hi everybody and welcome back to miss Angela's biology class I am Miss angler in today's video we are going to be looking at population growth curves I'm going to walk you through how to identify them and interpret them as well as how the different phases are affected by the competition in populations and how that also affects the numbers that we see in populations now terminology is really important in this section so you need to take note of how to correctly use the terms as well when you do your explanations now if you like this video don't forget to give it a thumbs up and make sure you are subscribed because I post new content every Tuesday and Thursday if you are in grade 11 and you're thinking about improving your marks you should think about getting a copy of my grade 11 study guide it has so many tips tricks um of how to get full marks in your finals and any of your tests and exams that you write I also have a grade 12 one if you're looking into the future as well which is filled with all of my top tips for finals as well and you can get a copy of that on my website which is Miss angler.co.za now today what we're going to cover is these two major growth curves that you can see in front of you we're going to look at exponential growth and we're going to look at a logistic growth there is a lot of things that are playing sort of in the background of these uh growth curves and what that means is you actually need a lot of knowledge about habitats competition and how basically survival of the fittest Works in order for these two graphs to be drawn now before we get into the deeper explanations what I want to cover is two major words that you see very very often one of them is going to be this uh guy over here carrying capacity like what actually is it um because it's really important for populations and two how do you make that carrying capacity and what is it how is it formed going to look at that now too so as I mentioned earlier it's a really really really big deal to know the difference between your terminology in this section and two of the words that are often confused with each other are environmental resistance and carrying capacity they are definitely not the same thing and too many people confuse them with one another and I'm going to show you the differences between the two and then I'm going to show you um how they interact with one another so let's start off with the environmental resistance aspect of our population now every single population when it's growing um in the beginning it doesn't have to worry about resources because the population is small so they flourish however over time the environment is going to resist the growth of the population because the the resources become limited over time and these resources can be things like water if food sunlight space shelter mates Etc and so the environment resists the growth of the population now when it resists the population it creates a level or a maximum amount of individuals that the ecosystem can support which is what we call the carrying capacity so in other words the carrying capacity is met when there is a balance between the biotic or the living population and its environmental resistance so the resistance is what makes the carrying capacity it's almost if you think of it like this the environmental resistance is the process and the carrying capacity is the product it's the thing that we create I want you to know also that when we look at a graph often the carrying capacity is included as the sort of like dotted line and you will notice that the population often goes above it and then sort of fluctuates around it and they even level off I want you to know that the carrying capacity can actually move down and it can also move up depending on the environmental resistance which means if there are fewer resources the carrying capacity moves down if there are more resources the carrying capacity will move up and so it all is a fine balance between the living components which is the population and then of course the non-living things like their space shelter water Etc so let's have a look at the first type of growth that we see in populations and it is called exponential growth and it is characterized by this sort of J shape that it produces when you plot the points of course we are plotting over time and population size now there are some key things that we need to keep in mind when we look at exponential growth and I want you to look out for these things when you have to explain what is happening so exponential growth often occurs in a population that is starting off quite small maybe they are new to the environment or maybe the environment itself is new and so they are starting off with maybe four or five breeding pairs there's not going to be a lot of individuals what's also important in the exponential growth curve is you must acknowledge that there is plentiful resources the reason that we know that is because if we look at the graph in the beginning this first sector over here the growth is quite um slow and gradual and that is because um the amount of breeding pairs are limited and reproduction takes time it and you know just in general populations take a little while to establish however everything after that is exponential like there's a there's a sharp increase over a very short period of time that is because there are so many resources available very little disease very little competition everybody has enough of everything to reproduce at their maximum and so that's why we see a high birth low death rate now the last thing I want you to keep in mind is exponential growth happens often in a new environment it basically means maybe this organism didn't live there before and now they're newly introduced or an area that used to have them has been cleared out more space has been provided to them and now they have um you know like an area to establish themselves and often we see exponential growth in organisms like bacteria we see these in like rodents and mice or butterflies there's like an explosion of growth and then what is missing in this graph which I'm going to include