Lesson video

Hello, I'm Mr. Jarvis and I'm with you today covering this lesson in the unit, Living Organisms and Their Environments.

Today's lesson is all about measuring the size and distribution of populations of organisms. By the end of today's lesson, you should be able to describe how to identify species, how to estimate the population size of a species, and how to investigate the effect of a factor on the distribution of species.

There are four key words to today's lesson.

They are classification key, population, sampling, and estimate.

Here are the definitions on the screen.

You can pause the video if you want to read through them, but don't worry, we will go through the definitions as we move through the lesson.

Today's lesson is broken down into three parts.

First of all, we'll look at how to identify organisms, then we'll move on to talk about sampling populations, and finally we'll talk about how to estimate population size.

So if you're ready, let's get started with our first part of the lesson, which is all about identification of organisms. Classification keys help us to classify or identify organisms based on observations of easily identifiable physical features.

They do this through a series of questions.

For example, does the animal have a backbone? If the answer's no, we know that it's an invertebrate.

If it's yes, we know that it's vertebrate and we know that there are different types of vertebrates that it could be, for example, a mammal, a bird, a reptile, an amphibian, or a fish.

So we can ask another type of question to see whether we can narrow down what sort of animal it is.

So we could say, does the animal have fur? If the answer to that is yes, then we know it's a mammal.

If the answer's no, we can ask other questions such as, does the animal have feathers? That would obviously help us to identify whether the organism was a bird.

Some organisms look like they may belong to several groups.

Consider a bat.

Here are some views.

Laura says, "It's definitely a bird.

It has wings." Andeep says, "It lives in the air and lives in trees, so it must be a bird." Jun says, "It has fur and it must be a mammal." And Izzy says, "It has wings and flies, so it can't be a mammal." What do you think? We can use a classification key to help us classify a bat.

Here's a classification key that I've designed.

Let's follow it through and determine what type of organism the bat is.

So my first question is, does it have wings? If the answer's yes, which it does, then does it have feathers? The answer here is no, and therefore we can say that the bat is a mammal.

When we carry out field work, we may need to identify species.

For example, if we looked at a school field, we may find these plants.

We can use a classification key to help us to identify them.

So let's use a classification key to identify one of the species that could be found on a school field.

Here's a classification key, and here's the organism that I'm trying to identify.

I'm going to pause and let you have a few seconds to see whether you can follow that classification key to identify what that species is.

You can also pause the video if you want a little bit more time.

Let's follow the key and see whether you've got the answer right.

I hope you've got an answer that we can test to see whether you followed the key correctly.

So the first question that we have is, are the flowers yellow? The answer to that question is obviously yes, they're yellow.

And so the next question that directs us to is, do the leaves have jagged edges? We can look at the picture and see that the edges are smooth, and so the answer to the question is no, and that means that our species is a buttercup.

Well done if you managed to follow that key through.

By using our answers, we were able to identify the correct species, and that's what we do when we're out in the field and we are looking at different organisms that we don't know what they are.

We follow classification keys to help us identify those species by looking at their physical features.

So here's a check.

While carrying out some field work, Sam found an insect.

I'd like you to use the classification key to identify the organism.

Here is the classification key.

You need to follow through the questions and decide whether the organism that's on the screen in the picture is A, a wasp, B, a hoverfly, C, a silverfish or D, an ant.

I'll pause for a few seconds, but you can also pause the video if you want a little bit more time to work through your classification key.

And if you do that, when you're ready, just press play and we'll check the answer.

Let's check your answer.

So our first question was, are wings present? The answer to that is, yes, there are.

There are clearly wings present in this organism.

Our next question is, is there a narrow waist? And we can see that between the main part of the body and the abdomen of this particular organism, the bottom part of the organism there is that thin waist.

And so the answer to that is yes.

And so the answer is A, this is actually a wasp.

Well done if you got that correct.

Now we're gonna move on to a task.

I'd like you to look at the pictures of the different organisms that we might find on a rocky shore, and I'd like you to design a classification key that could be used to help identify each of those organisms based on their physical features.

We have a dog whelk, we have a limpet, a sea anemone, a barnacle, and a flat periwinkle.

There is some additional information in the additional materials that might help you in terms of describing how the organisms live and what sorts of physical features they have.

