Lesson video

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

Today's lesson is all about pyramids of biomass.

Before we start today's lesson, you might want to just pause the video and go and grab yourself a calculator, as this might be useful as we go through today's lesson.

So just pause the video, and then restart it when you're ready.

By the end of today's lesson, you should be able to create and interpret pyramids of biomass.

There are three keywords in today's lesson.

They are pyramid of numbers, biomass, and pyramids of biomass.

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

You might want to again pause the video so you can have a read through of those definitions before we begin.

But don't worry, because we'll go through those definitions in a little bit more detail as we go through the lesson.

Today's lesson is broken down into two parts.

First of all, we're going to work out biomass, and then in the second part of the lesson, we're going to use that information to help us to construct pyramids of biomass.

So if you're ready, let's get started with our first part of the lesson, which is all about working out biomass.

So if you're ready, let's go.

Food chain and food web diagrams show the feeding relationships between populations of organisms. On the food chain diagram on the screen, we can see the feeding relationship.

Grass is eaten by eastern grey kangaroos, and eastern grey kangaroos are eaten by wedge-tailed eagles.

However, the food chain diagram doesn't show us the number of organisms at each stage or trophic level within the food chain diagram.

The organisms found in a food chain or food web diagram represent a population of organisms. And by counting those populations, the number of organisms at each trophic level, we can produce a pyramid of numbers.

So here's a food chain diagram, wheat, mouse, owl.

We can count the number of organisms within each population in that food chain diagram.

We've seen that there are 10 wheat plants, 5 mice, 1 owl.

We can draw some axes to draw a pyramid of number using those populations.

You can see, I've scaled the axes to allow us to plot the individual populations.

When we start plotting a pyramid of number, we start at the bottom of the pyramid with the producer.

There are 10 wheat plants.

They're plotted on the scales.

There are only 5 mice.

They're the next trophic level, the primary consumers.

There they are on our pyramid.

And finally, we have just 1 owl.

Let's plot that as our secondary consumer.

You can see that there is a pyramid, and that pyramid represents the number of organisms within that food chain diagram.

The problem with pyramids of number is that they're not always pyramids.

Here we have two producers.

We have a grass plant and an oak tree.

The grass plant and the oak tree both count as one organism.

But you can probably understand that a single grass plant isn't going to be much food for many animals, whereas an oak tree probably provides a lot of different types of animals lots of food.

So if we draw a pyramid of number using our population of organisms within the food chains, a grass plant will contain lots of individuals within the population, and so the bottom of our pyramid of number will be large, as shown on the screen.

The grass plant's eaten by snails, they're eaten by birds.

The pyramid is a pyramid shape.

However, the oak tree is just one single organism that supports a large number of caterpillars and a smaller number of birds.

You can see that the pyramid is what we call inverted.

It's not a true pyramid shape.

We can draw different pyramids for the same food chain that show the biomass of organisms rather than the numbers of them.

So, biomass is the mass of living material in one or more organisms. It's the biological mass.

So the biomass, the biological mass, of many caterpillars is much less than the biomass of the one oak tree that they live on.

So if we draw a pyramid of numbers, we can see that the pyramid of numbers is not a pyramid shape, but the pyramid of biomass is a pyramid shape.

That's because the oak tree has a much bigger biomass than the number of caterpillars.

Let's do a check.

Which of the statements are correct? And there's a hint here that there may be more than one.

So is it A, the number of birds is lower than the number of trees? Is it B, the number of birds is higher than the number of trees? Is it C, the biomass of the birds is lower than the biomass of the tree? Or D, the biomass of the birds is higher than the biomass of the tree? I'll pause for five seconds, and then we'll check your answer.

So the correct answer is B and C.

The number of birds is higher than the number of trees, and the biomass of the birds is lower than the biomass of the tree.

Well done if you got that right.

One way to estimate biomass is to measure the mass of an organism.

The mass of this caterpillar on the balance is 3.

21 grammes.

However, the mass includes all of the water within the caterpillar, and water is not a living material.

So, this is only a rough estimate of the biomass of the caterpillar.

