Lesson video

Hello and welcome to today's lesson.

This is the second lesson in the unit: Ecosystems. We're going to be looking today at predator-prey relationships.

And by the end of the lesson, you should be able to explain the relationship between predator and prey populations.

My name's Mr. Jarvis, and I am going to be teaching you today.

During the lesson, we're going to be looking at three key words.

They are predator, prey, and population.

Here are the definitions of these words.

I'll let you read them yourself, but if you need some more time, then please pause the video.

But I'll explain them to you as we go through the lesson.

So today's lesson is broken down into three parts.

First of all, we're going to look at what predators and prey are.

Then we're gonna move on to how populations change.

And then thirdly, we're going to look at how those changing populations impact on predators and prey.

So if you're ready, then let's go with our first part, predators and prey.

So consumers are organisms that eat other organisms in order to survive.

In a food chain diagram, there's often a lot of consumers.

So let's look at this food chain.

Grass is eaten by grasshoppers.

Grasshoppers are eaten by frogs.

Frogs are eaten by snakes.

Snakes are eaten by hawk.

You can see from the food chain, the grasshopper, the frog, the snake, and the hawk, all consumers.

Producers always begin a food chain diagram.

They provide the food for all of the other organisms that sit within the food chain.

In this food chain, the grass is the producer.

The grass is eaten by the grasshopper.

All the other organisms in the food chain, as we've already seen, are consumers.

We refer to the consumers by the position that they hold in the food chain.

So the grasshopper is the first consumer.

It's the primary consumer.

The frog eats the grasshopper.

It's the secondary consumer.

The second organism to eat within the food chain.

The snake is called the tertiary consumer.

That means the third consumer in the food chain, because it eats the frog that eats the grasshopper, that eats the grass.

And finally, we have the hawk.

The hawk is known as the quaternary consumer.

Quaternary means fourth, and the hawk is the fourth consumer in this food chain.

The primary consumer always eats the producer.

All of the other consumers eat other animals.

An animal that hunts and eats other animals is called a predator.

The animals that the predator catches and eats are called prey.

So let's look at this food chain in a bit more detail.

We already know that the grass is the producer, it's the start of the food chain.

But what about the predator and prey? So first of all, we're going to look at the primary consumer, the grasshopper and the secondary consumer, the frog.

The frog is a predator of the grasshopper.

The grasshopper is the prey of the frog.

Let's move on.

Let's look at the frog and the snake.

What do you think? But the snake is a predator of the frog and the frog is the prey of the snake.

We're now going to look at the snake and the hawk.

I'm going to give you a few seconds to think about which is the predator and which is the prey.

You should have said that the hawk is the predator of the snake and the snake is the prey of the hawk.

Well done if you got that right.

The hawk isn't prey for anything.

It sits at the top of the food chain.

An organism at the end of the food chain diagram is referred to as a top or apex predator.

And an apex predator isn't prey for any other organisms. So in this food chain, the hawk is the apex predator.

So let's check what you've understood from the first part of this lesson.

I'd like you to look at the following food chain diagram from a desert.

I'd like you to match the organism with the correct statement.

So first of all, the food chain is cactus, wood rat, rattlesnake, fennec fox.

Those are the organisms that appear in the boxes on the left-hand side of the screen.

And I'd like you to match them with the statements from the right-hand side.

The statements are, is a predator and a prey, is an apex predator, is prey, and is neither a predator or prey.

You may need to pause the video at this point to allow you to match the organism with the correct statement.

When you're ready, press play.

So here are the answers that you should have got.

The cactus is neither a predator or a prey.

Why? Because it's a producer.

It makes the food for the rest of the food chain.

The wood rat is the primary consumer.

And so it feeds on the cactus.

It doesn't hunt for food, but it is prey for the rattlesnake.

So the wood rat is prey.

The rattlesnake eats the wood rat, which is a consumer, and it's hunted by the fennec fox.

So the rattlesnake is both a predator and a prey.

And finally, the fennec fox is at the top of the food chain.

And we've seen that organisms that sit at the top of the food chain are called apex predators.

How did you do? I hope you did well.