You'll need to pause the video, refer to the additional materials, write down your classification key, and then we'll come back and I'll give you an example of something that you might have designed.

Good luck.

There are lots of different examples that we could put up on the screen now.

I've just written a classification key that's one example of what you might have had.

So here's my example.

First of all, does the animal have a shell? If the answer to that is no, that immediately allows us to identify a sea anemone.

The sea anemone is the only organism in terms of physical features in our list that didn't have a shell.

If the answer's yes, then we've got four other organisms that have shells, and so that doesn't help us.

So our next question is, is the shell snail-like? Does it look like a snail? If the answer's yes, we can then decide, actually there's two organisms that look like a snail.

We've got our dog whelk and our flat periwinkle.

So is the shell really sharply pointed? If the answer to that is yes, then we've identified the dog whelk.

You can see in the top picture highlighted in green that the snail shell, the dog whelk shell comes to a very nice point.

If the answer's no, as is the case with the flat periwinkle, it's much more rounded, then we've decided that we've identified a flat periwinkle.

That still leaves us with two organisms to identify, the limpet and the barnacle.

So I'm gonna go back to my, is the shell snail-like? The answer to that question is no for both the limpet and the barnacle.

And so now I'm going to ask, are there visible plates at the top of the shell? And we can see if the answer's yes, looking at the diagram and the picture on the screen highlighted in green, you can see some plates in the opening of the barnacle, and that gives us the classification of that organism.

If the answer's no, then we've a limpet.

So we've pulled together a classification key that would enable us reasonably quickly to identify those five rocky shore organisms. Well done if you got something that was similar and enabled you to identify those organisms from the physical features yourself.

That brings us to the second part of today's lesson, which is all about sampling populations.

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

Classification keys are really important tools for ecologists when they carry out field work because they help us to identify the populations of organisms that live in a habitat.

It's really difficult to work out the actual population size because it's constantly changing as a result of births, deaths and migration into and out of the habitats.

To count all of the individuals in a population would take a hugely long time to do and it would be really difficult and really expensive to do as well.

So sampling is one way to survey organisms and it gives us information about the populations in a habitat.

So to give you an example, if we look at the number of fruit bats in a still image on the tree on the screen, it's really difficult to count how many there actually are.

So we can sample the food bats to decide how many organisms there might be in that population.

And sampling is a useful way to get an estimate of the size of the population.

To estimate means to give an approximate value.

So here's a check.

Ecologists use sampling techniques because A, it's more accurate than counting all of the individuals in a population.

B, it provides a good enough estimate of a population size and C, it's quicker and easier to sample populations.

I'll pause for a few seconds and then we'll check your answer.

The correct answer is B and C.

Ecologists use sampling techniques because it provides a good enough estimate of population sizes and it's quicker and easier to sample populations than it is to try and almost do the impossible thing of counting every single organism in a habitat.

Well done if you've got those right.

Ecologists use lots of different ways to sample organisms, and that depends on the type of habitat and the species that's being sampled.

Here's a couple of examples of different sampling techniques that ecologists use.

They might use a quadrat to sample in a grassland and they might use a net to help sample either in a grassland for things like insects or in aquatic habitats such as rivers, ponds, and streams. A pooter is one piece of equipment that's used to help ecologists sample very small organisms such as insects, and typically they come from the underside of things like leaves.

It consists of a mouthpiece and the tube that goes to the mouthpiece is covered by a piece of muzzling to stop the small organisms going into your mouth, a jar to hold the collected specimens in.

And what and basically happens is it works like a mini vacuum.

You suck through the mouthpiece and those organisms then get sucked into the jar a bit like this.

Once the organisms have been identified and counted, you should make sure that you return them carefully back to the habitat that you got them from.

So pooters are often used for sampling small, slower moving invertebrates.

For example, you might use it to sample greenfly from leaves of a plant.

To estimate the size of a population of greenfly on a plant, you'd need to estimate the number of leaves on the plant and sample a number of leaves to find the average number of greenfly on every leaf.

We'd use an equation such as estimated population size is equal to the average number of greenfly on a leaf multiplied by the estimated number of leaves on the plant, and that would give us an estimated population size.

Have a look at this short video of how a pooter is used.