However, often, the estimate is good enough to make a useful conclusion about the habitat.

It's really difficult to measure biomass accurately, and there are two reasons for this.

First of all, to measure the biomass, we need to dry it out, we need to remove all of the water, and that means that we have to kill it.

And secondly, if we were gonna measure the biomass within a trophic level, we need to capture all of the organisms so that we can measure the individual biomass and add them all together.

And that's a really difficult, almost impossible thing to do.

Here's another check.

Here's a mouse on a balance.

What's the mass of the mouse? Is it A, 25.

56 kilogrammes? Is it B, 2.

556 grammes? Is it C, 2,556 grammes? Or is it D, 25.

56 grammes? I'll give you five seconds, and then we'll check your answer.

And the answer to the question is D, 25.

56 grammes.

Well done if you got that right.

Here's another check.

Which of the following explains what the term biomass means? Is it A, how many organisms are present, B, the mass of living material in one or more organisms, or C, the mass of water in one or more organisms? Again, I'll give you five seconds as a pause, and then we'll check your response.

And the answer to this one is B, biomass is the mass of living material in one or more organisms, the biological mass.

And remember, that doesn't include the water.

So to estimate the biomass of a population, we can use the number of organisms within the population and the average mass of organisms in the population.

We can estimate the average mass of organisms within a population by sampling it, and that means to select some of the organisms to get an average.

So let's sample the masses of a population of Oak pupils.

Here are our class of Oak pupils.

We've got six pupils, and we're going to select three of them within our sample.

To calculate the average, we need to add the measurements together that we've taken, so the masses of each of the three individuals in the class, and divide by the number of measurements that we've taken.

So let's take their masses.

Let's measure the masses.

Aisha is 50 kilogrammes, Alex is 55 kilogrammes, and Sofia is 45 kilogrammes.

We need to now add the masses together.

If you want to have a go for yourself, I'll give you a few seconds to calculate the masses.

You can also pause the video if you want to, and then when you're ready, just press play.

You should have added 50 plus 55 plus 45, and that gave us 150 kilogrammes as the total mass for the three pupils in our sample.

The number of measurements that we've taken is three, so we now need to divide 150 by 3.

Again, if you want to, I'll just pause for five seconds to allow you to work that out for yourself.

And the answer there is 150 divided by 3, and so we've got an average, or a mean, of 50 kilogrammes.

So the average mass of individuals in the Oak pupil population is 50 kilogrammes.

To estimate the biomass, we can use the equation, the number of individuals in the population multiplied by the average mass.

So we know that there are six individuals in the population, and we know that their average mass is 50 kilogrammes.

So, do you want to work that out? I'll give you five seconds.

So the total biomass in this population is 300 kilogrammes.

Well done if you worked that out for yourself.

So let's do a check.

Lucas has been recording the masses of a sample of grasshoppers in the school field, and here are the masses of the grasshoppers that he's managed to sample.

He's measured five samples.

One is 200 milligrammes, one is 190 milligrammes, one is 210 milligrammes, the fourth is 205 milligrammes, and the fifth, 195 milligrammes.

So the total mass is 1,000 milligrammes.

The number of samples is five.

I'd like you to work out the average mass of the grasshoppers.

Is it A, 195 milligrammes, B, 200 milligrammes, C, 205 milligrammes, or D, 210 milligrammes? You'll probably need to pause the video at this point, but I'll give you five seconds.

And remember, you can use that calculator if you want to.

So the answer to the question is 200 milligrammes.

It's 1,000 divided by 5.

Well done if you got that right.

We're now gonna move on to a task.

Here's a grassland food chain diagram, grass, rabbit, fox.

The number of organisms in each population has been counted, and these are shown in a pyramid of number.

You can see the pyramid of number on the screen now.

Scientists have been working to determine the average mass of an individual organism.

And the average mass has been calculated as grass, 4 grammes, rabbit, 2 kilogrammes, and fox, 8 kilogrammes.

You need to estimate the biomass of each population within this food chain.

I'll pause the video at this point.

