[Music] hi and welcome back to freesciencelessons.co.uk by the end of this video you should be able to describe how to use quadrats and transects to determine the abundance and distribution of species in an ecosystem in a recent video we saw that living organisms are affected by a number of abiotic factors and remember that abiotic factors are not living abiotic factors include the light intensity the temperature and the levels of moisture the levels of abiotic factors can vary from place to place and this affects the number of different organisms that we find in any location so in this video we're looking at how scientists determine the number of organisms in an area to do this scientists use a technique called sampling and there are two ways to do this these are called random sampling unsampling along a transect we're going to start by looking at random sampling random sampling is used to compare the numbers of organisms in different areas to do this we use a quadrat and i'm showing you a quadrat here a quadrat is simply a wooden or plastic square to use a quadrat we place it on the ground and then count the number of organisms inside the quadrat so this technique can be used to sample plants or slow moving animals imagine that scientists wanted to investigate the following hypothesis fewer ferns are found in light conditions than in dark conditions so in this case we're comparing two different areas one with light conditions and one with dark conditions that means that we need to use random sampling i'm showing you here the plants found in light conditions we've got dandelions daisies and ferns so we're going to use a quadrat to carry out random sampling and count the numbers of these three different types of plants in random sampling the quadrat is placed at random locations across the area and one way to do this is to use random numbers to select the different locations we'll see how to do this in the next video when we look at the required practical on sampling you then count the numbers of each organism in the quadrat so here's the first random location and now we count the numbers of the different plants in the quadrat now we move the quadrat to a different random location and count the number of plants here and we keep doing this until we've sampled a large number of random locations now it's very important that we place the quadrat a large number of times this makes it more likely that we get valid results if we place the quadrat only once this might not give us a sample which accurately represents the whole area for example if i only place the quadrat here we can see that this region has fewer plants than the other areas so we need to place the quadrat a large number of times to get valid results okay so here were the dog conditions and again we randomly place the quadrat a number of times and count the numbers of the different plants so this technique allows us to compare the numbers of the different species of plants in the light and dark conditions from this we consider our hypothesis is correct we do find fewer ferns in the light conditions compared to the dark conditions now we can also use random sampling to estimate the total population size of a species in an area to do that we use this equation the total population size equals the total area divided by the area sampled multiplied by the number of organisms of that species counted in our sample and we look at this equation again in the required practical on this topic coming up we're going to look at how to carry out sampling along a transect okay now sometimes we want to investigate whether the numbers of species change as we move across a habitat and to do this we use sampling along a transect a transect is simply a line such as a tape measure or a piece of rope and we place the transect so it runs across the habitat that we're looking at we then use a quadrat to count the number of organisms at intervals on the transect this picture shows scientists carrying out sampling along a transect on a rocky shore we can see the transect line on the quadrats so remember that we use sampling along a transect to see how the numbers of organisms change across a habitat a good example is a sand dune and i'm showing you a sand dune here the species of plants that we find on a sand dune change as we move inland from the sea and we can measure that using sampling along a transect so here's our sun dune and you can see that we've got the sea the beach and the dune this simple sand dune only has four species of plants however a real sand dune can have quite a few we've got simple grasses small bushes small flowers and trees so we're going to use sampling along a transect to see how the numbers of these organisms change across the sand dune first we place a tape measure on the dune running from the beach inland we now place our first quadrat at the start of the transect and we count the different plants in the quadrat in this case on the beach there are probably no plants at all we then move the quadrat closer in land by a set distance for example two meters and count the number of plants again and we keep doing this as we move further inland now it's important that we carry out this transect a number of times to get valid results so to do that we move the tape measure along and repeat the whole process and this way we can get valid data on how the numbers of the different species change across the dune remember you'll find plenty of questions on random sampling and sampling along a transect in my vision workbook and you can get that by clicking on the link above [Music]