Lesson video

Hello, I'm Mr. Jarvis, and I'm gonna be taking you through today's lesson, which is in the unit biodiversity.

The second lesson in this unit is called pyramids of number.

During today's lesson, you are going to learn how to create and interpret pyramids of number to show the size of a population at each trophic level.

There are five key words that we're going to look at in today's lesson.

These are herbivore, carnivore, trophic level, omnivore, and pyramid of number.

Here are the definitions coming up on the screen of each of those words.

You might want to take a little bit of time here to pause the video and read through them for yourself, but don't worry because we're going to go through each of those definitions as we come to them during today's lesson.

Today's lesson is broken down into two parts.

First, we're going to look at what food chain diagrams are, and then we're gonna move on to talk about how we construct pyramids of number.

So if you're ready, let's get started with our first part to today's lesson, which is all about food chain diagrams. A food chain diagram shows how food is passed from one type of organism to another within a habitat, through a process that we know as feeding.

Here's an example of a food chain diagram.

Wheat is food for a mouse, and a mouse is food for and owl.

The arrows show the direction that the food is transferred along the food chain.

That food chain diagram can be simplified into simply wheat, mouse owl.

All food chains start with a producer and producers make all of the food for the rest of the organisms in the food chain to eat.

So here's another example of a food chain.

Oak tree, aphid, ladybird, blue tit, wildcat.

The oak tree is the producer.

It makes all of the food for the rest of the food chain to eat.

The aphid, the ladybird, the blue tit and the Wildcat are all consumers.

So let's do a quick check.

Which type of organism always starts a food chain and why? is it A, a producer as they eat other organisms, B, a consumer as they eat other organisms, C, a consumer as they make their own food, or D, a producer as they make their own food? I'll give you five seconds.

You can pause the video if you want to, and then we'll check your answer.

The correct answer for this question is D.

A producer is the type of organism that always starts a food chain.

And why? Because they make their own food.

Well done if you got that right.

So let's look at that food chain again.

Oak Tree, aphid, ladybird, blue tit, wildcat.

Within this food chain diagram, the oak tree is the producer, and we know that producers make their own food from lightened chemicals in their surroundings.

For example, the carbon dioxide that's in the air, the oak tree makes the food for the whole of the food chain diagram.

So the aphid, the ladybird, the blue tit, and the wildcat all rely on the oak tree to get them the food that they need.

And remember, the organisms within a food chain diagram represent populations of those organisms. So we're talking about populations of oak tree, populations of aphid, populations of ladybird and so on.

The aphid is a consumer.

The first consumer within the food chain is called the primary consumer.

And a consumer eats other organisms to get the food that they need.

They can't make the food for themselves.

Primary consumers are always herbivores and herbivores, eat producers or plants.

So the Ladybird is also a consumer.

The second consumer in the food chain is called the secondary consumer.

And secondary consumers always eat primary consumers, the herbivores.

Secondary consumers though, are carnivores and a carnivore eats other animals to get the food that they need.

The blue tit is the third consumer in the food chain.

And the third consumer in the food chain is called the tertiary consumer.

And tertiary consumers always eat secondary consumers.

Tertiary consumers are also carnivores.

They rely on eating other animals to get the food that they need.

The carnivores that eat other carnivores are called predators, and the organisms that they eat are called prey.

And finally, the wildcat.

The wildcat is also a consumer.

The fourth consumer in the food chain is called the quaternary consumer.

And they always eat tertiary consumers.

Quaternary consumers are also carnivores.

They eat other animals to get the food that they need.

The last predator in a food chain diagram, which the wildcat is, is known as the apex predator.

So we've seen that organisms get their food in different ways.

Look at the pictures on the screen.

We have a tree getting its food from the light and the chemicals within its surroundings.

We've got a chicken and a giraffe eating plant, material, grain or leaves, and we've got carnivores, a seagull and a lion, both eating other organisms that are animals.

Each of the feeding groups that we've seen within the food chain diagram, and by that I mean the producer, the primary consumer, the secondary consumer, the tertiary consumer, and the quaternary consumer is called a trophic level.

And a trophic level is the position of an organism within a food chain diagram.

Let's look at the words trophic level.

Trophic comes from the Greek word meaning to feed and level means a position or rank.

So it's the rank of feeding that we're talking about, the position of feeding within a food chain diagram, which is called the trophic level.

The first trophic level is always a producer.

The second trophic level is always a primary consumer.

Here's another food chain diagram, lettuce, snail, blackbird, cat.

The first trophic level is the producer, and that's the lettuce.

The second trophic level is the primary consumer.

That's the herbivore.

What do you think the blackbird is? Which trophic level is it? What type of consumer is it? And is it a herbivore or a carnivore? I'll give you five seconds to think.