for us now is what we call a drop-off point and I'm going to draw it in for you essentially what happens sometimes is a population will dramatically explode they'll use all their resources and then the numbers just completely drop off and that might be a breeding the cycle locusts go through these breeding Cycles where in the next season that comes around their population dramatically increases and then they drop off completely and that is because certain organisms go through these breeding Cycles where they've got lots of resources they use all the resources and then they die and then they repeat it all over again in the next breezing breeding season Now we move into the next growth curve which follows exponential growth and that is logistical now logistic growth again has a very very defining shape you'll notice it has an S shape to it and it's actually made up of two components and what I mean by that is it's actually got a couple of phases to it the first phase we've actually just looked at which is the exponential growth which happens in the beginning here but then later on it's followed by the logistical part which is here when it starts to level off and flatten now again you are going to need to be able to draw these graphs label them and you need to explain what's happening in these populations and so I'm going to give you some things to look out for that you can use to explain yourself in an exam now the most defining feature of a logistical growth curve is of course the carrying capacity as we can see it over here which we spoke about in the beginning is the maximum amount of individuals that we can support and in electricity physical growth curve we have approached the carrying capacity which means we've gotten up to the maximum number and we've now settled at the maximum number sometimes logistical growth curves can actually do a little bit of a wave all the way along which is also very acceptable because that can happen so they are not always perfectly flat at the top is what I'm trying to say and what I would like you to keep in mind so when you are explaining physical growth these are the things I want you to keep in mind as you explain them number one we need to know that this is actually an older population which means the population has been around for a while it's established itself it's gone through already it's exponential growth phase it's now moved into a different phase a more stable one and so because it's past its exponential phase it means that it's not using up all the resources rapidly and it's living very close to the carrying capacity if not actually on it itself now there are two more phases of growth that I'm going to speak about soon when we look at the different phases that happen before we get to the plateau and when I say Plateau I'm referring to how the graph can actually start to level out and become flattened like that now that is due to the carrying capacity which as we mentioned is created by environmental resistance now sometimes what can happen as I mentioned to you is we get what we call an overshooting event which would look something like this if I were to just take away this line here and I'm going to continue the graph what it means is that sometimes you overshoot the carrying capacity you drop down past it and then you come back up and then you flatten out this bit over here is called the overshooting event and essentially it means organisms are rapidly reproducing making mini babies and then all of a sudden the ecosystem doesn't have any resources to support them so the ecosystem and the individuals take a collapse but the ecosystem recovers and the population can start producing reproducing again and got back up to where they were before now eventually the population is going to level out and that is because remember when you are explaining why does it level out it levels out because environmental resistance has created a carrying capacity which limits the amount of individuals the ecosystem can support and therefore we end up with a plateau or a stable population curve now as I mentioned to you earlier you will need to be able to draw these graphs and label them but not just label them with their time and population numbers you must also be able to identify the phases of growth in particular logistical growth because it actually has four phases that you need to be able to identify so first of all we're going to start off with this first phase here which is the lag phase now the reason why it's called the lag phase is because there are few reproducing couples and the problem with the lag phase is it takes time to reproduce it takes time to find a partner to reproduce with so in the beginning the population growth is very slow and we call it a lag then we move into exponential growth now exponential growth means that there are many resources and of those resources there is also many breeding pairs and that is why you exponentially grow because at first you were breeding with two individuals that became four four that then became eight eight that then became 16. in the beginning that's actually quite slow but after that becomes 32 and then 64 and you'll see that there's a rapid increase uh in growth and that is because you have so many more adults who are mature enough to reproduce and they start replacing themselves exponentially while their own children are still alive also making babies and so you have this compacting effect where you add more and more individuals The Next Step that we see is the transitional phase and in your textbook it might be called something different like the decelerating phase that's also an acceptable way to describe it as well but as the name suggests this is when growth starts to slow down and the reason for that is because number one you are approaching the carrying capacity which if you remember from the beginning of the video that is the maximum amount of individuals that we can carry which is shown here by the started line this red dotted line and as you approach that maximum amount you are going to slow down in growth because you are running out of resources so the