Hello and welcome to today's lesson which is in the unit, "Living Organisms and their Environments".

My name's Mr. Jarvis and I'm gonna be taking you through the lesson today.

Today's lesson is all about estimating population size using quadrats and it's a practical base lesson.

By the end of today's lesson, you should be able to use quadrats to estimate the population size of a species.

There are four keywords in today's lesson.

They are population, sampling, quadrat and abundance.

You can pause the video if you need some time to read through the definitions that are on the screen, but don't worry, because we will deal with the definitions as we come to the words in the lesson.

Today's lesson is broken down into two parts.

First of all, we're going to look at sampling using a quadrat and then we're gonna move on to measuring the population size of a species using practical methods.

So if you ready, let's get started with our first part of the lesson, which is all about sampling using a quadrat.

Ecologists study the organisms and surroundings within the ecosystem.

They look at how the living or biotic factors and the non-living or abiotic factors impact the community of organisms. They investigate how changes to the environment impact organisms within their ecosystem.

In order to investigate the impact of these changes, ecologists need to be able to measure the size of different populations.

And remember, a population is the number of organisms of one species that live in a particular place.

So here's an example.

A population of bees.

Counting all of the individuals within a population can be difficult, expensive and time consuming.

If you can imagine trying to count this population of bees that's on the screen, A, it would be difficult because of the chance of being stung, it would also be really difficult, because the bees would be flying around and so you can see that it would be really time consuming to do.

Ecologists use sampling to help them estimate the size of a population and sampling is a really useful technique.

It's used to study a small part of the population and that's because counting all individual organisms can almost be impossible to do as we saw with the previous slide in the population of bees and counting all individuals in a population would be very time consuming.

You can see this ecologist sampling a population of reeds.

To count all of the reeds within this lake would be a very time consuming process.

So let's move to a check.

Why do ecologists use sampling rather than counting all individuals in a population? Is it A, sampling is completely accurate and always representative of the whole population, B, estimates are the most accurate way of counting the population in a habitat or C, it takes too long and it's not possible to count all individuals within a population? I'll pause for a few seconds.

You can pause the video too if you need more time and then we'll check your answer.

Good luck.

Let's see how you did.

You were asked to tell me why ecologists use sampling rather than counting all individuals in the population.

The correct answer here is C.

It takes too long and it's not possible to count all individuals within a population.

Well done if you got that right.

Quadrats are one method that scientists use to survey or sample populations of organisms. They're square frames that are made of wood or metal.

Here's a picture of one.

They vary in size, but often each side is 50 centimetres or 0.

5 metres square.

So here you can see the quadrat.

The area of these quadrats is 0.

5 metres by 0.

5 metres and that gives a total area of 0.

25 metres squared.

Quadrats are useful for sampling plants and animals that are slow moving or sedentary.

Sedentary means that they stay still.

Some examples of animals that might be sampled include a sea anemone, mussels, barnacles and starfish.

And many animals that are sedentary are found on rocky shores.

Can you think of any examples for yourself? Some examples might include snails or wood lice.

Here's a check.

Which of the following populations of organisms might you use a quadrat to sample? A, buttercup plants, B, flies, C, frogs, D, snails.

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

The correct answer is buttercup plants and snails.

Buttercup plants are easy to count, because they grow and they stay still.

Snails move really slowly and so you could use a quadrat to sample them.

Well done if you got both of those answers.

Quadrats can be used to sample populations in some habitats, estimate the number of different species that live in a habitat, which is a measure of what we call biodiversity, the number of different species that live in the same place and they can be used to estimate the percentage coverage of a species in a habitat and that's helpful when plants are small and really difficult to count.

Quadrats are usually divided into smaller squares and this makes it easier to count the organisms that are present.

When using a quadrat, there are two important things to remember.

First of all, sample size is really important.

The more samples you take, the more valid your results will be.

And secondly, samples must be chosen at random, and this avoids what we call bias.

And bias is when a person carries out an investigation and affects the outcome.

So for example, using a quadrat, if you were sampling daisy plants, you might pick the quadrat up and put it onto the daisy plants so that you could count them.

That's bias.

So here's a question.

When using a quadrat, what two important things should you remember? A, enough samples need to be taken to give valid results.

B, samples should be selected to record all species present, C samples should be taken randomly to avoid bias and D, five samples is enough to give reliable results.