That's right.

The third trophic level is the blackbird.

It's a secondary consumer and it's a carnivore.

And finally, the cat is the fourth trophic level, the tertiary consumer, and also a carnivore.

I'd like you to look at this food chain diagram, and I've taken the organisms away and just called them A, B, C, and D.

So we've got organism A is food for organism B, is food for organism C, is food for organism D.

And there's a few questions here that are all based on the same food chain diagram.

So first of all, which trophic level does organism C occupy? Is it A, a, producer, B, a primary consumer, C, a secondary consumer, or D, a tertiary consumer? I'll give you five seconds and we'll check your answer.

The correct answer is C, a secondary consumer.

Well done if you've got that one.

Here's another.

Same food chain diagram, organism A, organism B, organism C, organism D.

Which of the following words correctly describes organism? C? Is it A, a, producer, B, a herbivore, C, a, carnivore, or D, an apex predator.

Again, five seconds to get your answer.

And the correct answer this time is C, a carnivore.

Here's another question based on that same food chain diagram, which organism is the apex predator? Is it A, organism A, B, organism B, C organism C, or D, organism D? Again, five seconds.

And then we'll check to see if you got it right.

And the answer this time is D.

Organism D is the apex predator.

Well done if you got that one.

And the last question, same food chain diagram, which organism is the producer? Is it A, organism, A, B, organism B, C organism C, or D, organism D? Five seconds and then we'll check your answer.

And the answer to this one is A, organism A is the producer which makes the food for the rest of the food chain.

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

A food chain diagram is a simple version of feeding relationship.

Actually feeding relationships are much more complex.

More complex feeding relationships are shown by food web diagrams. They show how all of the food chains interact with each other.

And here's an example.

So you can see in this food web diagram, we have wheat being eaten by a number of different organisms. Each of the pathways are food chain diagrams. What food chains can you spot in the food web diagram? I'll pause for five seconds so that you can spot some of the food chains for yourself, or you can stop the video and then press play when you are ready to carry on.

How many did you spot? We're gonna look at the food chains that were included in the food web diagram, but I'm going to also highlight the trophic levels of each organism.

And so at the bottom of the slide, you can see that there are the trophic levels of producer, primary consumer, secondary consumer, and tertiary consumer.

I've also labelled those trophic levels, which are carnivores and herbivores.

So let's look through our food chains within this food web diagram.

First of all, we have wheat, human.

Moving along, we've got wheat, pheasant, human.

Another food chain is wheat, pheasant, hawk.

A fourth food chain is wheat, beetle, thrush, hawk.

A fifth food chain is wheat, locust, thrush, hawk.

Well done.

If you spotted all five of those food chain diagrams. We can see that each of those food chains start with a producer, and the producer is always the wheat.

Let's look at another organism within the food chain diagrams on the screen.

Let's look at the human.

So in the first food chain on the screen, we can see it's a simple one.

Wheat, human.

Humans eat the wheat.

Humans are eating the producer, and that means that humans are in this food chain diagram, a herbivore.

Let's look at the second food chain, wheat, pheasant, human.

And in this food chain, the human is a secondary consumer.

It's not eating the producer, it's eating the primary consumer, which is a pheasant.

The pheasant is an animal, and that means that the human in this food chain is a carnivore.

So let's look at that in a bit more detail.

We've got wheat, human, where the human is a herbivore and wheat, pheasant, human, where the human is a carnivore.

In most communities that are shown in a food web diagram, organisms eat more than one thing.

And organisms that eat both producers and consumers are called omnivores.

So omnivores eat both plants and animals.

Most humans are omnivores because generally speaking, humans eat plants and animals.

Let's do a check of that last part of the lesson.

I'd like you to look at the food web diagram that's on the screen.

There's gonna be a few questions based on the same food web.

The first check is which word describes the monitor lizard? I've highlighted the monitor lizard in a purple box.

Is it A, a producer, B, a herbivore, C, a carnivore, or D, an omnivore.

Five seconds to get your answer and then we'll check.

The answer to this is a carnivore.

Why is the mono lizard a carnivore? Because it's eating a locust.

We can look at the arrow that's pointing to the mono lizard and the locust is another animal.

And remember, organisms that eat other animals are called carnivores.

Well done if you've got that.

Same food web diagram for the second check.

This time, which word describes the deserts bush? Here's a desert bush in the purple box.

Five seconds to get your answer.

Is it A, a producer, B, a herbivore, C, a carnivore, or D, an omnivore? Good luck.

And the answer to this one is A, a producer.

The desert bush is making the food for all of the food chains that it's part of.

Another question with the same food web diagram.

This time we are looking at the locust.

Which word describes the locust? Is it a, a producer, B, a herbivore, C, a carnivore or D, an omnivore.

