Lesson video

Welcome to this fourth lesson in the unit ecosystems. This lesson is all about food webs.

And my name is Mr. Jarvis and I'm gonna be teaching you today.

By the end of today's lesson, you should be able to explain what is shown by a food web diagram and also to explain the effects that a change in the size of a population could have on other populations within the same community.

During the lesson, we are going to be using five key words.

They are habitat, community, population, food chain, and food web.

Here are the definitions that we're going to be using for those words.

I'll give you a few seconds to read those for yourself.

You may need to pause the video while you do so and then restart it when you're ready.

But don't worry, I'll explain those definitions as we go through the lesson.

Today's lesson is going to be divided into three parts.

First of all, we are going to look at what habitats and communities are.

Then we're gonna move on to look at what food web diagrams are.

And finally, we're going to look at those changes in populations and how they impact a food web.

So if you're ready, let's get started with the first section, which is all about habitats and communities.

Let's go.

A habitat is a place where an organism lives.

The habitats have to provide organisms with three things.

They are food, shelter, and a place to reproduce.

There are lots of different types of habitat that you might come across.

The largest habitat is the ocean, but there's lots of smaller habitats that you can find, for example, under logs or leaves in a woodland.

And we call really small habitats microhabitats.

Can you think of other habitats that you might have encountered? I'll give you a few seconds to think.

So what did you come up with? Perhaps you thought about a woodland or a grassland or even a tropical rainforest.

You might have thought smaller and thought about a pond in the back garden or a rock pool.

You may have even thought smaller still and thought about under stones and rocks or perhaps in a temporary pool.

All of those places are habitats if they provide organisms with food, with shelter, and with a place that they can reproduce.

All of the different organisms that live in the same place form what we call a community.

A community is a group of populations, and remember, a population is the number of organisms of the same type that live in the same place.

So here's an example of a community that lives in a coal reef habitat, and you can see there are lots of different types of organisms in the picture.

What populations can you see in that picture that make up the community? Again, you might want to pause the video here while you spot the different types of organisms and different populations that we can see in this picture.

Then when you're ready, restart and we'll see some of the populations that you might have found.

So how many different populations did you spot? You may have spotted that there are different populations of fish in the diagram.

I've circled four different types that I've been able to spot in the picture.

You may have even thought, actually, there are different organisms that are there too.

There are different types of coral.

Now, corals are types of animals, and you can see I've circled three different types of coal that are in the picture.

You may have even spotted more.

Well done if you did.

Members of the same community may have similar adaptations that help them to cope with where they live.

So let's use our coal reef habitat again.

And in this habitat, in an ocean habitat, organisms have to be adapted to live in salty water.

In an arctic habitat, organisms have to be adapted to help them keep warm.

There are lots of interactions within a community.

For example, some organisms help each other survive.

So here we've got a picture of a bee pollinating a crocus.

Some organisms help others with jobs like cleaning.

For example, the oxpecker here in the picture helps to clean parasites from the buffalo's skin.

Or organisms may rely on each other as we've seen before in our food chains, and are linked together by those feeding relationships.

And here we've got a picture of a heron that's caught and eating a fish.

So let's see how well you've remembered the first part of today's lesson.

Which of of the following images and descriptions best represents a community? Is it A, a crab on a sandy beach, B, koi carp in a pond, or C, bushes, lion, giraffe, bison, ostrich and bush buck in Africa? I'll give you a few seconds.

The correct answer is C.

Remember, a community are the groups of populations that live together in the same habitat, in the same environment.

In picture A, we've got an individual crab on a sandy beach.

In picture B, we've got a population of koi carp.

It's koi carp in a pond.

But in picture C, we've got all the different populations of lion and bison and bush buck that living in Africa.

Well done if you got that right.

So now let's move on to a practise question.

First of all, habitats provide organisms with three things.

What are they? Secondly, I'd like you to write a short paragraph to explain what a community is.

And finally, I'd like you to think of a habitat and explain how the populations within this habitat might interact with each other.

You'll need to pause the video at this point and then when you're ready, you can start the video and we'll check to see how well you did.

Good luck.

So how well did you do? There were three parts to this task.

The first was a question.

Habitats provide organisms with three things.

What are they? You should have answered food, shelter, and a place to reproduce.

Well done if you got those.

The second task was to write a short paragraph to explain what a community is.

This is what you should have had, something along these lines, that a community is made up of all of the organisms or all of the populations that live within a habitat.

And the third part to this task was to get you to think of a habitat and explain how the populations within the habitat might interact with each other.