Again, I'll pause for a few seconds and then we'll check to see which two answers you've selected.

The correct two answers are A, enough samples need to be taken to give valid results and C, samples should be taken randomly to avoid any bias.

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

There are several ways that can be used to make sure that samples are taken at random and examples include using a random number generator to decide the number of steps you take before placing the quadrat on the ground, using two measuring tapes to create two axes, forming a sampling area, and then use a random number generator to create coordinates within that area to place the quadrat.

Which of the following are safe methods of randomly sampling using a quadrat? A, throw the quadrat and see where it lands.

B, use a random number generator to select the position of a quadrat sample or C, spin around with your eyes closed and let go of the quadrat and see where it lands.

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

The correct answer here is B.

You should use a random number generator to select the position of the quadrat sample.

The other two methods do select random samples, however, you should never throw a quadrat.

You need to have some rules for counting when you sample using a quadrat and this is so that you don't count an individual more than once.

It doesn't really matter what your rule is as long as it makes sure that you don't count individuals more than once when you're sampling.

So one example method is that we count an organism if it's totally inside the quadrat or it touches the top and or left edges of the quadrat.

And if we have an organism that is not fully inside and touches the bottom or the right edges of the quadrat, we don't count it.

And this way we don't end up counting an organism more than once.

So which of these three plants in the picture on the side of the screen would you count using the rule above? Just have a look and a think.

Let's start with the top plant.

Would you count this? Yes, you would, and you'd count it because it's inside or it touches the top or left edges of the quadrat.

What about the middle plant? Would you count this? Again, the answer is yes, you would, because it's fully within the quadrat square.

And finally, what about the bottom plant? Would you count that one? The answer here is no, because the plant is not fully within the quadrat and it is touching the bottom edge.

So we don't count that plant using our rules.

The quadrat is randomly placed on the ground as we've seen using a random number generator.

We're going to show you what this looks like.

So first of all, I'm going to mark out my sample area using two tape measures.

The tape measure on the diagram on the screen along the bottom is my x-axis.

The tape measure going up the side is my y-axis.

And I'm going to use a random number generator to select two numbers to find a coordinate where I place my quadrat.

So I'm gonna take a bag of random numbers and I'm going to pick a number out of the bag.

And so the first number I've picked is the number four.

That's going to be my X coordinate.

So I go four metres along the tape measure on the x-axis and that forms my coordinate.

You can see the dotted line that I've drawn on the screen.

I then place the number back in the bag and draw another random number.

This is gonna form our Y coordinate.

This time we've chosen the number six.

So the Y coordinate is number six.

The dotted line that shows the coordinate is shown on the screen and where those two individual coordinates intersect is where we place our quadrat.

We then are going to count the number of organisms within our quadrat and then repeat until we've got a large enough sample size.

You can work out the abundance of a species using this method.

And abundance is the number of organisms of a species in a habitat or ecosystem.

It tells you how common or abundant a species is.

We can calculate the mean number of organisms per quadrat or per metre squared using this sampling method.

So here's an example looking this green flowered plant.

The number of plants in each 0.

25 metre square quadrat are as follows.

Four, five, three, eight, two, seven, six and one.

We can now find out the mean by using this equation.

The mean is equal to the number of organisms counted, divided by the number of samples.

You can work this out for yourself.

Pause the video if you want to try and work it out.

The number of organisms we've counted is 36 and the number of samples is eight.

And so our equation is 36 divided by eight and that gives us a mean of 4.

5 or four and a half organisms per 0.

25 metres squared or per quadrat.

If we wanted to find out how many organisms there were per metre squared, we'd need to multiply that number by four.

Let's do a check.

What does the term abundance mean? Is it A, the number of a species found in a habitat? Is it B, the number of different species found in a habitat? Or is it C, how organisms interact with each other and their environment? I'll pause for a few seconds and then we'll check your answer.

Abundance is the number of a species found in a habitat.

Well done if you got that right.

Let's move on to a practise task.

Alex has sampled the daisy plants in the school playing field.

He used random sampling to gather the data to avoid bias.

He used a 0.

5 metre by 0.

5 metre quadrat for all his samples.

So the area of that quadrat is 0.

25 metres squared.

I'd like you to use the data in the table to work out first of all the mean number of daisy plants per quadrat in the school field and then the mean number of daisy plants per metre squared in the school field.