There are lots of different types of nets that are used to sample different habitats, And kick-sampling is one type of sampling technique that's used to sample aquatic organisms. In particular, kick-sampling's used to sample the organisms that live on the riverbed and we use a kick-net to do this.

And you can see an example of a kick-net in the picture are on the right hand side of the screen.

The method's used to find out what species live in the river bed of a river, and the presence of some organisms help ecologists to evaluate the quality of the water.

One of those species that helps to identify good water quality are dragonfly larvae.

So this is how you would kick sample in a riverbed.

You'd place the net on the riverbed with the opening facing the flow of the water, and we can see the flow of the water indicated by the blue arrows.

Then we'd gently kick the stones on the riverbed near to the opening of the net.

The idea is, is that we don't kick the stones into the net, but we disturb the stones and the flow of the river takes the aquatic organisms into the net.

We then empty the net into a white tray, and that helps us to really easily see the organisms and helps us to identify them.

Of course, remember we need to put some water in that tray too so that we're providing the right conditions for those organisms that we've caught.

And as always, when we've finished sampling, we return those organisms back to the place that we got them from.

So here's a check.

Why is kick-sampling used to sample populations in a river habitat? Is it A, because it provides information about the river's biodiversity? Is it B, it provides information about the quality of water, or C, it provides information about how fast the river flows.

I'll pause for a few seconds and then we'll check your answer.

The correct answer is A and B.

Kick-sampling is used to sample populations in a river habitat and that provides information about the river's biodiversity, the number of different species that are living there, and it provides information about the quality of water through species that indicate that the water quality is either good or not so good.

So let's move to a task.

An oak tree has 10,000 leaves.

Using a pooter, Aisha randomly samples 10 leaves to estimate the number of greenfly that are on the tree.

Her results are shown in a table and that table is in the additional materials.

What I'd like you to do is to use the data in the additional materials to work out the average number of greenfly on each leaf.

Remember, you need to show your working, and then I'd like to estimate the population.

That's the number of greenfly on the tree, and again, you'll need to show your working.

The additional materials have got the equation there to help you work out the number of the estimated population of greenfly on the tree.

You'll need to pause the video at this point, work out your answers, and then we'll check them to see if you got them right.

Good luck.

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

I asked you, first of all to work out the average number of green fly on each leaf and reminded you to show your working.

So the total number of greenfly is the first thing that we needed to do, and that was 10 plus eight plus seven plus zero, plus 23 plus 12 plus six plus a 13 plus 16 plus 10, and that gives us a total of 105.

Then we needed to look at our number of samples and if we count the number of samples, there were 10.

To work out the average number of greenfly per leaf, we need to take the total number of greenfly counted, which was 105, and divide it by the number of samples, which was 10, and that gives us an average number of greenfly per leaf of 10 and a half, 10.

5.

To estimate the number or population of greenfly on the tree, we needed two pieces of information.

First, the average number of greenfly per leaf, that's 10.

5, and secondly, the number of leaves on the tree, and we estimated that to be 10,000.

So the estimated population is 10.

5 multiplied by the number of leaves, 10,000.

So there were 105,000 greenfly on the tree.

Well done if you worked out that answer.

That brings us to the third and final part of today's lesson, which is all about estimating population size.

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

Sampling, as we've seen, is a useful way for ecologists to estimate the population size of species in a habitat, to measure the biodiversity, that's the number of species within a habitat, and when we're using a quadrat to estimate the percentage cover of a species.

Populations of plants are easier to estimate because plants stay in the same place.

Populations of animals are more difficult to estimate because they move around and sometimes they come out at certain times of the day.

For example, at night.

Stag beetles that's shown in the picture is mostly active at dawn and dusk.

One method that ecologists use to estimate the population size of animals is called mark-release-recapture, and it involves capturing a sample of animals, carefully marking them and then releasing them back into their population and then giving it time to mix and recapturing a sample to see how many marked individuals there are.

It's really important when we undertake something like a mark-release-recapture experiment, that when we sample the animals, we do it humanely, and when we mark them, it shouldn't harm them or make them more vulnerable to predators.

Let's look at a mark-release-recapture experiment.

First of all, we need to decide on a suitable sampling method to catch our animals.

I'm going to use snails in my example.

