foxes (predators)
201612015
201720030
20189025
20194010
202011018
Predator and prey numbers (example data)20162017201820192020Number of animalsorange: prey (rabbits)red: predator (foxes)
Prey and predator numbers over five years: predator numbers follow the prey numbers.

Key ideas

  • Competition: organisms need the same limited resource.
  • Predator numbers rise after prey numbers rise, and fall after prey numbers fall.
  • Look at the trend over several years, not one year only.
  • The data here is example data.
Worked example 5.3.1
In which year were rabbit numbers at their lowest? Write the year.

Method. Find the lowest rabbit number.

  1. Look. The lowest rabbit number is 40.
    40
  2. Name it. That is in 2019.
    2019
  3. Write the answer.
    2019

Check. Foxes were also lowest in 2019. ✓

Worked example 5.3.2
What is the difference between the highest and lowest fox numbers? Write the number.

Method. Subtract the lowest from the highest.

  1. Read. The highest fox number is 30 and the lowest is 10.
    30 and 10
  2. Calculate. The difference is 30 minus 10.
    30 - 10 = 20
  3. Write the answer.
    20

Check. 10 + 20 = 30. ✓

Worked example 5.3.3
Between 2018 and 2019, did the fox numbers rise or fall? Write the word.

Method. Compare the two years.

  1. Compare. Foxes fell from 25 in 2018 to 10 in 2019.
    25 to 10
  2. Decide. So the number fell.
    fell
  3. Write the answer.
    fell

Check. Rabbits also fell over the same years. ✓

Watch out

Watch out: the predator numbers follow the prey numbers with a delay. A rise in predators does not happen in the same year as a rise in prey.

Exam tip

Exam tip. Describe the trend with both populations: “when prey numbers fall, predator numbers fall a year later”.

Try it (with your teacher, or a grown-up at home). Plot the rabbit and fox data on squared paper, one line for each. Write one sentence describing the pattern. Use example data only; do not draw conclusions about any real wood from it.

Exercise 5C  Competition and predator-prey relationships

1
FWhat is the word for organisms that need the same limited resource? Write the word.
2
FName one resource that plants compete for. Accept any one: light, water, minerals, or space.
3
FUsing the table, how many rabbits were there in 2016? Write the number.
4
FUsing the table, how many foxes were there in 2017? Write the number.
5
FUsing the table, between which two years did the rabbit numbers rise the most? Write the years.
6
FUsing the table, what is the difference between the highest and lowest rabbit numbers? Write the number.
7
FUsing the table, in which year were foxes at their lowest? Write the year.
8
FUsing the table, did the fox numbers rise or fall between 2019 and 2020? Write the word.
9
FWhat type of relationship does this table show: competition or predator-prey? Write the words.
10
FName the predator in the table. Write the word.
11
PSOmar says that more foxes always means fewer rabbits in the same year. Is Omar right? Write yes or no.
12
PSWorking scientifically: how many years of data are in the table? Write the number.
13
RExplain why foxes may fall in number after rabbits fall. Write one sentence.
5.4

Decomposers and the carbon cycle

Year 8
8B7: decomposers break down dead matter and release nutrients; the basic carbon cycle
Video lesson: Decomposers and the carbon cycleComing soon
Story. Sam finds a rotting log under a tree. Inside, the wood is soft and crumbles. “Where does the wood go?” he asks. His teacher says, “Bacteria and fungi break it down. They return nutrients to the soil, and carbon dioxide to the air.”

Decomposers are microorganisms, mainly bacteria and fungi, that break down dead organisms and waste. Decomposition releases nutrients into the soil, which plants can use again. Decomposers also respire, releasing carbon dioxide.

The carbon cycle moves carbon between the air and living things. Plants take carbon dioxide from the air in photosynthesis. Animals eat plants and pass the carbon along the food chain. Living things return carbon dioxide to the air by respiration, and decomposers return it when they break down dead matter.

air (carbon dioxide)plantsanimalsdead matterphotosynthesiseatendecomposersrespiration
The carbon cycle, with carbon dioxide in the air, plants, animals, dead matter and decomposers.

Key ideas

  • Decomposers are mainly bacteria and fungi.
  • Decomposition releases nutrients into the soil.
  • Photosynthesis takes carbon dioxide out of the air.
  • Respiration and decomposition return carbon dioxide to the air.
Worked example 5.4.1
Name two groups of decomposers. Write the words.

Method. Recall the groups.

  1. Recall. The main decomposers are bacteria and fungi.
    bacteria and fungi
  2. Check. Both bacteria and fungi break down dead matter.
    break down dead matter
  3. Write the answer.
    bacteria and fungi

Check. Both are microorganisms. ✓

Worked example 5.4.2
Which process takes carbon dioxide out of the air? Write the word.

