Lesson video

Welcome to today's eighth lesson in the unit Ecosystems. My name's Mr. Jarvis, and I'm gonna be teaching you today.

Today, we're gonna be learning all about bioaccumulation.

By the end of today's lesson, you should be able to describe how toxic materials can accumulate and affect organisms within a food chain diagram.

There are three key words during today's lesson.

They are pesticide, toxic, and bioaccumulation.

Here's the definitions of those words coming up on the screen.

I'll give you a little bit of time to read through those, but you can always pause the video if you want to and restart it when you're ready.

So today's lesson is going to be built in two parts.

First of all, we're going to look at using chemicals in farming, and then we are going to look at how those chemicals build up in organisms within food chain diagrams. So, if you're ready, let's get started with our first section, using chemicals in farming, Producers begin all food chains, and that's because they make food from light and chemicals that they have in their surroundings.

All of the organisms within a food chain diagram rely on producers for food, and this includes humans.

Humans grow crops, they're the producers to provide food for the population and the animals that we farm for food.

Wheat that humans grow provides food for humans and food for the animals that we farm for food.

So you can see that the wheat crop that's in the picture on the left hand side of the screen is being harvested.

Those wheat seeds are used to make bread from the grain of the wheat seed and to feed things like chickens that we farm for food.

Farmers want to grow as much crop as they can as cheaply as possible, but sometimes organisms are in competition with the farmer for this food, and that's because there's more than one food chain diagram that the crop is part of.

Let's use an example to illustrate this.

So we have a crop of lettuces in a field that a farmer is growing in the picture on the screen.

Here's the food chain diagram.

Lettuce is food for humans.

However, there are other food chain diagrams that are taking place within that crop of lettuces.

Here's one.

Lettuce is food for caterpillars.

Caterpillars are food for thrushes.

Thrushes are food for hawks.

And here's a third lettuce, aphids, ladybird, goldfinch, owl.

Here are those food chains again, and you can see we have are two primary consumers, the caterpillar and the aphids, eating the producers, the farmers' crops.

And the farmers' crop is the lettuce.

The more food means that population of consumers increase.

So a large field of lettuces helps to increase the population of caterpillars and aphids.

An organism that competes with humans for food is known as a pest, and pests can consume as much as one third of the crop that humans grow.

One way that farmers protect their crops from being eaten is by using chemicals called pesticides.

And pesticides are a group of chemicals that are toxic.

A toxic substance is one that acts as a poison to living organisms. Here on the screen on the right hand side is the symbol for a toxic substance.

Some pesticides are persistent in the environment and that means that they don't break down quickly.

And it also means that we have lots of toxic pesticides hanging around in the environment, which potentially can cause harm to living things.

Let's check to see how well you've understood that first section.

The class I've been learning about this hazard symbol, which is found on the labels of some chemicals.

Which member of the class is correct? Is it Izzy, who says the symbol tells you that the chemical is corrosive and may kill you? Is it Jacob that says the symbol tells you that the chemical is hazardous, or is it Andeep, who says the symbol tells you that the chemical is poisonous or toxic? I'll pause for five seconds while you think about your answer, and then we'll check to see how well you did.

So the correct answer is Andeep.

The symbol tells you that the chemical is poisonous or toxic.

Well done if you've got that right.

So there are different types of pesticides that farmers can use.

Herbicides, sometimes also known as weed killers, are toxic to plants.

Insecticides are toxic to insects, and fungicides are toxic to moulds and fungi.

So again, we're going to check to see how well you've understood that first section.

We're going to go through four check questions.

All of them are multiple choice, a, b, c, or d.

So first of all, what's a herbicide? Is it a, a chemical that's toxic to moulds? b, a chemical that's toxic to insects? c, a chemical that's toxic to plants, or d, a chemical used to kill organisms that might eat or cause damage to a crop? Which one do you think, a, b, c, or d? I'll give you five seconds to give yourself an answer.

So the correct answer is c, a herbicide that is a chemical that's toxic to plants.

Here's a second question.

What type of chemical is toxic to moulds? Is it a, herbicide, b, pesticide, c, insecticide, or d, fungicide? Again, I'll give you five seconds.

So this time the correct answer is d, a chemical that's toxic to moulds is a fungicide.

Here's question three, what's a pesticide? Is it a, a chemical that's toxic to moulds? b, a chemical that's toxic to insects? c, a chemical that's toxic to plants, or d, a chemical used to kill organisms that might eat or cause damage to a crop.

