hi everybody welcome back to miss Angela's biology class I am Miss angler in today's video we are going to do the introductory episode to population ecology and this is going to be one of many videos I will make on this topic some of the other topics that we will still cover will be things like different types of growth curves Predator prey relationships and so much more but in this particular video we're going to look at what a population is how to count it and what are the factors that affect the size of that population now if you are new here don't forget to give this video a thumbs up and make sure your notifications are turned on and you're subscribed because I post new videos every Tuesday and Thursday so the first most important component of our understanding is what exactly is a population so when we speak about population ecology we are talking about a group of individuals that are the same species and they are found in the same place at the same time and so that's why when we talk about populations we can talk about populations that are perhaps in a country like a country's population but we can also talk about the human population too and that's all the individuals together now when we look at population ecology we specifically focus in on the things that affect their numbers so what is making the population bigger what is making the population smaller and there are four key components that you need to know and understand how they affect the population starting off with this picture here we have a little group of small birds let's say they are a group of sparrows and we need to see what affects their population so the first thing we look at is births now births have a positive influence on the population which means that they add more individuals however on the other side deaths is going to be a negative influence and that then means that we minus individuals from that population then we have immigration now immigration is often confused with its sister immigration but essentially immigration is when individuals are moving into a population so this has a positive influence on the population numbers you adding individuals whereas immigration is removing individuals and a nice way to tell the difference is immigrations like exit so it's leaving immigration sounds like in like they're moving into the population now an important component of population ecology is being able to actually count populations but the problem is when it comes to Counting organisms is that they don't all behave in an easy way for us to actually count them and so we've got two main ways that we're going to learn now about how we can count organisms depending on whether they move around or if they stand still and make it easier to count we're going to start off with organisms that move around and we're going to start with the method of Mark and recapture using a specific formula and I'm going to use these snails to demonstrate how you actually calculate this now what we do is we use this particular formula we see alongside and you should always write this out in an exam because you always get like a mark for writing the formula but let's break down the letters and I'm going to show you how to apply to our snail population here so n is the estimated population that's what we're trying to solve for M is the total amount of organisms we captured the first time C is the total amount of organisms we captured on the second time hence the name Mock and then recapture and R is the total marked individuals that we caught on the second time that were already marked and you'll see now what that looks like once I use it in this snail example foreign so applying it to our snails let's say these are the snails that I have caught in my first grouping so I've caught them now and what I need to do is I need to actually Mark all of them so I would go along and I would Mark each of them on their shell and it's important and you would need to know this also for tests and exams that when you mark your snails your animals whatever you've caught you need to make sure that the way in which you mark them doesn't hinder their movement it doesn't hurt them in any way and when you put them back into nature you need to put them back where you found them or relatively close to that and when you do the second capture so when I go out again let's say two to three days later I need to do it in a very short period of time I can't come back months and years later to do the second capture so let's say for example I've gone out now and I've caught all these snails and we have one two three four five by four so we've got 20 into visuals here so in my first capture I have got and let's put it into our formula n equals 20 multiplied by and I'm going to do our second capture so I'm going to come back three days later and I'm going to count my next group of snails so I've gone out and I've caught my second group of snails but you'll notice that this second group of snails has some that are counted and already have a little green mark on them and then others don't what I need to do is I need to again count the total population that I've caught the second time around so the second time around I still caught 20 but now we need to divide this number by how many individuals were already recaptured so let's actually just count we've got one two three four five six seven eight nine ten eleven snails that were already marked which means that I've counted them actually for the second time now what you need to do is take this and calculate it in your calculator now this gives us a population of 36 comma 3 6 but it's important that you don't leave the answer as a decimal because you can't have like a third of a snail you need to have a whole number and so you actually need to round down which I know conflicts with what we do in maths we generally round up if it's more than or less than but in this instance we're always always going to round down it doesn't matter if this was 36 comma 5 you must round down so you get a whole number and so this is the estimated number of snails in this particular area of 36. now the next counting method is what we call quadrat sampling and basically we're using squares to count how many individuals are in that specific sample area now quadratic sampling is great for organisms that don't move very much or are completely immovable um because you use a plastic or a metal square a one by one meter Square to see how many individuals are inside of that square and so when you place it down you don't want them to be able to move very quickly because otherwise they'll just walk over your quadrat and out of your area so this is great for things that move slowly or don't move at all one thing to keep in mind though when you are quadrat sampling and you can have a look at the diagram on the left here is if we are counting for example these crabs you're going to count the crab that is mostly inside of the quadrat if you get an individual who's on the edge or it's got most of its body on the outside you're not going to count it so in this example you should know that there are five crabs in this quadrat and not six but let's actually apply it to the picture we see on the right hand side which is a representation of a field of sunblowers each one of those dots represents a sunflower and I need you to know that one of these blocks is one by one meter and it's going to form that uh square meter Ridge that we see um in the diagram it's really important to figure out what it is there are 10 by 10 blocks so that means that this is 10 meters and this is 10 meters and we need to know the size of the area we are working with when we do the calculation and when we calculate the estimated population size now the next thing we need to do is we would take a a random selection of samples of the quadrats and so each of these squares represents one quadrat and I'm going to select five one two three four five right those are five random samples and now we need to count how many individuals are in each so in quadrat one we have one two three in quadratic two we have one two three in quadrat three we have again one two three in quadrat four we have one and two and in quadrat five we have one two three four right so you're gonna take all these individuals now you're going to add them to get it to get a total so we've got nine ten eleven twelve thirteen fourteen fifteen so in total we have 15 individuals we are now going to take those 15 individuals and we are going to divide them by five to get an average of how many are in each of the quads and that gives us three we then take that 3 and what we need to do is we