Here are a couple of examples.

So in a grassland, bees may pollinate flowers and organisms eat each other for food.

Or in a river, water plants may provide shelter and a breeding ground for fish and invertebrates.

I hope that you got something like that.

Well done if you did.

We are now going to move into the second part of today's lesson, which is learning all about what food web diagrams are.

So if you're ready, let's move on.

So food chain diagrams are models and they show how food passes from one organism to the next.

We have a food chain diagram on the screen.

The leaf is eaten by the lady bird.

The lady bird is eaten by the spider.

Remember those arrows point in the direction of where the food passes.

So the leaf is eaten by the lady bird.

The lady bird is eaten by the spider.

Producers, that's the leaf, make their own food.

They always begin a food chain diagram.

And consumers, animals that eat other organisms for food, rely on producers as they're not able to make the food for themselves.

So producers provide all of the food for all of the food chain.

Communities are made up of lots of food chains, and most consumers eat more than one single type of food.

So our model of a food chain diagram is actually too simple as it shows just one feeding relationship.

So for example, an oak leaf is eaten by an aphid, is eaten by a lady bird, is eaten by a blue tit, is eaten by a wildcat.

Each organism, however, could be eaten by consumers that are not shown in this food chain.

Food chain diagrams show just one simple feeding relationship.

Food webs are much better models and this is because feeding relationships are really complex.

And a food web diagram is a set of linked food chains.

Many producers and many primary consumers are eaten by more than just one population.

Let's have a look at an example.

So here are some food chain diagrams from a woodland.

We have oak leaves is eaten by snails, is eaten by hedgehogs, is eaten by foxes.

Oak leaves are eaten by earthworms, are eaten by hedgehogs, are eaten by foxes.

Oak leaves are eaten by voles, are eaten by weasels, are eaten by foxes.

And oak leaves are eaten by vol, are eaten by owl.

So let's see if we can link these food chains together.

I'm going to start with the same producer because we've got the same producer in each of our food chains.

That's the oak leaves.

And I'm gonna take our simplest food chain first that's at the bottom of the screen.

Oak leaves is eaten by vol, is eaten by owls.

So let's look at this food chain and change the way that we draw it.

So we've got oak leaves, vol, owl.

We've still got a simple food chain.

Let's now add the one above.

Oak leaves, vol, weasel, fox.

We've already got oak leaves is eaten by vols, but let's then move and add a new arrow.

Vol is eaten by weasel, and weasel is eaten by fox.

We've added two of those food chains together and you can now see that there are different food chains represented here.

Let's add the next one up.

Oak leaves is eaten by earthworm, is eaten by hedgehog, is eaten by fox.

So oak leaves is eaten by earthworm, is eaten by hedgehog, is eaten by fox.

And finally, let's add the very last food chain.

Oak leaf is eaten by snail, is eaten by hedgehog, which we've already got in our food web, is eaten by fox, again, we've already got.

So we've now combined those four food chains and we have a food web diagram in a woodland.

So let's see how well you've understood the second part of today's lesson.

The class are asked why a food web diagram is a better model for showing feeding relationships than a food chain diagram.

Here are the answers that the class gave.

Who do you think is right? Alex says producers and primary consumers are usually eaten by more than one organism in the habitat.

Sophia says a food web shows that there is a larger population of producers in a habitat, and Andeep says a food web shows the decrease in populations of organisms as food is transferred in a habitat.

Who do you think is right? I'll give you five seconds.

If you need to pause the video because you need more time, that's fine.

Just remember to restart when you're ready.

So the correct answer is Alex.

Producers and primary consumers are eaten by more than one organism within the habitat.

Well done if you've got that one right.

Let's move on.

So food webs give a more realistic picture of feeding relationships that take place within a habitat.

They showed that organisms feed on more than one other type of organism.

Here is a food web diagram.

You can see all of the different feeding relationships that are taking place.

They're represented by the arrows.

This food web is made up of lots of individual food chains.

Now we know that a food chain always starts with a producer.

Producers make their own food and the food that producers make provides the food for all of the organisms within a food chain.

So let's find a producer in the food web and let's then see what food chains we can see.

You might be able to see that grass as a producer is food for rabbits and that rabbits are food for foxes.

You might also be able to see the food chain that fall in oak leaves are food for worms. Worms are food for thrushes and thrushes are food for buzzards.

Can you see any other food chains in the food web? I'll pause for five seconds so that you can see what food chains you can find for yourself.

So let's look at one of the organisms in this food web.