You need to make sure you show your working in your answer.

You'll need to pause the video at this point and then when you've worked out your answers to those problems, you can press play and we'll check to see if you got them right.

Good luck.

How did you find that? I hope that you got through that okay.

Let's see if you got the right answer.

So question one was to use the data in Alex's table of results to work out the mean number of daisy plants per quadrat in the school field and you needed to show your working in your answer.

So we've got our data, we've got a table of the number of plants that we've counted per quadrat.

To work out the mean, we're going to use the equation sum of the samples divided by the number of samples.

So we need to first of all add five plus six, plus one, plus 11, plus 10, plus eight, plus six, plus four, plus 14, plus three, plus nine, plus seven, and then divide it by the number of samples, which is 12.

So doing that equation, it's 84 divided by 12 and that gives us an answer of seven.

There are seven daisy plants per quadrat in the school field.

Well done if you got that correct.

The second question asked you to use the data in Alex's table to work out the mean number of daisy plants per metre squared.

And again, you are asked to show your working in your answer.

We know that Alex used a quadrat that was 0.

5 metres by 0.

5 metres and that means that the area of that quadrat is 0.

25 metres.

One square metre is made up of four 0.

25 metre quadrats.

When we found the average or the mean number of daisy plants per quadrat, we calculated that there were seven daisy plants.

So 0.

25 metres squared contains a mean of seven daisy plants.

To find out how many daisy plants there are in one square metre, we need to do a calculation of seven, the mean in 0.

25 metre squared, multiplied by four, the number of quadrats that make up one metre squared and that gives us a mean of 28 daisy plants per metre squared.

Well done if you got that answer correct.

That brings us to the second part of the lesson which is all about measuring the population size of a species.

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

We are going to measure the population size of a species, for example, we're going to look at daisy plants in a habitat.

You're going to need a quadrat, two tape measures which are at least 20 metres long, a clipboard, pencil and paper to record your results and some bags containing some number counters.

You can use pieces of card or pieces of paper, but they need to have the numbers from zero up to 20 written on them and it's a good idea to have two bags, one for your x-axis coordinate and one for your y-axis coordinate.

First of all, you need to think about how you'll record your results.

You need to design a table.

What is it that you need to record? Well, first of all, you need to record the quadrat number and then the number of the species that you count within each quadrat.

So your table should look something like this.

You're then going to have to go out into your sample area and lay out your tape measure in your survey field to make two sides of a square as we've got in this diagram on the screen.

Each side of the square should be at least 20 metres.

First of all, you need to decide which species you're going to sample in your habitat.

So if you're going to use your school field or a playing field, which species are you going to sample? Daisy plants, buttercup plants, dandelion plants? If you're not sure, choose a species that's relatively common and there's an identification guide in the additional materials that might help you to identify some of the more common species you're likely to find.

You also need to prepare two bags that contain numbered counters from naught all the way up to 20.

So here's an example that you might do and as I've said, you might prepare two bags, one with your X coordinate numbers and one with your Y coordinate numbers.

They can be pieces of card, pieces of paper, but they need to have the numbers clearly written on them and then put into the separate bags.

We're going to use the numbers and tape measures to locate our first position of our first quadrat in our sample.

We've got our numbers, the X coordinate numbers in one bag and the Y coordinate numbers in another bag.

And we've got our sample area set out using the tape measures, which go beyond 20 metres on the x-axis and the y-axis.

So selecting our x-axis coordinate, we take one number from the sample, that's the number four.

We can see four metres along the x-axis is at this point here.

We select then a number from the Y bag and our Y coordinate is 14.

Here is our 14 metres along the Y coordinate.

Where the two intersect is our first point and we put the bottom corner of our quadrat at that intersection.

We then put the the numbers back into the bags, ready for our second sample.

Here's a check.

Laura used random numbers to select a quadrat location.

She picked the number four from the bag that gave the X coordinate and the number 18 from the bag that gave the Y coordinate.

Which position indicates where Laura should place her quadrat? Is it A, B, C, or D? I'll pause for a few seconds and then we'll check your answer.

The correct answer is B.

We've gone along the x-axis four metres and the y-axis 18 metres and so coordinate B is the place where Laura should place her quadrat.

Well done if you've got that correct.

Once we've got our coordinate, we're going to lay the quadrat on the ground at the intersection of where the coordinates are generated as in the picture here.

