Chapter 7 of 8

All 8 chapters
  1. 1Natural selection, readunderstood
  2. 2Evidence for evolution, readunderstood
  3. 3New species and extinction, readunderstood
  4. 4Classification, readunderstood
  5. 5The carbon and nitrogen cycles, readunderstood
  6. 6Measuring populations, readunderstood
  7. 7Interdependence, readunderstood
  8. 8Biodiversity and people, readunderstood

S4.3.7 Chapter 7

Interdependence

Why do the numbers of foxes and rabbits in a wood rise and fall in a pattern, year after year?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain predator and prey population cycles from a graph
  • Explain how living and non-living factors affect a community

All the living things in one habitat make a community. The living things in it depend on each other. This is interdependence. They depend on each other for food, for shelter, and for jobs such as pollination. Remove one kind and the others change.

A food web with arrows from food to feeder, so energy flows along the arrows. Grass and shrub are on the bottom row. The rabbit and the grasshopper eat both of them. The fox eats the rabbit. The frog eats the grasshopper. The tawny owl eats the frog and the rabbit.shrubgrassgrasshopperrabbitfrogfoxtawny owlenergy flows along the arrows
A food web. Energy flows along the arrows, from the food to the feeder. Decomposers feed on every dead thing in it, but they are not drawn.

Look at the web. In this web, the fox eats only rabbits. If the rabbits fell in number, the foxes would soon lack food. The tawny owl eats both frogs and rabbits, so it has another choice. Living things that eat the same food also compete. Rabbits and grasshoppers compete for grass.

A predator is an animal that hunts and eats other animals. The animals it eats are its prey. Predator and prey numbers affect each other. Let us follow a wood with rabbits and foxes.

Draw it first

Draw the two lines

  1. A graph of animals in a wood over 12 years. The axis along the bottom is labelled time in years, with a scale from 0 to 12 in steps of 2. The axis up the side is labelled number of animals, with a scale from 0 to 200 in steps of 40. The first picture shows only the line for rabbits. The second adds the line for foxes. Both lines rise and fall in a regular pattern.02468101204080120160200rabbitstime in yearsnumber of animals

    1 The number of rabbits rises, falls and rises again in a regular pattern. These are example numbers.

  2. A graph of animals in a wood over 12 years. The axis along the bottom is labelled time in years, with a scale from 0 to 12 in steps of 2. The axis up the side is labelled number of animals, with a scale from 0 to 200 in steps of 40. The first picture shows only the line for rabbits. The second adds the line for foxes. Both lines rise and fall in a regular pattern.02468101204080120160200rabbitsfoxestime in yearsnumber of animals

    2 The foxes follow the same pattern, but each peak comes a little later than the rabbits' peak. The fox line is drawn larger than real life so that both lines can be read. Real predators are far fewer than their prey.

Why does this happen? When rabbits are many, the foxes have plenty of food. They survive and breed, and the number of foxes rises. But foxes need time to breed and grow, so the foxes peak a little after the rabbits.

Many foxes eat many rabbits, and the rabbits fall. Now there is less food, so the foxes starve and fall. With fewer foxes, more rabbits survive, and the rabbits rise again. The cycle repeats.

Four boxes joined by arrows in a ring. The boxes read: prey increase; predators increase; prey decrease; predators decrease. The words on the arrows, going round, are: food; eaten; hunger; fewer eaten.preyincreasepredatorsincreasepreydecreasepredatorsdecreasefoodeatenhungerfewer eaten
The predator and prey cycle. Real cycles are less regular, because weather, disease and other food also act.

Worked example

Use the graph of rabbits and foxes. The rabbits peak in year 3 and the foxes peak in year 4. How many years after the rabbit peak is the fox peak?

  1. The rabbits peak at 180, in year 3.
  2. The foxes peak at 60, in year 4.
  3. The foxes peak 4 − 3 = 1 year later.

Answer: 1 year later

Worked example

Use the same graph. In year 2, how many more rabbits than foxes are there?

  1. Read the rabbit line at year 2. It is at 160.
  2. Read the fox line at year 2. It is at 40.
  3. The difference is 160 − 40 = 120.

Answer: 120 more rabbits

Predators do not only harm prey. They keep prey in check. Suppose all the foxes were removed. The rabbits could rise until they ate all the grass, and then many would starve or die of disease.

The factors that affect a community are of two kinds. Biotic factors are living ones: food, predators, disease and competition. Abiotic factors are non living ones: temperature, light, water, soil pH and, in water, the amount of oxygen.

A table with the columns Living and Non living. The living factors are food, predators, disease, competition. The non living factors are temperature, light, water, soil pH, oxygen in water.LivingNon livingfoodtemperaturepredatorslightdiseasewatercompetitionsoil pHoxygen in water
Factors that affect a community. The living factors are also called biotic, and the non living ones abiotic.