So I might have used something like a pitfall trap to sample sampler organisms. And a pitfall trap is in essence a cup that's sunken to the ground with some food in that attracts animals.

And we leave it for a short period of time to collect animals from the population.

Once we've caught our animals, we then need to put a harmless mark on them.

And remember that mark also needs to ensure that the organism isn't more vulnerable to predators.

So I've used a red mark here so that you can see it on the screen.

I'd probably use a brown mark if I was doing this in the wild because the red mark might make the snail stand out to a predator like a bird.

Once we've marked our sample that's been caught, then we release them and it's important that we then give them time to mix back into the population.

After a couple of days, we collect a second sample and we count the number of organisms that have got marks on them within the sample.

And you can see here that we've got four organisms that are marked within our second sample.

So what we then look at is the number of organisms that we caught in our first sample, and that was 10.

In the second sample, we also caught 10, and the number of those organisms which were marked from the first sample was four.

To estimate the population size, we use an equation, and that's the number in the first catch multiplied by the number in the second catch, divided by the number of marked animals in the second catch.

So that would be equal to 10 in the first catch, 10 in the second catch, which is a hundred, and we divide by the number of marked animals in the second catch, which was four.

A hundred divided by four is 25.

So we would estimate that our population of snails is 25 organisms. The information that's gathered by sampling is important in conservation and monitoring the health of a habitat.

Sampling can be the first step in deciding whether a species are is at, for example, risk of extinction.

And by evaluating data gathered over time, ecologists can see if conservation efforts are successfully helping populations of species to increase.

One example is the chequered skipper butterfly.

It became extinct in the UK in 1976.

Scientists have been releasing populations from Belgium back into the UK, and these are being really closely monitored, and they're showing that the chequered skipper is actually starting to grow in population size in a number of different places in the UK.

So complete the following.

Ecologists use sampling because A, it is a cheaper and quicker way of knowing how many individuals are in a population than by counting them.

Is it B, ecologists use sampling because it can help to monitor populations over time and see how successful the conservation work has been, or is it C that they use sampling because it can help to identify species that are at risk of extinction? I'll pause for a few seconds and then we'll check your answer.

The answer to the question is, A, it is a cheaper and quicker way of knowing how many individuals there are in a population than just simply by counting them.

But it's also B, because it helps ecologists to monitor populations over time to see whether conservation work successful.

And it's also C.

It can help identify species that are at risk of extinction.

Well done if you spotted that it was all three answers that were correct.

We're going to move on to our final task of today's lesson.

Fish can be caught in nets and marked using temporary fin tags.

You want to find out how many perch are in a large lake.

To do this, you carry out a mark-release-recapture experiment.

You catch and mark 90 perch and you release them back into the lake.

You return two days later and catch 50 perch, nine of which are marked.

I want you to estimate how many perch are in the lake, and I'd like you to make sure that you show all of your working.

As a reminder, this is how you work out the estimated population size.

You'll need to pause the video, work out your answer, remembering to show all of your working, and then press play, and we'll check to see whether you got the answer right.

How did you do? I set you the task to work out how many perch are in the large lake following your mark-release-recapture experiment.

I gave you the facts that you caught and marked 90 perch.

You returned two days later and caught 50 more perch, and nine of those were marked, and I gave you the equation to use to work out the estimated population size of the perch within the lake.

So here's what you should have had.

Your estimated population size was 90, the number of perch in the first catch multiplied by 50, the number of perch in the second catch, and that should have been divided by nine because that was the number of marked animals that you had in the second catch.

So let's work out the answers.

90 multiplied by 50 is 4,500.

4,500 then needs to be divided by nine, and that gives us an answer of an estimated population size of 500 perch in the lake.

Well done if you got that right.

That brings us to the summary of today's lesson.

We've seen that classification keys are sets of questions that help us to identify species.

We've seen that scientists or ecologists use sampling to gather information about populations that live in different habitats and that they can use different sampling techniques to help estimate population sizes.

Different types of sampling are used in different types of habitats and for different types of organisms. And two examples that we've seen are pooters and kick-nets.

Animal population sizes can be estimated using methods such as mark-release-recapture, and it's an important part of helping to monitor populations as part of conservation projects.

Thanks as always for learning with me.

It's been great learning with you and I hope that you enjoyed the lesson.

So until next time, bye-Bye for now.