But now let's give you some practise.

So Sam has five cards, each of which has an organism on it.

Those organisms are seal, algae, polar bear, cod, and shrimp.

I'd like you to help Sam by organising these cards into a food chain diagram.

Once you've organised the food chain, then I'd like you to draw it out and then label it with some of the words from the box below.

Not all of the words that are in the box are right answers.

So you need to carefully select which of the words are correct and add those to your food chain diagram.

You'll need to pause the video, write your food chain, and use the words in the box to label that food chain.

And then when you're ready, come back and we'll check your answer.

Good luck.

So how did you do? You were asked to use the cards to draw and label a food chain diagram.

Well, this is the right order of the food chain diagram.

Algae, shrimp, shrimp, cod, cod, seal, seal, polar bear.

There's the food chain diagram on the screen.

And I've used one of the words, a model in my title.

It's a food chain diagram and a food chain diagram is a model.

It's a simplification of what's happening in the real world.

Some of the words that you could have used to label those food chains, the algae are producers.

The shrimp is a primary consumer and it's prey of the cod.

The cod is the secondary consumer.

It's prey for the seal, but it's also a predator of the shrimp.

The seal is the tertiary consumer, the third consumer in the food chain.

It's prey for polar bears and it's a predator of the cod.

And finally, the polar bear is the quaternary consumer.

It isn't eaten by anything else because it sits at the top of the food chain.

And so another label is that it's the apex predator.

There were two words that you shouldn't have used.

They are top consumer and top producer.

I made those up.

Well done if you got that right.

But if you got some of it wrong, don't worry.

Just try and correct your answers and then hopefully next time round you'll get it all right.

So we're now gonna move on to the second part of our lesson, which is all about changing populations.

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

A population is the number of one type of organism in a specific area.

So for example, the number of humans that live in Leeds, or the number of goldfish in a pond, or the number of wheat plants in a field.

All of those are examples of populations.

They're one type of organism in a specific area.

Food chain diagrams are models and a model is something that simplifies things.

Food chain diagrams are models that show populations of organisms. So often we only show one organism when we talk about food chains, but actually we're talking about the population of that organism as a whole.

Populations are always changing, and they always change because new individuals are introduced to the population when an organism reproduces.

The number of organisms in a population reduces because they die, either because of disease or predation, or changes to the environment.

And populations can change upwards or downwards because of migration.

And migration means to move into or out of an area.

So for example, the number of seagulls at the seaside may increase or decrease as more seagulls fly in or fly out of that area.

There are lots of factors that can impact the size of populations, and these include predation, disease, different seasons.

So think about the number of flies that you see, for example, in the summer, and how that compares with the number of flies that you might find during the winter.

When it's colder, there's less flies.

Natural disasters also have an impact.

Things like volcanic eruptions, floods, wildfires.

And human activities can impact the size of populations too through activities such as hunting and fishing, cutting down forests, pollution that we put out, climate change, but also in a positive way through conservation.

So the destruction of the places that organisms live, as a result of natural disasters or human activities, will have a big impact on the populations that live there.

Natural disasters and large-scale human activities tend to have very large and very rapid impact on the populations.

Say for example, floods.

Here you can see some pictures of cows that were living in a field that have been flooded, but actually there's lots of organisms that are living in that area.

The size of those populations will reduce rapidly as a result of that flood.

So for example, the organisms that are eating the grass suddenly have very little food to eat.

Diseases can also reduce the population size significantly.

However, diseases usually reduce the population over a longer period of time and kill less of the population than a natural disaster does.

So here is a graph that shows the size of a population against time.

And you can see that with a natural disaster, the population decreases really quickly in a short space of time.

For a disease, usually the reduction of population is smaller and it takes place over a much longer period of time.

Human activity such as fishing and farming have also caused many populations to decline.

And humans are helping populations to recover through conservation.

Conservation means to restore the places that organisms live, and that's one way that humans help populations to grow.

So for example, here on the left-hand side of the screen, you have some farmland, huge field with no hedgerows, no places for things like insects to live.