Here are two examples. Plants that need much light grow well in the open, and plants that cope with shade grow under trees. Light is an abiotic factor. In a pond, warm water holds less dissolved oxygen than cool water. In a hot spell, the fish may lack oxygen.

Factors act together. A hot dry year may lower the number of rabbits by reducing the grass. This gives the foxes less food. A change in one factor can run through the whole community.

Try it

Does light change what grows?

You need: a square frame, 0.5 m by 0.5 m, made of sticks and string, paper and a pencil, a sunny patch and a shady patch of ground, with the owner's leave.

Be safe: Mind the sharp ends of sticks and wear a hat in the sun. Pick nothing, and leave every plant and animal as you found it. Wash your hands afterwards.

  1. Choose a sunny patch of ground and a shady patch near it. The soil should be much the same.
  2. In each patch, lay the frame in 5 places chosen at random. Count the plants inside it each time.
  3. Work out the mean count for the sunny patch and for the shady patch.
  4. Compare the two means. Then list other things that might differ, such as water.

What you will see: The sunny patch and the shady patch usually hold different numbers of plants, and often different kinds. Light is one abiotic factor. Water and soil may differ too, so be careful when you say that light is the cause.

If you cannot try it, this is what happens.
A table with the columns Place and Mean. The sunny patch has a mean of 6 plants. The shady patch has a mean of 2 plants. These are example data.PlaceMeansunny6shady2

A wrong idea many people have

If we remove all the predators, there will be more prey, and that will be good for the prey.

Why it feels right: Predators kill prey, so it seems that fewer predators must help the prey.

The test that breaks it: Without foxes, rabbits can rise until they eat all the grass. Then many starve or die of disease. Predators keep the number of prey in check.

What is true: Predators and prey keep each other in check. Removing a predator can lead to a boom in prey, and then a crash when the food runs out.

Key words

community
all the living things of every kind in one habitat.
interdependence
living things depending on each other, for food, shelter and other needs.
predator
an animal that hunts and eats other animals.
prey
an animal that is hunted and eaten by a predator.
biotic factor
a living thing that affects a community, such as food, a predator, disease or competition.
abiotic factor
a non living thing that affects a community, such as temperature, light, water or soil pH.

Back to your guess

The answer is B: When rabbits are many, foxes get more food and increase. Then many rabbits are eaten and fall, and foxes fall after them.

Predators and prey affect each other. The predators rise a little after the prey, because they need time to breed. Then they eat so many prey that the prey fall, and the predators fall after them.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Look at the graph of mice and owls. Which statement is true?

    A graph of animals over 12 years. The axis along the bottom is labelled time in years, with a scale from 0 to 12 in steps of 2. The axis up the side is labelled number of animals, with a scale from 0 to 200 in steps of 40. The line for mice rises and falls in a regular pattern. The line for owls follows the same pattern, with a smaller peak.02468101204080120160200miceowlstime in yearsnumber of animals
    Tap your answer.

    1. Not this one. Read the years of the peaks. The mice peak first. Many mice give the owls more food, and then the owls rise. Try another.

    2. Yes, that is right. The mice peak in year 3 and the owls peak in year 5. The predator needs time to breed after the prey rise.

    3. Not this one. Compare the peaks. The owl line peaks later than the mouse line. Try another.

    See the answer

    Each owl peak comes after a mouse peak.

    The mice peak in year 3 and the owls peak in year 5. The predator needs time to breed after the prey rise.

    • If you chose A: Read the years of the peaks. The mice peak first. Many mice give the owls more food, and then the owls rise.
    • If you chose C: Compare the peaks. The owl line peaks later than the mouse line.

  2. Why does the predator peak come after the prey peak?

    Tap your answer.

    1. Not this one. Speed is not the reason. The delay is about breeding. Predators need time to breed and grow when food becomes plentiful. Try another.

    2. Not this one. It is the other way round. Many prey give the predators plenty of food, and then the predators increase. Try another.

    3. Yes, that is right. More prey means more food, but young predators take time to be born and to grow up. So the predators peak a little later.

    See the answer

    Plentiful prey give the predators food, but the predators need time to breed and grow.

    More prey means more food, but young predators take time to be born and to grow up. So the predators peak a little later.

    • If you chose A: Speed is not the reason. The delay is about breeding. Predators need time to breed and grow when food becomes plentiful.
    • If you chose B: It is the other way round. Many prey give the predators plenty of food, and then the predators increase.

  3. Which of these is a non living (abiotic) factor that affects a community?

    Tap your answer.

    1. Yes, that is right. Soil pH is not alive. Temperature, light, water and soil pH are non living factors.

    2. Not this one. A disease is caused by a living thing, such as a virus or a bacterium. So it is a living (biotic) factor. Try another.