Five seconds.

And then we'll check your answer.

And the answer this time is be a herbivore.

Last one.

This time we're looking at the baboon.

The baboon is again in the purple box on the screen.

Which word describes the baboon? Is it a producer, a herbivore, a carnivore, or an omnivore? Five seconds.

And then we'll check.

And the answer this time is an omnivore.

And it's an omnivore because if you look at the arrows that are pointing towards it, grass is food for a baboon and so is a scrub hare.

So the baboon eats a producer, the grass, and a primary consumer, an animal, the scrub hare.

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

Now it's time for a practise task.

Here's another food web diagram.

This time it's from the C.

You need to look at the food chains within the food web diagram and decide whether each organism is a producer, a herbivore, a carnivore, or a omnivore.

You need to design a table to present your answers.

So pause the video, draw a table with the headings, producer, herbivore, carnivore, and omnivore, and then decide which organisms fit into each of those categories.

When you've completed your table, then you can press play and we'll check your answers.

Good luck.

So how did you do? I hope you didn't find it too tricky.

I asked you to look at the food chains within the food web diagram from the sea.

For each organism decide whether they're producers, herbivores, carnivores, and omnivores and present your answers in a table.

So here are the answers that you should have got.

So the producers are the phytoplankton and the seaweed.

They make the food for all of the organisms within the food web diagram.

The herbivores are the organisms that only eat the producers, and they are zooplankton, clam, and tang fish.

The omnivores eat both plant and animals.

And the only omnivore in this food web diagram is the triggerfish.

The triggerfish eats phytoplankton and seaweed, but it also eats clams. And so it eats both plants and animals.

And the carnivores in the food web are the pufferfish, the tuna, the mantaray and the tiger shark.

That's because they only eat other animals.

Well done if you've got those right.

That brings us to the second part of the lesson today, and that's all about constructing pyramids of number.

So if you're ready, let's get started.

So we've seen that food chain and food web diagrams show the feeding relationships that exist within a community.

And each organism within a food chain or food web represents a population of that organism.

So to remind you, a population is the number of organisms of the same type within a specific area.

So an example is the population of fish in a pond, a community are all the populations that exist in a particular area.

So here we've got a community of bushes and giraffe and bison and ostrich and bushbuck that are on the African plane.

Let's look at this food chain.

Grass, caterpillar, shrew, falcon.

That food chain just tells us the feeding relationships, but it may take many producers to feed one herbivore, so lots of grass plants to feed one caterpillar.

Likewise, it may take many herbivores, caterpillars, to feed one carnivore the shrew.

And it may need many carnivores, many shrew, to feed another carnivore, an apex predator.

We can count how many organisms there are at each trophic level and draw boxes to represent the populations.

So let's look at the food chain grass, caterpillar, shrew, falcon.

We can count the number of organisms in each population and we'll record them underneath the food chain diagram.

We've got 1,000 grass plants, 100 caterpillars, 10 shrew, and one falcon.

So let's draw some boxes to represent the population.

First of all, we're going to start with the start of the food chain, the grass plant.

Let's draw a box.

We've got a thousand grass plants, so it's a big box.

The number of producers is large.

We are then going to draw a box on top of this to show the number of caterpillars.

That's the second level.

The number of primary consumers.

And the box is smaller because there's less caterpillars in this food chain.

At the next level, we're going to draw the number a box to show the number of secondary consumers, the shrew.

And again, the box is smaller because there are less shrew than there are caterpillars.

And finally, the fourth level is the number of tertiary consumers, the number of falcons, which in this case is only one.

And you can see that we've got smaller and smaller boxes less sitting on top of each other because there are less and less organisms within this food chain at each trophic level.

If we look at the shape of it, it forms a bit of a pyramid.

And this type of diagram is therefore called a pyramid of number.

We can plot how many organisms there are at each trophic level using a scale and draw boxes to represent the populations.

In the last example, we just drew boxes that were slightly smaller at each level, but we can actually draw them to scale.

Here we've got wheat, mouse, owl as our food chain diagram, and we've counted the number of organisms in each population.

There are 10 wheat plants, five mice, and one owl.

So the first thing that we need to do is to draw a scale.

And we can see that we need at least 10 as our maximum population size, and we want the boxes to sit in the middle of each other as we draw them at each level.

So I've drawn a middle line and five one side, five the other which makes 10.

Here's our wheat plants, five on one side, five on the other.

That's 10 organisms. There are five mice.

And so let's draw a box on top of that that represent the mouse population.

And you can see we're going to two and a half for each side of the line, which gives us a population total of five.

And finally we've got the owl population, which is just one.

And you can see with 0.

5 and 0.

5 each side of the line, which gives us that population of one.