We're gonna look at the fox.

The fox is an apex predator, but what does the fox eat? We can see from the feeding relationships that the fox eats rabbits, mice, and thrushes.

What do mice eat? I'll give you a few seconds to think about it.

So what you should have come up with is that mice eat grass and wild flowers.

Well done if you've got both of those correct.

So let's do a check to see how well you've understood that last section.

Izzy has drawn a food web diagram to show feeding relationships of organisms in her garden community.

I'm gonna ask you a number of questions that link to that food web diagram that Izzy has drawn.

The first question is how many food chains are shown in Izzy's food web diagram? You may need to pause the video at this stage to be able to count them all.

When you're ready, restart the video, and we'll check your answer.

So the correct answer is five.

Well done if you got that right.

Here are the food chains.

Cabbage, caterpillar, chaffinch, cat.

Cabbage, caterpillar, thrush, cat.

Lavender, caterpillar, chaffinch, cat.

Lavender, caterpillar, thrush, cat.

And finally, lavender, bee, thrush, cat.

Well done if you've got all those right.

Using the same diagram, what eats cabbage? Again, pause the video if you need to.

So caterpillars eat cabbage.

Well done if you got that one right.

What do thrushes eat? Again, I'll give you five seconds.

Pause the video if you want to and when you're ready, press play.

So thrushes eat caterpillars and bees.

Well done if you got those right.

And the final question, which organisms are both predators and prey? Again, pause the video if you need to and when you're ready, restart.

So the answer to that one is chaffinch and thrush.

The chaffinch and thrush are both prey of the cat.

The chaffinch and the thrush are both predators of the caterpillar, and the thrush is a predator of the bee.

Well done if you've got all of those right.

So now let's do a practise task.

You need to look at the food web diagram that's on the the right hand side of the screen and use it to answer the following questions.

Question one is what eats grass? Question two, what do caracals eat? Question three, locusts are prey for which two predators? And question four, I'd like you to write down the four longest food chains in the food web.

I'd also like you to try and label the organisms in your food chain if you can.

Good luck with that task.

You'll need to pause the video and then when you're ready, press play, and we'll check to see how well you've done.

So how well did you do? Let's check your answers.

Question one was what eats grass? Well, the oryx, baboon, and scrub hares all eat grass.

Question two was what do caracals eat? Caracals eat oryx and shrike.

Question three, was locusts a prey for which two predators? The answer to that question were monitor lizards and shrike.

And then question four was to write down and label the four longest food chains in the food web.

Let's see whether you got these right.

Here they are.

Desert bush, locust shrike, lanner falcon.

Desert bush, locust, shrike, caracal.

Grass, scrub hare, baboon, cheetah.

And finally, grass, scrub hare, baboon, lion.

Well done if you got those four longest food chains in the food web and even more well done if you managed to label them firstly with the word's producers, primary consumers, secondary consumers, and tertiary consumers.

And even better if you spotted that the primary consumers are prey, the secondary consumers are predators and prey, and the tertiary consumers are apex predators.

A big well done if you got all of those right.

That brings us to the third and final part of today's lesson, which is all about how population changes can impact a food web.

So if we're ready, let's move on.

In a food chain diagram, changes to one population can have a large effect on other organisms within the food chain.

Let's look at food chain diagram.

Lettuce, snail, thrush, hawk.

If the number of thrushes decrease, think for a moment about how this will impact the other organisms within the food chain.

I'll pause, let you think, and then we'll come back and see what the impacts might be.

So if there's a decrease in the number of thrushes, then the snail population is likely to increase because there are fewer thrushes to eat them.

The number of lettuces in the population will decrease because there are more snails eating the lettuces, and the number of hawk will also decrease because there are fewer thrushes for them to eat.

You can see how the impact of one organism impacts the whole food chain.

We know that food web diagrams show more complex feeding relationships.

Here is the food web that we saw earlier in the lesson, and you can see that we've got thrushes within the food web.

If the population of thrushes decrease in this food web, what impact is there on other organisms within the food web? Well, first of all, foxes will eat more mice and more rabbits because there are fewer thrush to eat.

The buzzards will eat more rabbits, mice and snails, again, because there are fewer thrushes for them to eat.

Less worms will be eaten, and so the population of worms will increase.

And the population of rabbits and mice will decrease, so grass and wild flowers won't be eaten so much and their populations will increase.

So you can see that the impact of one population reducing can impact many other organisms within the food web diagram.

Removing just one organism from a food chain diagram can have many effects.

This is because the organism can be in several different food chains.