But in the right-hand side picture, there's lots of smaller fields, and each of the smaller fields is bordered by a hedgerow.

And that gives organisms places to live.

We're conserving the places that those organisms live in to help them to survive and to reproduce and increase their population.

So now we're gonna come to a check to see how well you've understood that last section.

And I'd like you to complete the following sentences using words from the box below.

So a something is the number of one type of organism that lives in a particular place.

These are constantly changing.

Predators reduce the size of populations.

The also make it more difficult for some populations to survive at certain times of year.

Natural disasters such as wildfires and can decrease populations.

And something can also reduce numbers of organisms. Humans impact the size of populations through their activities.

They can help increase populations by activities.

And here are your words.

Population, seasons, fishing, floods, prey, conservation, diseases, changing, and predator.

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

Good luck.

So how well did you do? Let's see what the right answers are.

So a population is the number of one type of organism that lives in a particular place, and they're constantly changing.

Predators reduce the size of prey populations, and the seasons can also make it more difficult for some populations to survive at certain times of year.

Natural disasters such as wildfires and floods can decrease populations.

Diseases can also reduce the number of organisms. Humans impact the size of populations through their activities, and they can help increase populations by conservation activities.

I hope you did well and didn't find it too tricky.

So let's move on to a practise task.

And this one is all about the muskox, a picture of which is in the screen.

Muskox live in Alaska.

They have a really pungent, musky smell.

And there were once many of them.

However, they were hunted by humans almost to extinction between 1900 and 1930.

Humans hunted them for their hide, their meat and as trophies.

Regulations on hunting have been in place which has helped to conserve the species.

However, they haven't yet recovered to the numbers that were seen before 1900.

What I'd like you to do is to sketch a graph that shows the population of muskox from 1880 through to today.

I'd like you to label the graph to show when hunting began and when conservation work started to take place.

You need to label your axes clearly.

If you need a bit of help, this is what your axes should look like.

You'll need to pause the video.

And then when you're ready, when you've sketched and labelled your graph, I'd like you to restart and we'll check your answer.

Okay, so let's see how well you've done.

On the left-hand side of the screen, you've got the key events in our story about the muskox.

On the right-hand side, I've drawn my axes for the graph.

I've labelled the x-axis, the one at the bottom with the years, and the y-axis, the one along the side with the population of muskox.

Remember, when we're sketching a graph, it doesn't have to be accurate in terms of the points, it has to tell us the story that's going on.

So we know that the population of muskox was high before 1900, and then they were started to be hunted by humans for trophies, for meat and for their hide, for their skins.

This reduced the population quickly between 1900 and 1930 almost to extinction.

So the graph should have rapidly declined between 1900 down to very low numbers at around 1930.

At 1900, you should have labelled your graph to show that that's when hunting began.

And after 1930, humans started to conserve the species by putting restrictions on their hunting.

And as a result, the numbers of muskox started to increase again, but not to the numbers of 1900.

So your graph should have increased, but shouldn't have increased to the point that it's as high as it was in 1900, something like this.

And so you can then label that part of the graph to show that conservation work was taking place.

I hope that you got something that looks similar to my graph.

Okay, so we're ready now to start the third part of our lesson, which is all about predator-prey relationships.

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

So predators and prey are closely linked to each other.

The numbers or populations of each organism depend on each other.

For example, if there's a large population of predators, the number of prey will fall quickly.

And the reason for that is that the prey will be eaten.

What do you think would happen to the population of predators if the prey population has decreased? I'll give you five seconds, or you can pause the video, and then I'll see whether you get the right answer.

So if the population of prey has fallen, because so many predators are eating the prey, then actually there's not much food for the predators.

And as a result, the population of predators will also get smaller quickly, because there's simply not enough food to keep them alive.

We're gonna look at that in a graph.

So here we have a graph with the axes of time along the bottom, the x-axis and the size of the population along the y-axis.

So let's look at that last example.

The prey population increases and there's more food for the predators.

So the population of predators also begins to increase.

However, the more predators there are, the more prey are eaten, and so the prey population starts to decrease quickly.

As a result of less prey, the predator population then starts to decrease too.