Method. Recall the process.

  1. Recall. Plants take carbon dioxide out of the air in photosynthesis.
    photosynthesis
  2. Check. Photosynthesis is the only process that removes carbon dioxide from the air.
    removes
  3. Write the answer.
    photosynthesis

Check. Photosynthesis needs light. ✓

Worked example 5.4.3
Which process returns carbon dioxide to the air from living things? Write the word.

Method. Recall the process.

  1. Recall. Living things release carbon dioxide by respiration.
    respiration
  2. Check. Respiration happens in every living cell, so it returns carbon dioxide from all living things.
    every living cell
  3. Write the answer.
    respiration

Check. Respiration happens in every living cell. ✓

Watch out

Watch out: decomposers are not producers. They do not make their own food; they feed on dead matter.

Exam tip

Exam tip. In the carbon cycle, name the process that moves carbon: photosynthesis takes it out, respiration and decomposition put it back.

Try it (with your teacher, or a grown-up at home). Look at a compost heap or a pile of fallen leaves with a grown-up. Say which decomposers you think are there, and why it may feel warm. Do not dig into the compost or touch anything with your bare hands.

Exercise 5D  Decomposers and the carbon cycle

1
FName two groups of decomposers. Write the words.
2
FWhat useful substance is released into the soil by decomposition? Write the word.
3
FWhich process takes carbon dioxide out of the air? Write the word.
4
FWhich process returns carbon dioxide to the air from living things? Write the word.
5
FWhat is the name of the process in which decomposers break down dead matter? Write the word.
6
FDoes burning fuel add carbon dioxide to the air? Write yes or no.
7
FWhat sugar do plants make from carbon dioxide and water? Write the word.
8
FAre decomposers producers? Write yes or no.
9
FName one place where decomposers live. Accept any one: soil, leaf litter, or compost.
10
FWhere does the carbon in a plant come from? Write the words.
11
PSRuby says decomposers only break down plants. Is Ruby right? Write yes or no.
12
PSWorking scientifically: a pupil tests whether decomposers need air. Name one control variable. Accept any sensible control: the same amount of leaves, or the same temperature.
13
RExplain why a compost heap gets warm. Write one sentence.
5.5

Sampling with quadrats and transects

Year 8
8B7: sampling with quadrats and transects; estimating a population from a sample
Video lesson: Sampling with quadrats and transectsComing soon
Story. Omar’s class wants to know how many daisies are in a field. “We cannot count every one,” he says. His teacher says, “Count in some quadrats placed at random, find the mean, and then estimate for the whole field.”

A quadrat is a square frame, for example 1 metre by 1 metre, placed on the ground to sample a small area. A transect is a line along which samples are taken at regular points. Quadrats are placed at random to avoid bias, so that the sample is fair.

To estimate a population, find the mean number per quadrat, then multiply by the total area: estimate = mean x area.

The table gives the counts in ten quadrats.

QuadratDaisies counted
14
26
35
43
57
65
74
86
95
105
one 1 m x 1 m quadrat (example)
A 1 metre square quadrat with daisies marked as dots.

Key ideas

  • A quadrat is a square frame that samples a small area.
  • Place quadrats at random to avoid bias.
  • Estimate the population: mean per quadrat multiplied by the total area.
  • More quadrats give a better estimate.
Worked example 5.5.1
What is the mean number of daisies per quadrat in the table? Write the number.

Method. Add the counts, then divide by 10.

  1. Add. The total of the ten counts is 50.
    50
  2. Divide. The mean is 50 divided by 10.
    50 / 10 = 5
  3. Write the answer.
    5

Check. 5 x 10 = 50. ✓

Worked example 5.5.2
The field is 200 square metres. Using the mean of 5, estimate the number of daisies. Write the number.

Method. Multiply the mean by the area.

  1. Multiply. Estimate = mean x area = 5 x 200.
    5 x 200
  2. Calculate. The estimate is 1000 daisies.
    5 x 200 = 1000
  3. Write the answer.
    1000

Check. The estimate is 1000 daisies. ✓

Worked example 5.5.3
Name the sampling line along which samples are taken at regular points. Write the word.

Method. Recall the word.

  1. Recall. A line along which samples are taken at regular points is a transect.
    transect
  2. Check. A quadrat samples a small square; a transect samples along a line.
    along a line
  3. Write the answer.
    transect

Check. A transect can cross a habitat gradient. ✓

Watch out

Watch out: placing quadrats where you choose, such as the sunniest corner, causes bias. Use random positions.

Exam tip

Exam tip. Show the mean first, then multiply by the area. Give the answer with its unit, for example daisies.