Think for a moment, why might taking apex predators from a food chain have a huge impact on a habitat? I'll give you a few seconds to think.

So taking away the apex predator from a food web can have a huge impact on the habitat.

Let's look at one food chain.

Lavender, bee, thrush, cat.

Let's take the cats away.

The population of thrushes will increase because the thrushes are not being eaten by the cat.

The thrushes, now more in number, will eat more bees so the bee population will decrease.

That could be good news for the lavender because less lavender is being eaten because there are fewer bees in the population.

However, bees are really important for the lavender to help them pollinate.

So without the bees, they'll find it harder to reproduce.

So the number of lavender plants may actually decrease.

You can see taking away an apex predator can have a huge impact on the producer population, and the producer population is important in food chains and food webs because we know that producers make the food for all other organisms within the food chain.

Introducing organisms can be dangerous too.

Rats where introduced to New Zealand in the late 1700s as ships arrived from Europe.

Ground nesting birds were very common in New Zealand at that time because there were very few predators that ate their eggs.

The rats quickly found that the eggs were a good source of food and their population increased quickly.

They also eat invertebrates and plants that are native to New Zealand.

And as the rats ate more eggs, the bird populations declined, and many ground nesting birds are now endangered because of the rats.

The New Zealand government still treat rats as pests and are always trying to reduce the number of rats in the population to help conserve the ground nesting birds like the kiwi.

So time now for our final check of today's lesson.

I'd like you to look at the food web diagram.

Imagine all the lavender died.

Read the statements and decide how you feel about each suggestion.

Are you sure that the statement is right? Do you think it's right? Do you think it's wrong? Or are you sure that it's wrong? Decide for each of the following statements.

Statement A, the population of bees could decrease.

B, the population of blackbirds could decrease.

C, the population of cabbages could decrease, and D, the population of caterpillars could decrease.

You may need to pause the video.

When you're ready, press play and we'll check your answers.

Good luck.

Let's check how well you did.

I hope you are confident with your answers.

So the population of bees could decrease is what statement A says.

That's correct.

This is because lavender is food for bees.

If there's no lavender, then there's no food for the bees to eat and their population could decrease.

Statement B says the population of blackbirds could decrease.

That's also true because bees are food for blackbirds.

And so if the number of bees decline, the number of blackbirds will decline because there's less food for them to eat too.

Statement C says the population of cabbages could decrease.

That's also true because caterpillars feed on both lavender and cabbage.

If there's no lavender, then more caterpillars will eat more cabbages and the population of cabbages will reduce.

And because of this, the population of caterpillars could also decrease because the cabbage population is decreasing, which means there's less food for the caterpillars to eat.

Well done if you've got all of those answers.

Let's move on to our final task for today.

Look at the food web diagram.

Imagine all the chaffinches died.

Read the statements then write a sentence to explain why you think the statement is right or wrong.

Statement one is the population of caterpillars could increase.

Statement two, the population of cabbages could decrease.

Statement three, the population of blackbirds could decrease.

And finally, statement four, the population of bees could increase at first and then decrease.

Pause the video, write your answers, and then press play, and we'll check to see how well you did.

Good luck.

So let's imagine what happens when all the chaffinches died.

Statement one said the populations of caterpillars could increase.

That's correct, because their predator, the chaffinch has disappeared.

Statement two said the population of cabbages could decrease.

That's also correct because the population of caterpillars has increased, so more cabbages will be eaten.

Statement three says the population of blackbird could decrease is also correct.

This is because more blackbirds will be eaten by the owls for food.

This is because the chaffinches have all died.

And statement for the population of bees could increase at first and then decrease is also correct.

This is because owls will eat more blackbirds.

So there are fewer predators for bees.

An increase in the caterpillar and bee populations will decrease the lavender population, so the bee's food will become more scarce and the population will eventually decrease.

Well done if you've got all those answers and reasons correct.

So finally, we're going to look at what we've covered in today's lesson about food webs.

We've seen that a community is made up of all the different organisms that live in a habitat.

Interactions that take place between different organisms take place within a community, which include feeding relationships.

We've seen that food web diagrams are models that give a more realistic picture of feeding relationships that take place within a habitat.

And finally, we've seen that changes in the populations of organisms affect all organisms in the food chain.

changes also impact populations in the food web.

This is because the food web is made up of connecting food chains.

Thank you for all your hard work in today's lesson, and I hope that you've had a good time learning with me today.

Look forward to seeing you again soon.

Take care for now.

Bye-bye.