As the predator population gets smaller, less prey are eaten and the prey population begins to increase again.

And at the same time as the number of prey increase, there's more food for the predators and the predator population begins to increase too.

So you can see that there is a real relationship between the number of predators and the number of prey.

When there's more prey, the predators have more food.

But then as the prey numbers fall, the predators have less food and their numbers decrease too.

So let's look how the populations of predators and prey fluctuate over a long period of time.

We're going to use this graph, which is called a predator-prey model.

Remember, a model shows something that's more difficult to understand in a simpler way.

On the graph are two lines that represent the population sizes of the predators in green and the prey in red.

At the start of the graph, the red line, the number of prey is high.

This means there's lots of food for the predators to eat, and the number of predators starts to increase.

At the same time, the number of prey start to decrease.

This means there comes a point where there's not enough food for the predators to eat.

And so their numbers start to decrease.

As the number of predator numbers decrease, then the number of prey start to increase.

And so we get this fluctuation up and down of the size of populations of predators and prey.

In practise, this is much more complex.

Here's a real-life example.

We're using the food chain diagram of grass, Arctic hare and Canadian lynx.

Here's the population of Arctic hare between 1845 and 1925.

And you can see that the populations are changing up and down over time.

The Canadian lynx is the apex predator, it's the final predator in the food chain.

And its population also fluctuate, but you can see it synchronises with the population of Arctic hare.

When the population of Arctic hare are high, the number of lynx start to increase.

When the population of Arctic hare is low, then the numbers of a Canadian lynx decrease.

This is because there isn't enough food for them.

So let's see how well you've understood that.

The greenfly population increases in the summer when the weather is warm.

And this is because the greenfly reproduce more.

Greenfly are food for ladybirds.

Which of the following statements completes the sentence? As the population of greenfly increases, does the population of ladybirds decrease? B, the population of ladybirds stays the same, or C, the population of ladybirds increase.

I'll give you five seconds to think about your answer.

If you need longer, pause the video and then we'll check to see whether you've got the right answer.

So how well did you do? Let's see.

As the population of green fly increases, the population of ladybirds increase.

And the reason for that is because there's more food for the ladybirds.

So there's lots of greenfly and there's lots of food for the ladybirds, and so they reproduce more.

Well done if you got that answer correct.

Let's go to our final practise task of this lesson.

So a class of learning about predator-prey models.

Can you complete what each class member is saying about the populations? So Andeep says, if the population of predators increases, the prey population will increase, decrease, stay the same, what do you think, and why? Complete the sentence.

Jacob says, if the population of predators decreases, the prey population will, what do you think? Will it increase, decrease? Will it stay the same? And why? Laura says, if the population of prey increases, the predator population will, what do you think? And why? Complete the sentence.

And finally, Izzy says, if the population of prey decreases, the predator population will, so will it increase? Will it decrease? Will it stay the same? What's your thoughts? So you need to write down those sentences and complete them and give a reason for your answer.

You'll need to pause the video at this point.

When you've got all your answers written, press play and we'll check to see how well you did.

Good luck.

So how well did you do? Let's find out.

First of all, Andy said, if the population of predators increases, the prey population will decrease.

And it will decrease because there's lots of predators that will hunt the prey for food.

Jacob said, if the population of predators decrease, the prey population will increase, because the prey are not being hunted as much.

Laura said, if the population of prey increases, the predator population will increase, because there's lots of food available for them to eat.

And finally, Izzy said, if the population of prey decreases, the predator population will decrease, because there's less food for the predators to eat.

Well done if you got those right.

So that brings us to the end of today's lesson on predator and prey relationships.

During this lesson, you should have learned that predators are animals that hunt and catch animals for food.

The animals that the predators catch are called prey, and that populations of organisms can be impacted by factors such as the seasons, natural disasters, but also positively by conservation.

Populations of predators and prey are very closely linked.

They rely on each other.

And there's a cycle between populations of predators and populations of prey, which can be shown in a predator-prey model.

I hope that you've enjoyed today's lesson and I look forward to learning with you again soon.

Take care for now.

Bye.