    3. Not this one. A predator is a living thing, so it is a living (biotic) factor. Try another.

    See the answer

    The pH of the soil

    Soil pH is not alive. Temperature, light, water and soil pH are non living factors.

    • If you chose B: A disease is caused by a living thing, such as a virus or a bacterium. So it is a living (biotic) factor.
    • If you chose C: A predator is a living thing, so it is a living (biotic) factor.

  4. Look at the food web. A disease kills all the rabbits. Which animal loses its only food?

    A food web with arrows from food to feeder, so energy flows along the arrows. Grass and shrub are on the bottom row. The rabbit and the grasshopper eat both of them. The fox eats the rabbit. The frog eats the grasshopper. The tawny owl eats the frog and the rabbit.shrubgrassgrasshopperrabbitfrogfoxtawny owlenergy flows along the arrows
    Tap your answer.

    1. Not this one. The tawny owl also eats frogs, so it still has food. Try another.

    2. Yes, that is right. The fox eats only rabbits in this web. The tawny owl has the frog as another food, and the frog eats grasshoppers.

    3. Not this one. The frog eats grasshoppers. The rabbits are not its food. Try another.

    See the answer

    The fox

    The fox eats only rabbits in this web. The tawny owl has the frog as another food, and the frog eats grasshoppers.

    • If you chose A: The tawny owl also eats frogs, so it still has food.
    • If you chose C: The frog eats grasshoppers. The rabbits are not its food.

  5. Why may fish in a pond have trouble breathing in a hot spell?

    Tap your answer.

    1. Not this one. It is the other way round. Warm water holds less dissolved oxygen than cool water. Try another.

    2. Not this one. Temperature is an abiotic factor with a big effect. It changes how much oxygen the water can hold. Try another.

    3. Yes, that is right. Fish take oxygen from the water. In warm water, less oxygen is dissolved, so the fish may have too little.

    See the answer

    Warm water holds less dissolved oxygen than cool water.

    Fish take oxygen from the water. In warm water, less oxygen is dissolved, so the fish may have too little.

    • If you chose A: It is the other way round. Warm water holds less dissolved oxygen than cool water.
    • If you chose B: Temperature is an abiotic factor with a big effect. It changes how much oxygen the water can hold.

  6. A poison kills most of the bees in a meadow. What is most likely to happen to the flowers that bees pollinate?

    Tap your answer.

    1. Yes, that is right. The flowers and the bees depend on each other. Bees get nectar, and the flowers get their pollen carried.

    2. Not this one. Many flowers depend on animals such as bees to carry pollen from flower to flower. Without it, they make fewer seeds. Try another.

    3. Not this one. Bees carry pollen to the next flower. This is how the flowers are pollinated, so fewer bees means less pollination. Try another.

    See the answer

    Fewer flowers are pollinated, so they make fewer seeds.

    The flowers and the bees depend on each other. Bees get nectar, and the flowers get their pollen carried.

    • If you chose B: Many flowers depend on animals such as bees to carry pollen from flower to flower. Without it, they make fewer seeds.
    • If you chose C: Bees carry pollen to the next flower. This is how the flowers are pollinated, so fewer bees means less pollination.

  7. From an earlier chapter

    Which method would you use to estimate the number of snails in a garden?

    Tap your answer.

    1. Yes, that is right. Snails move, so we mark a sample, release it, catch a second sample and count the marked ones.

    2. Not this one. Quadrats suit plants and animals that stay still. Snails move about, so they would leave the frame. Go to S4.3.6. Try another.

    3. Not this one. That is not a fair sample. A few snails in one spot do not stand for the whole garden. Go to S4.3.6. Try another.

    See the answer

    Capture and recapture

    Snails move, so we mark a sample, release it, catch a second sample and count the marked ones.

    • If you chose B: Quadrats suit plants and animals that stay still. Snails move about, so they would leave the frame. Go to S4.3.6.
    • If you chose C: That is not a fair sample. A few snails in one spot do not stand for the whole garden. Go to S4.3.6.

  8. A rabbit population falls from 200 at its peak to 50. By what percentage did it fall?

    See the answer

    75%

    It fell by 200 − 50 = 150. 150 ÷ 200 = 0.75, which is 75%.

  9. A wood has 240 rabbits and 8 foxes. How many rabbits are there for each fox?

    See the answer

    30 rabbits for each fox

    240 ÷ 8 = 30 rabbits for each fox.

Keep it

Copy this into your exercise book.

Predator and prey numbers rise and fall in cycles, and the predator peak is a little later. Living factors: food, predators, disease, competition. Non living: temperature, light, water, soil pH, oxygen.

Draw: A graph with two lines that rise and fall, and the second peak a little after the first.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.