You can then work out your answer, and then we'll check to see how well you've done.

Good luck.

You were asked to estimate the biomass of each population in the food chain grass, rabbit, fox.

You should have used this equation.

Let's see how you did for each of those three populations.

Let's look at the producer, the grass, first.

So the grass has 200,000 organisms in the population, and each organism has an average mass of 4 grammes.

200,000 multiplied by 4 gives us 800,000 grammes.

The rabbit has a population of 100, and each rabbit has an average mass of 2 kilogrammes.

So 100 multiplied by 2 gives us 200 kilogrammes.

And then finally, the fox population, there are 5 individuals in this population, and each has an average mass of 8 kilogrammes.

8 times 5 is 40 kilogrammes.

So well done if you estimated the biomass of each of those populations correctly.

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

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

Pyramids of biomass are constructed in the same way as pyramids of number, and we estimate the biomass at each trophic level and draw boxes to represent the populations.

So, the first level is the biomass of the producers.

We draw that at the bottom of our pyramid.

The next level is the biomass of primary consumers.

The third level is the biomass of secondary consumers.

And the fourth level is the biomass of tertiary consumers.

And we can see it forms our pyramid shape.

To construct a pyramid of biomass, we can count or estimate the number of individuals in each trophic level in the food chain.

We can measure the mass of a sample of individuals within each population within the habitat.

And we can calculate the average mass of individuals for each population within that habitat.

And that gives us the estimate of the total biomass of organisms at each trophic level within the habitat.

Let's look at this really simple food chain diagram, bamboo, giant panda.

We're going to try and estimate the total biomass for these two populations.

In order to do that, we need to know some pieces of information.

We need to know the number of individuals in each population, and we need to know the average biomass of an individual within each population.

If you've got your calculator handy, get ready to try and work this out for yourself as we go through this example.

So, we've counted the number of individuals of bamboo.

We've counted 100,000 individuals making up the bamboo population.

We've also determined through sampling that the average biomass of an individual plant is 40 kilogrammes.

So do you think you can work out the total estimated biomass of this bamboo plant population? Remember, to do that, you need to multiply the number of individuals by the average biomass of each individual.

I'm gonna pause for five seconds while you try and do that calculation, using your calculator if you need to, and then we'll see whether you got the answer right.

So, the correct answer is 100,000 multiplied by 40, and that should give you a total estimated biomass of 4 million kilogrammes for the bamboo population.

Well done if you managed to calculate that one correctly.

Let's move on to the giant panda.

We've counted the number of individuals within this population, and we've seen that there are 40.

The average biomass of each individual panda is 110 kilogrammes.

I'm gonna pause again to see whether you can work out the total estimated biomass of the giant panda population.

And the calculation you should have done here is 40 multiplied by 110, and that gives you a total of 4,400 kilogrammes as your estimated biomass of the giant panda population.

Let's look at those two estimated biomasses and see whether we can plot them in a pyramid of biomass.

We're going to use a scaled pyramid.

So that means we need to put a scale on the bottom that allows us to plot the total biomass for each population.

We've got 4 million kilogrammes that we need to plot as our maximum number.

So, using our centre line as a starting point, I've gone 2 million kilogrammes on each side.

We're then going to plot the bamboo at the bottom of the pyramid of biomass because it's the producer.

And then at the second level, we're going to plot the giant panda.

That's the primary consumer.

Here's our pyramid of biomass.

Here's a check.

Scientists have counted 10,000 penguins in an area of the Antarctic.

They've sampled the penguins and estimated that the average biomass for each individual is 30 kilogrammes.

What's the estimated biomass of the penguin population? Is it A, 3,000 kilogrammes, B, 30,000 kilogrammes, C, 300,000 kilogrammes, or D, 3 million kilogrammes? If you need to pause the video at this point to use your calculator, that's fine.

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

And the correct answer is C, 300,000 kilogrammes.

Well done if you got that right.

Let's look at this woodland food chain diagram, oak tree, caterpillar, blackbird, buzzard.