need to multiply by the total amount of quads we have which was 10 by 10 which means we have a hundred and that means our estimated total population size is 300 Sun Flowers again everybody if you ever get a decimal answer you always round down now once we start counting these populations we can use them to plot graphs and we can create a number of how many individuals we have over every single year and we can start to see what the population is doing with its numbers now it's interesting to know that when we do these counts there is actually a maximum number that an ecosystem can support and that's where the carrying capacity comes in it is the maximum amount of individuals an ecosystem can support and if it goes over that number you often see a decline in the population now what actually is the carrying capacity it is everything from the resources available like food shelter space that creates a maximum number and if we have a look at the graph here we have time and we have the number of rabbits in a population and then we have the carrying capacities that's the maximum amount of rabbits that we can support you'll notice in this graph the number of rabbits starts to slowly increase and then dramatically increase because it's got lots of resources and then when it gets to the top here and it reaches the carrying capacity it levels out it flattens out and that's because there are fewer and fewer and fewer resources because there are more and more rabbits using those resources but there's a little bit more to this story if we look at the same graph again and we insert the carrying capacity here as the dotted line the carrying capacity can actually move down and it can move up depending on how many resources and individuals they are so what I mean by that is if there are far too many individuals the carrying capacity can move down permanently and that's because you have removed so many resources that it can move down however if an area is opened up maybe there is a new piece of land that now plants and animals can go into and maybe humans have moved away from that area so a whole new ecosystem opens up that means the resources increase and so the carrying capacity can also increase but something also that maybe we are not aware of is when we talk about carrying capacity we must also know that the graph is never this smooth like we see in the first drawing in actual fact if this is the carrying capacity when a population is is approaching that line so it goes slowly at first and then when it gets to this top part of the carrying capacity it often actually overshoots it and what that means is there's so many individuals they're rapidly reproducing there's lots of resources and then all of a sudden there are way too many of them and what does that cause it causes a dramatic dip in the population once it's done this overshoot it actually self-corrects really well and doesn't drop all the way back down again it will come back up and then level out a lot better if it doesn't do this perfect leveling out what can also happen is it can go up and overshoot and then fluctuate gently around the carrying capacity and that's often the most common and realistic growth curve is where you fluctuate slightly above and slightly below the carrying capacity over time what we also need to talk about is what is keeping the population curve below the carrying capacity so what's pushing down on it when we look at these diagrams and often in textbooks you'll see these arrows pointing down and that represents what we call environmental resistance and environmental resistance refers to basically the limit of resources that are available in an ecosystem and the environmental resistance resists the population growing and so it does play an important role essentially in creating the carrying capacity so just to clarify the difference between the two words the carrying capacity is the maximum amount of individuals an ecosystem can support and that carrying capacity is created by the environmental resistance which is linked to the resources and the amount of resources that are available and it is the force that is pushing down on a population and it prevents them from overshooting the carrying capacity and staying above the carrying capacity now what we need to also focus in on is these factors these things that are influencing the size of a population the things that create the carrying capacity that create the environmental resistance and that is where the density dependent factors come into play and the density independent factors now when we speak about density dependent factors the examples that we use are things like food space shelter predation and disease and these are the things that are linked to density dependent because it depends on how large or how dense a population is so so sort of think of it like this um when we talk about the food dependency right if there are more individuals so there's a higher density there will be a greater need for food likewise if there is more individuals there's going to be actually more disease so these are the factors that control a population and they are based on how big the population is another one for example is predation the more there is of prey the more predators and ecosystem will then provide to maintain the population numbers on the other hand we have density independent factors now these are factors that are not linked to the size of the population and they are rather linked to natural phenomenon and the two main ones are climate and natural disasters so so sort of think of it like this um it doesn't matter how many individuals they are in a population um the natural disaster that comes along let's say it's a tsunami um the tsunami that wipes out the population it it didn't matter if it was a big population or a small population so when we control that population size it didn't matter how big it was it was independent and this natural disaster occurs and it affects the whole population likewise with things like climate so think think like drought or floods again that's not directly linked to the size of the population if it's a big population it's not going to create some kind of flooding situation so when these independent factors occur they are independent of how big the population is and regardless of the the population size they will equally be affected every individual there's no survival of the fittest necessarily there is no bigger or better or adapted they are all equal and they all succumb to the drought the flood or the tsunami now as always I like to end off my lessons with the terminology recap and you can use all of these terms to create flashcards and to study from or to mind map out your ideas or a blurting method is really good for this kind of thing so you can write down the word and everything you remember about it but we first thought of all looked at population which remember is a particular group of organisms in the same place at the same time and they are the same species it's important that you know the difference between population and the definition for species then we looked at how birth and death have a negative or a positive impact on a population by adding or subtracting individuals we also looked at how immigration which is the movement of individuals into a population versus immigration which is individuals leaving a population how they impact the overall population numbers we looked at two methods that we count individuals we use Mark recapture for organisms that move around found a lot and we have to capture them more than than once we used a quadrat method for organisms that are either really slow moving or they don't move at all and we count in average and we get an estimate we spoke about the carrying capacity which was the maximum amount of individuals an ecosystem can support and that carrying capacity is based off of the environmental resistance that occurs that is when the environment resists the populations growth and finally we looked at the two types of density factors that can influence the size of the population which was the density dependent which is linked to the size itself the bigger the population the more problems the less food the less shelter whereas density independent factors are not linked to the size of the population and are rather linked to things like natural disasters climate change things that are not necessarily linked to the size of the population now if you like this video don't forget to give it a thumbs up and make sure you are subscribed with your notifications turned on because I post every Tuesday and Thursday and I will see you all again soon bye