But this gives us a scaled pyramid of number.

Pyramid of number don't always take into account the size of organisms at each level.

And to give you an example, here are two producers.

A grass plant makes its own food for food chains, and an oak tree makes its own food that provide foods for food chains.

The grass plant and the oak tree both counts one organism, one grass plant, one oak tree.

However, the oak tree provides food for many more primary consumers than a single grass plant can.

And that means that some pyramid of number have shapes which are not pyramids.

So here's an example of a food chain diagram.

Elm tree, greenfly, spider robin, fox.

Let's count the number of organisms we've got one elm tree, 10,000 greenfly, 2000 spiders, 100 robins and five foxes.

We're going to draw a scale and then plot those organisms on that scale.

And we can see that the producer at the bottom of this food chain diagram only has one organism.

The greenfly has 10,000.

So there's a big box that we draw for the greenfly, the secondary consumer, the spider 2000.

We can plot that.

And it's a smaller box than the greenfly, but much bigger than the elm tree.

The robin, only a hundred, a smaller box than the greens, than the greenfly and the spider and the fox, again, a smaller box as a quaternary consumer because there's only five organisms. But you can see it doesn't really make a pyramid.

It actually makes what we call an inverted pyramid of number, where the number of organisms at the bottom of the food chain is very small because the producer is large and provides food for lots of smaller organisms. So let's check to see how well you understood that last section.

A pyramid of number shows, A, how many organisms there are in a food web.

B, the number of individual organisms at each trophic level in a food chain.

C, the number of populations of organisms at each trophic level in a food chain.

Or D, how big the organisms are at each trophic level within a food chain.

I'll give you five seconds and then we'll check your answer.

So the correct answer is B, a pyramid of number shows the number of individual organisms at each trophic level in a food chain.

Well done if you got that.

Let's move on to a practise task.

Jacob has drawn three pyramids of number for this food chain, oak tree, caterpillar, goldfinch, tawny owl.

Here's A.

Here's B.

Here's C.

Which of Jacob's pyramid of number best represents this food chain? Write an explanation for your answer.

You'll need to pause the video at this point, and then when you've got your answer, press play and we'll check to see how well you've done.

Good luck.

So which of Jacob's pyramid of numbers best represented the food chain? And the answer was B, it's this shape pyramid of number.

So at the bottom of the food chain, we've got the producer, the oak tree, and there's not going to be many of those.

We've then got the primary consumer, the caterpillar.

There's going to be lots of caterpillars on each oak tree.

So the numbers are gonna be high.

The number of goldfinches that are eating the caterpillars are gonna be smaller, and the number of tawny owls as a tertiary consumer are gonna be small too.

So that's the shape.

So here are the reasons why.

There are small number of oak trees.

There's lots of caterpillars that eat the oak trees leaves.

There are fewer goldfinches to eat the caterpillars, and there are fewer tawny owls to eat the goldfinches.

Well done if you've got those right.

Here's another practise task.

I'd like you this time to draw a labelled pyramid of number for the following food chain.

Include a scale in your pyramid.

The food chain is grass, grasshopper, frog, owl.

And you can see I've put the number of organisms in each population in brackets underneath each organism.

So you need to now pause the video again, draw your pyramid of number and label it.

And when you're ready, press play and we'll see how well you've done.

Good luck.

How did you get on with that? I hope that you didn't find it too tricky.

So you are asked to draw a labelled pyramid of number for this food chain.

Let's start with what your pyramid of number should look like.

You should have started with a scale so that you can plot the numbers as accurately as possible.

I've gone to 8,000 on each side of my midway line.

That's because there are 16,000 grass plants.

So here are our grass plants.

The grass plants are producers.

I've labelled my box.

The second level of a my pyramid are the grasshoppers.

They're the primary consumers.

They're 6,000 of those.

So the box should be smaller.

Smaller still are the number of frogs, which are the secondary consumers.

And finally, smaller still, the number of owls, only five of those as tertiary consumers.

So your pyramid of numbers should look something like this.

Well done if you did that and got it correct.

That brings us to our summary of today's lesson, which has been all about pyramids of number.

We've seen that food chain and food web diagrams show the feeding relationships between organisms. We've seen that producers make their own food and provide food for the rest of the food chain.

We've learned about herbivores being animals that feed on producers and carnivores consume other animals.

And an organism that feeds on both animals and plants is called an omnivore.

We've looked at what atrophic level is the position that an organism has within a food chain diagram.

For example, a producer, a primary consumer, a secondary consumer, and so on.

And finally, a pyramid of number is a diagram that shows the number of organisms in a food chain at each of those trophic levels.

Thanks for all of your hard work today.

It's been great as always being with you, and I look forward to seeing you all again soon.

Bye for now.