We've gathered some information about the population of organisms at each trophic level within this food chain.

That's summarised in the table that's now on the screen.

You can see first of all that we've counted the number of individuals in each population.

That's shown in the first row of the table.

There's 1 oak tree, 10,000 caterpillars, 200 blackbirds, and 6 buzzards.

We've also sampled the population to work out the average biomass of an individual within each population.

And you can see that in the second row of the table.

Each individual oak tree has an average biomass of 1,500 kilogrammes, caterpillar, 3 grammes, blackbird, 100 grammes, and buzzard, 1 kilogramme.

We can use the number of individuals that are in the populations to draw us a pyramid of number.

In order to work out and draw a pyramid of biomass, we need to calculate the estimated biomass of the population, and we've seen in the first part of the lesson how to do that.

We multiply the number of individuals by the average biomass of each individual.

We can do this for each of the four populations.

Let's try the oak tree first.

We have one individual, and its average biomass is 1,500 kilogrammes.

So the calculation that we need to do is 1 multiplied by 1,500 kilogrammes, and that gives us an estimated biomass of the total population of oak trees as 1,500 kilogrammes.

I'm going to see if you can work out the estimated biomass of each of the populations of caterpillars, blackbirds, and buzzards.

So you'll need to pause the video at this point.

You can use a calculator if you need to.

Work out the estimated biomass of each population, and then when you've got some answers, press play, and we'll see if you got the answers right.

Good luck.

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

So for the caterpillars, we've got 10,000 individuals, and each of them weigh 3 grammes.

So 10,000 multiplied by 3 grammes gives us 30,000 grammes.

Well done if you got that one.

The blackbirds, we have 200 individuals.

Each of them weighs an average of 100 grammes.

So our calculation is 200 multiplied by 100 grammes, and that gives us a total of 20,000 grammes as our estimated biomass of the blackbirds.

Well done if you managed to get that one.

And finally, the buzzards.

We've got 6 buzzards.

Each of them has an average biomass of 1 kilogramme.

That gives us a calculation of 6 multiplied by 1 kilogramme.

That's 6 kilogrammes as our estimated biomass of the population.

Well done if you managed to get that one right too.

So now we've got two pieces of information about our food chain diagram.

We've got the total number of individuals in each population, and we have an estimate of the total biomass of each population of organism that's found within that food chain diagram.

Looking at the total biomass, you may notice that the caterpillar and the blackbird are both recorded in grammes, and the oak tree and the buzzard are both recorded in kilogrammes.

So the first thing that I'm going to do is to convert our grammes into kilogrammes, and we do that by dividing by 1,000 because there's 1,000 grammes in a kilogramme.

So let's look at the caterpillar.

30,000 grammes divided by 1,000, that gives us 30 kilogrammes.

And the blackbird, 20,000 grammes divided by 1,000 gives us 20 kilogrammes.

So now we've got all our units the same.

And what we're going to do is we're going to draw, first of all, a pyramid of number for the food chain diagram, and then we're gonna compare it to a pyramid of biomass for the same food chain diagram.

We're gonna start with our pyramid of number.

We know that our maximum number is the number of caterpillars, because that's 10,000.

So you can see that I've gone 5,000 each side of my zero line.

We're gonna start with the producer at the bottom.

That's the oak tree.

There's 1 oak tree, there are 10,000 caterpillars, there are 200 blackbirds, and there are 6 buzzards.

You can see our pyramid of number isn't a true pyramid.

Let's compare it with a pyramid of biomass.

And you can see that our maximum number here is 1,500 kilogrammes, so we're going to go 750 kilogrammes either side of our zero line.

We're going to again start with our oak tree at the bottom, then the caterpillar, then the blackbird, and finally at the top of our pyramid, the buzzard.

And you can see that in the pyramid of biomass, there is a pyramid shape.

So pyramids of biomass are better than pyramids of number as they're always a pyramid shape.

However, pyramids of biomass only show the biomass of populations at each trophic level at a single point in time.

To give an example, the biomass of an oak tree will change during the course of a year.