Which one do you think this time? Is it a, b, c, or d? Here's five seconds.

The correct answer to that one is d, a pesticide is a chemical that's used to kill organisms that might eat or cause damage to a crop.

Well done if you got that right.

And here's the last one.

What type of chemical is toxic to insects? Is it a, a herbicide, b, a pesticide, c, an insecticide, or d, fungicide? Five seconds to get yourself an answer.

And the correct answer to this one is c.

It's an insecticide.

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

So farmers use the right pesticide to help control the pests.

Insecticides are usually used to kill pests that eat crops.

And as I've already said, some insecticides don't break down very easily in the environment.

Some do, but some stay around in the surroundings for a long time.

Some insecticides don't break down in the bodies of organisms either, and so they remain in the organism's body even after they die.

Insecticides in the surroundings can get passed on to other organisms in the food chain.

For example, if a caterpillar's been sprayed with an insecticide and the blue tick eats it, the blue tick eats the pesticide and the caterpillar, or they can get washed into rivers and can end up in the sea where small amounts can be absorbed into organisms' bodies.

So here's an example again of a farmland that can be sprayed and it's right next to a river, and if the farmer overuses the pesticide, it can run off, wash into the river, and end up in the sea.

So let's do another check question.

Why might insecticides affect non-target organisms like birds? Is it a, insects affected by the insecticide are eaten, b, insecticides wash into rivers, or c, insecticides are sprayed onto the non-target organism? I'll give you five seconds to think about your answer and then we'll check to see whether you got it right.

So the correct answer is a, insects affected by the insecticide are eaten.

Well done if you got that.

So now we're going to do a practise task.

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

I'd like you to explain how a farmer might ensure that the producer is able to grow without being eaten or damaged by insects.

And then secondly, what might happen to the population of pheasants if the insects, that's the locust and the beetles, are all killed? You'll need to pause the video at this point, write down your answers, and then when you've done that, press play on the video and we'll check to see how well you did.

Good luck with that.

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

Looking at the food web diagram, you were asked first of all to explain how a farmer might ensure that the producer is able to grow without being eaten or damaged by insects.

Now, the producer in this food web diagram is the wheat.

Remember, a producer always starts a food chain diagram or a food web diagram, so the farmer could use an insecticide, and we know that insecticides are toxic to insects.

So the producer, the wheat, is eaten by insects, the locusts and the beetles, and the insecticide will kill the locusts and beetles, and that will mean that the farmer is protecting his crop against being eaten or damaged by the insects.

Question two is what might happen to the population of pheasants if the insects, the locusts and beetles, are killed? Well, there are less locusts and beetles eating the wheat because the insecticide has killed them, and that means that there's more food for pheasants to eat.

We know when there's more food available for a population, the population grows and that as a result, the pheasant population might increase.

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

That brings us to the second part to today's lesson, and that's all about how these chemicals build up in the food chain.

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

So we've seen that pesticides can get washed into rivers and out to sea, and in the water, the producers, the water plants, absorb a very small quantity of this pesticide.

Let's look at this as an example.

Here's a food chain diagram from a river.

You can see we have pondweed, grass carp, perch, and otter.

The pondweed is the producer.

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

The grass carp is the primary consumer, the first consumer in the food chain, and it's also the prey.

Prey is something that's hunted and eaten by other animals.

It's prey for the perch.

The perch is a secondary consumer.

It's also the predator of the carp and the prey of the otter.

And finally, we have the otter.

The otter is the tertiary consumer.

It's the apex or top predator in this food chain.

So let's imagine our farmer is spraying insecticide on the crop plants and some of that insecticide gets washed into the rivers and ponds.

Think about our food chain.

The insecticide washes into the river and ponds, and each pondweed plant absorbs a small amount of the insecticide.

The pondweed is eaten by the grass carp, and the grass carp eat lots of plants.

They also consume the insecticide that was in the pondweed.

The grass carp is food for the perch, and each perch eats lots of grass carp and each of the grass that it eats contains some of that insecticide.

And finally, the perch is food for otters, and each otter eats lots of perch.

The insecticide in the otter becomes very concentrated indeed, and it can reach toxic levels in the otter, causing the otter to die.

Let's look at this slide in another way.

Here's our food chain diagram for the river, and I'm going to use a red dot to indicate the units of insecticide that are within each individual organism within this food chain diagram.