We are then going to count and record the number of the chosen plant species that are within our quadrat.

And so if we look using our rule of the left-hand side and the top of the quadrat counting and the bottom and the right-hand side not, then we're going to count none in the first row, two in the second, one in the third, one in the fourth, and none in the fifth row.

So there are four plants inside this full quadrat.

We're then to select another set of numbers from the bags.

So here's 18 and for the y-axis two and we've got our coordinates where we're going to place our quadrat.

What we're going to do now is to watch a short video to see this in action.

We can estimate the population of the plant species by using the equation, estimated population is equal to the total area divided by the area sampled, multiplied by the number of plants sampled.

To work out the area sampled, we need to know the size of our quadrat.

Usually that's 0.

5 metres by 0.

5 metres.

And if we make 10 quadrat samples, then that number needs to be multiplied by 10 the number of samples.

So 0.

5 metres multiplied by 0.

5 metres, multiplied by the number of samples, which is 10, gives us 2.

5 metres squared.

The total area for the survey is therefore 20 metres by 20 metres.

That's the size of our sample area.

We stretched out the tapes 20 metres on the x-axis and 20 metres on the y-axis.

20 by 20 gives us 400 metres squared.

And the total of 50 daisy plants were counted in the 10 samples.

So using that, we can use the estimated population equation.

The total area is 400 metres squared.

The area sampled is 2.

5 metres squared and the number of daisy plants we counted is 50.

So we can work that out as 160 multiplied by 50 and that's 8,000 daisy plants.

Here's a check.

Aisha used a quadrat to estimate the number of dandelion plants in a school field.

Aisha took five random samples from the school field.

How could Aisha improve the investigation so that a valid estimate of the dandelion plant population can be made? Is it A, weigh the dandelion plants, B, gather more samples using the quadrat or C, count the leaves of the dandelion plants? I'll give you a few seconds and then we'll check your answer.

So the correct answer here is B.

Aisha could have improved her investigation by gathering more samples using a quadrat.

Five samples is probably not enough to get a valid estimate.

We're now going to do our final practise task.

Jun carried out a quadrat investigation to estimate the number of ribwort plantain plants in a school field.

The school field has an area of 500 metres squared.

Jun used a square quadrat with the sides of 0.

5 metres.

He carried out five quadrat samples and he recorded 12, eight, two, seven and 11 ribwort plantain plants.

First of all, you need to design a table to show the number of ribwort plantain plants that were recorded in each quadrat.

Then I'd like you to calculate an estimated population of the plantain plants in the school field and remember to show your working.

You'll need to pause the video at this point and then when you're ready, press play and we'll check your answers.

So you were asked first of all to design a table to show the number of ribwort plantain plants that were recorded in each quadrat.

You should've designed something like this.

Your table heading should be the quadrat number and the number of ribwort plantain plants.

Then you were asked to calculate an estimated population of ribwort plantain plants in the school field and show your working.

So the information you were given is that quadrats were 0.

5 by 0.

5 metres and the school field has an average area of 500 metres squared.

So you're going to use the equation, estimated population is equal to the total area divided by the area sampled, multiplied by the number of plants sampled.

So that works out as the total area is 500 metres squared, that's the school field area.

The area you sampled is 0.

5 by 0.

5.

That's the area of one quadrat.

And we're going to need to multiply that by five to find the area of five quadrats.

And the number of plants we sampled as we've got in the table is 12 plus eight, plus two, plus seven, plus 11.

So simplifying that down, it's 500 divided by 0.

25 by five, multiplied by 40.

And 0.

25 multiplied by five is 1.

25.

So that gives us a calculation of 400 multiplied by 40, and that's 16,000 ribwort plantain plants as an estimated population.

Well done if you got that answer correct.

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

Today, we've looked at how quadrats are one type of sampling technique which we can use to study plants and slow moving or sedentary animals.

They can be useful in helping us estimate the size of a population, and this is known as abundance.

When we sample, quadrats should be placed randomly and not thrown to avoid bias.

Random number generators can help to decide where quadrats should be placed.

And when carrying out an investigation to estimate a population size, it needs to be methodical.

Enough samples need to be gathered so that we get reliable and valid results.

I hope you've enjoyed today's lesson.

It's been great having you along again and I look forward to seeing you soon.

Take care for now, bye-bye.