Why do you think that is? Yes, that's right, because during the winter, its lost its leaves, and the leaves contain biological material which contribute to the biomass.

In the summer, those leaves grow, and so the biomass increases.

Can you think of any other organisms that might change their biomass during the course of a year? So animals that might change their biomass during the course of the year are those that hibernate over winter.

When they're hibernating, they're not eating, and so they're using their food reserves.

So in essence, their biomass is getting less and less over the course of the winter.

Let's do a check.

A hedgehog hibernates during the cold winter months.

What happens to the biomass of the hedgehog during hibernation? Is it A, the biomass increases as the hedgehog is asleep, B, the biomass decreases as the hedgehog isn't eating, or C, the biomass stays the same as the hedgehog isn't moving or eating? I'll give you five seconds, and then we'll check your answer.

So the correct answer is B, the biomass decreases as the hedgehog isn't eating.

Well done if you got that right.

So let's move on to a practise task.

Lucas has gathered some information about three organisms within a food chain.

They are oak tree, aphid, ladybird.

Here's the data that Lucas has gathered.

He's gathered the number of individuals in each population in the food chain, and he's sampled the population to calculate the biomass of one individual on average of each population within the food chain.

You need to use the information to first of all draw a scaled pyramid of number, and then secondly, to draw a scaled pyramid of biomass.

Here's a hint just to remind you that 1,000 grammes is equal to 1 kilogramme.

You'll need to pause the video at this point, draw your pyramid of number and your pyramid of biomass, and then when you're ready to check your answer, press play, and we'll see how well you've done.

Good luck.

So how did you do? First of all, I asked you to draw a scaled pyramid of number.

Well, we've got 1 oak tree, 10,000 aphid, and 200 ladybirds.

So, the first thing that you should have drawn with your pyramid of number is scaled axes.

There are 10,000 aphids, so you need to go at least 5,000 each side of your zero line.

We're going to plot our pyramid of number, starting with our producer, our first trophic level.

There's only 1 oak tree in the population of producers.

We're then going to plot the number of aphids, our primary consumers.

There they are, 5,000 either side of the zero line.

And then finally, the number of ladybirds is 200, and there's our ladybird population.

So you can see, it's an inverted pyramid.

It's not a true pyramid shape.

Let's move on to our scaled pyramid of biomass.

The first thing that you needed to do here was to calculate the biomass of the population.

And remember, we do that by multiplying the number of individuals in the population by the biomass of one individual.

So, oak tree, 1 multiplied by 2,000 kilogrammes gives us 2,000 kilogrammes as the biomass of the population.

To work out the biomass of the population of aphids, 10,000 multiplied by 1 gramme, and that gives us 10,000 grammes.

Now remember, 1,000 grammes is equal to a kilogramme, so you should have converted that to 10 kilogrammes.

And finally, 200 ladybirds multiplied by 2 grammes for the biomass of one individual, and that gives us a total of 400 grammes.

We can convert that by dividing by 1,000, and that means that the total biomass of ladybirds is 0.

4 kilogrammes.

We then need to draw our scale pyramid, and we can see that our maximum biomass is 2,000 kilogrammes.

So we're going to go 1,000 either side of our zero line, remembering to use our units, kilogrammes.

And then we're gonna draw our producer, the oak tree, the aphid, and then the ladybird.

And you can see the pyramid of biomass is a pyramid.

Well done if you got those right.

So in summary, what we've looked at in today's lesson is that food chain and food web diagrams show the feeding relationships between organisms. Pyramid of numbers can be used to show the number of organisms at each trophic level.

But the pyramid of numbers don't take the size of organisms into account, and that means that some of the pyramids are unusual shapes.

Biomass is the mass of living material in one or more organisms. And remember, we're measuring the mass of living material, and that doesn't include water.

And the pyramids of biomass are diagrams which show the biomass of living material at each trophic level within a food chain diagram.

And we've also seen how we can estimate biomass by including the water in our calculations.

I hope you've enjoyed today's lesson.

Thanks for learning with me, and I hope to see you again really soon.

Bye-bye for now.