Let's imagine that each pondweed plant contains one unit of insecticide.

We know that in a food chain diagram, the individual organisms actually represent a population.

And so here is our population of pondweed.

Each of those pondweed plants contains one unit of insecticide.

Pondweed is food for grass carp, and so the grass carp are going to swim around and eat the pondweed.

Let's see what happens to the insecticide when the grass carp eats the pondweed.

Every time a member of the pondweed population is eaten, the insecticide within the grass carp increases.

We know that the grass carp isn't on its own.

It's a population.

And if all of the grass carp eat the same number of pondweed plants, then all of the population will contain five units of insecticide within their bodies.

Grass carp are eaten by perch.

Let's see what happens when the perch eat the grass carp.

You can see that the insecticide is passing from the grass carp into the perch.

And again, the perch isn't an individual organism, it's a part of a population.

And so the otters that eat the perch then accumulate the pesticide within the body of the otter, and eventually the insecticide reaches a level that becomes toxic to the otter and the otter can die.

The buildup of pesticide in organisms in this way is called bioaccumulation.

Bio means relating to living organisms and accumulation means to build up.

And we've seen in that last slide how, within each of the living organisms, the pesticide has built up and built up as they've eaten more and more food.

One type of insecticide that has built up in food chains, as in the previous example, is a pesticide called DDT.

Farmers used DDT to kill insects and Peregrine falcon populations died quickly.

Scientists couldn't really understand at first what was happening, but they discovered that DDT was building up in the food chain.

That's an example of bioaccumulation.

Accumulation means to build up, bio means living organisms. So it was built up in the living organism, the peregrine falcon.

The DDT caused the bird's eggs to have really thin shells.

And when the birds sat on the eggs to incubate them and to hatch them, the eggs broke.

DDT as a result was banned in Britain.

And since DDT having been banned, the peregrine falcon population has increased.

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

What does bioaccumulation mean? Does it mean an increase in the concentration of a chemical within a habitat? Does it mean an increase in the number of organisms in a habitat over time, or does it mean an increase in the concentration of a chemical within an organism over time? I'll give you five seconds.

When you're ready, we'll check your answer.

So the correct answer is C, bioaccumulation is an increase in the concentration of a chemical within an organism over time.

Well done if you got that right.

So now we're gonna move on to a practise task.

In the late 1950s, scientists recorded a high number of dead seed eating birds like pheasants and partridges.

They also recorded deaths of lots of apex predators, such as falcons and foxes.

High levels of a pesticide called dieldrin was found in the bodies of these organisms. Before seeds were sown by farmers to produce new crops, the seeds were dipped into dieldrin to stop pests from eating them.

Here's a picture of some barley seeds that were dipped in the pesticide before being sown.

So your task is to write a paragraph to explain why you think seed eating birds and apex predators died.

You'll need to pause the video, write your paragraph, and then when you're ready, press play and we'll check your answer.

Good luck.

How did you get on with that? I hope it wasn't too hard.

So you are asked to explain why you think seed eating birds and apex predators died.

Seeds were coated in pesticide to protect them from the pests, and when they were sewn in the fields, the seeds were eaten by seed eating birds, such as the pheasants and the partridges.

As the birds ate more and more seeds, the level of pesticides in their bodies increased, and we call that bioaccumulation.

Well done if you used that word.

In some birds, the pesticide levels became toxic and they died.

In others that ate the seeds, so containing some of the pesticide, they were eaten by apex predators such as foxes and falcons.

And as the predators ate more and more seed eating birds, the level of pesticide within their bodies increased, too.

The pesticide levels in the bodies of the predators continued to increase.

That's bioaccumulation again, and eventually became toxic to them and they died.

Well done if you got that right.

That brings us to the final part of the lesson today, which is a summary of what we've covered.

So we've seen that farmers use chemicals to grow crops, and those chemicals are important in providing food for humans and the animals that we farm.

However, some chemicals like pesticides don't break down quickly in the environment and they can get washed into rivers and seas.

If they're consumed, pesticides can pass along the food chain, and as more pesticide is eaten, it increases the concentration in the body.

And that's something that we call bioaccumulation.

If a pesticide reaches a certain level, it becomes toxic to organisms, and this can cause organisms to die.

So thank you once again for joining us in this lesson.

It's been great teaching you again today, and I look forward to seeing you all again soon.

Take care for now.

Bye.