[Music] hi and welcome back to three science lessons Kole you K by the end of this video you should be able to describe how to measure the population size of a common species in a habitat you should then be able to describe how to use sampling techniques to investigate the effect of a factor on the distribution of this species and this is required practical so it's important that you learn all the details we're going to start by estimating the total population of daisies in a field like this one to do that we're going to use random sampling and we looked at that in the last video remember that in random sampling we use a quadrat like this one quadrats come in a range of different sizes we're going to use one that's not point five meters by null point five meters first we place two tape measures at right angles these tape measures should be 20 meters in length we don't need two bags and each bar contains the numbers 1 to 20 in this practical students work in groups of three the first student removes a number from the bag imagine that this number is 8 this student now moves to the 8 meter point on one of the tape measures the second student now select a number from their bark aren't most of that point on the other tape measure imagine that they've got the number 12 now the third student places a quadrat on the ground at the 8 meter by 12 meter point like this the students now record the number of daisies in the first quadrant the students now return the numbers back to the box and then repeat this process nine more times for a total of ten samples imagine that the students counted 300 daisies across the ten quadrats we now need to estimate the total population of daisies in the whole field to do that we use this equation the total population size equals a total area divided by the area sampled multiplied by the number of organisms of that species counted in the sample in this case the total area is 20 meters multiplied by 20 meters which gives us four hundred square meters each quadrat was not twenty-five by not twenty five meters and we threw the Quadra ten times this gives us an area sample of 9.5 x naught point five multiplied by 10 which is two point five square meters and the number of daisies counted in the sample was three hundred putting these numbers into the equation gives us a total Daisy population of 48,000 daisies okay now I should point out that this is an estimate it's possible that our 10 quadrat throws do not represent the whole area for example there might be regions within the area with a much higher or much lower number of daisies than the average if we think that's the case then we should increase the number of quadrats rolls to cover a greater percentage of the area okay known this require practical we also need to measure the effect of a factor on the distribution of a species in this case we're going to look at the effect of light intensity on the distribution of daisies I'm showing you here another field of daisies but this field also contains a large tree in this case we need to use a transect line to see how the number of daisies changes from the tree outwards to do this we place a tape measure up the tree we then use a quadrat to count the number of daisies at the start of the transect we also need to record the light intensity at this point and to do this we can use a light meter or we can use an app now we move the Quadra one meter down the tape measure and repeat the measurements and we continue doing this all the way down the tape measure now it's possible that we will see a greater number of daisies as we move further from the tree that's because under the tree there'll be a lower light intensity remember that plants need light in order to photosynthesize however you also need to bear in mind that a tree will absorb a lot of water and minerals from the soil so it might be that light intensity is not the only abiotic factor but affecting the number of daisies remember your phone plenty of questions on this required practical in my vision workbook and you can get that by clicking on the link to both [Music] you [Music]