Try it (with your teacher, or a grown-up at home). With a grown-up, place a 1 metre quadrat, or a hoop, on a grass area at random. Count the daisies or the dandelions, and record the number. Repeat in three places. Do not pick any plant, and wash your hands afterwards.

Exercise 5E  Sampling with quadrats and transects

1
FWhat is a square frame used to sample a small area called? Write the word.
2
FWhat is a line along which samples are taken called? Write the word.
3
FQuadrats are placed at random to avoid what? Write the word.
4
FAdd the daisy counts in the first three quadrats: 4, 6 and 5. Write the number.
5
FFind the total of all ten quadrat counts. Write the number.
6
FFind the mean number of daisies per quadrat. Write the number.
7
FIf the field were 400 square metres, what would the estimate be, using the same mean of 5? Write the number.
8
FWhich quadrat had the fewest daisies? Write the quadrat number.
9
FWhich quadrat had the most daisies? Write the quadrat number.
10
FWhat is the range of the ten counts, that is the highest minus the lowest? Write the number.
11
PSAli counts only the daisies in the sunniest corner of the field. Is this a fair sample? Write yes or no.
12
PSWorking scientifically: name one thing that must stay the same for every quadrat. Accept any one: the size of the quadrat, or the same counting method.
13
RExplain why more quadrats usually give a better estimate of the population. Write one sentence.
14
RChallenge: a pupil estimates the number of daisies as 5 x 200, but the field is actually 200 m by 2 m. Explain the error. Write one sentence.

Chapter summary

SkillMethod
EcosystemHabitat, population, community; the community with its abiotic environment.
Food webArrows from food to eater; producers, consumers, predators and prey; energy lost at each level.
CompetitionOrganisms needing the same limited resource.
Predator-preyPredator numbers follow prey numbers, with a delay.
Carbon cyclePhotosynthesis takes carbon dioxide out; respiration and decomposition return it.
SamplingRandom quadrats; estimate = mean x area.

Can you…?

  • I can use the words habitat, population, community and ecosystem correctly
  • I can name producers, consumers, predators and prey, and read a food web
  • I can describe competition and predator-prey relationships using data
  • I can describe the role of decomposers and the carbon cycle in simple terms
  • I can estimate a population from quadrat samples and say how to make a sample fair

Key terms

habitat
the place where an organism lives
population
all the organisms of one species living in one area
community
all the populations living together in a habitat
ecosystem
a community together with its non-living surroundings
biotic
living factors that affect other organisms
abiotic
non-living factors, such as light, temperature and water
producer
an organism that makes its own food by photosynthesis
predator
an animal that hunts and eats other animals
decomposer
a microorganism, mainly a bacterium or fungus, that breaks down dead matter
quadrat
a square frame used to sample a small area
transect
a line along which samples are taken at regular points

Exercise 5Z  Check-up

1
What is the name for a community together with its non-living surroundings? Write the word.
2
Is light intensity biotic or abiotic? Write the word.
3
In the chain grass → rabbit → fox, what is the producer? Write the word.
4
What is the name for an animal that is eaten by another animal? Write the word.
5
What is the word for organisms needing the same limited resource? Write the word.
6
Name the group of microorganisms that break down dead matter, mainly bacteria and what other group? Write the word.
7
Which process takes carbon dioxide out of the air? Write the word.
8
Using the mean of 5 daisies per quadrat and a field of 200 square metres, estimate the number of daisies. Write the number.
9
Explain why a predator population falls after its prey population falls. Write one sentence.
10
What is the name for the non-living part of an environment that affects living things, such as light? Write the word.

Total 14 marks. Allow about 14 minutes.

Exercise 5X  Practice test questions

1
FName the living thing that makes its own food from light, carbon dioxide and water. Write the word.[1 mark]
2
FIn a food web, which way do the arrows point? Write the words.[1 mark]
3
FName one abiotic factor that affects the distribution of plants. Accept any one: light, temperature, water, or soil pH.[1 mark]
4
FName the process by which decomposers return nutrients to the soil. Write the word.[1 mark]
5
RExplain why food chains are usually short. Write one sentence.[2 marks]
6
FUsing the example data, how many rabbits were there in 2018? Write the number.[1 mark]
7
PSUsing the example data, how many more rabbits were there in 2017 than in 2019? Write the number.[2 marks]
8
PSA quadrat count of 4, 6, 5, 7 and 3 is recorded. Find the mean. Write the number.[2 marks]
9
RExplain why quadrats should be placed at random. Write one sentence.[2 marks]
10
FWhich process returns carbon dioxide to the air from living things? Write the word.[1 mark]
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