resources are becoming limited and because they're becoming limited you can have fewer babies which means that the reproduction rate is slower you do notice here that the line of the population goes just above the carrying capacity and then it dips down below and then it comes back up again and then dips down back a low again and that's actually quite normal um some tables or some textbooks will have the line go flat others will have it like fluctuate like this they're both correct but in real life circumstances most populations do very small fluctuations up and down which brings me to the final stage which is what we call the plateau or the stationary phase and essentially what that is when we're looking for things is a population that is fluctuating around the norm and what that means is you fluctuate uh around the carrying capacity and the reason why we become stable is because stability is a result of resources you have just the right amount of resources to support just the right amount of organism and so they balance out with one another the living components now balance with the non-living components and that's the maximum amount of individuals an ecosystem can support in a healthy way I want to remind you though that this carrying capacity can of course move down if we we lose habitat it can also move up if we gain habitat now I just wanted to include some examples of different organisms showing their different populations all three of these graphs are logistical growth and I know that because of our s shape that is forming it shapes slightly different in each of them and I wanted to quickly talk about why that is but essentially they make the same shape what we have here is a collection of different kinds of organisms we have a paramecium we have a defina and we have a fur seal now I wanted to show you the differences between them because it's quite interesting the emphasis or the difference in each of these organisms has to do with how long they live for and you will notice that for the first two examples they're a life cycle and their population cycle is in days whereas our first seal here is in use and that's interesting because what it means is different organisms have different rates of growth when you are unicellular like our first two organ organisms here the paramecium and the defenia they are unicellular which means that they can grow very very quickly another important thing to keep in mind here is these are both laboratory examples which means they are placed in Prime conditions and that means they have lots of food lots of space they don't have any diseases or anyone else to compete with you know not other species to compete with so that's also something to keep in mind versus our little fur seal here which is out in the wild and in nature that was what you need to keep in mind when you discuss these kinds of things and you discuss the steepness of the graph you will notice they all have a lag period which is when each of these populations is establishing themselves they all have an exponential growth point some steeper than others and that means technically the paramecium is very steep because this is over 5 and 10 days they have their greatest increase if you compare it to our seals they actually have their most exponential growth over here and that is about from let's say 1920 to 19 let's say 30. so over a 10 year period they have had their exponential growth versus five days so it's really important that you are able to read the graph properly when you're explaining how different types of animals uh grow they also all have a DC exhilarating Point as you can see here they're slowing down and then they all have a plateau or a point that they level off on now the final thing I want to discuss is what happens if you overshoot the carrying capacity but you don't recover and so we at this point have only discussed the following which is a population establishes itself in the lag phase or the establishing phase it exponentially grows it slows down and it becomes stationary for now that's what we have discussed and when it becomes stationary like that we need to remember that there is a portion of the carrying capacity that we can't see here which I was going to put here in a dotted line that is where that is in this newer graph though I have added in the death or the decline phase that you can see here and essentially that is illustrating what happens if the carrying capacity continues to drop and does not return back up to where it was before in other words there has been some major events now this major event could be independent or dependent on the density so what that means is maybe we have used up all the resources so all the food is gone or it might be independent which means it has nothing to do with how big the population is and that could be something like a natural disaster maybe like a flood or a fire and what can happen is an entire ecosystem can disappear the carrying capacity lowers and gets lower and lower and lower because it cannot support the organisms and they end off dying and becoming extinct so this would be at the bottom here an Extinction point this organism has become extinct in this particular graph this is a bacterial population so don't forget to make sure you familiarize yourself with what organism this is this is bacteria and this is what we see in hours so this is about a 10 hour period and this is very very very possible and in my experience of how it explain this is this is probably a resource issue if we were growing bacteria in a Petri dish they have a limited amount of space right which means they also have a limited amount of resources so once the bacterial population starts to grow and let's imagine each of these dots is the bacteria growing and reproducing once they take up all the space and all the resources in the dish what you will find is a mass sort of death event where all of these bacteria start to die and there's no way for new resources to be added in a Petri dish right because a petri dish is fixed it's not alive it's not growing it is a isolated space and so that is how you can overshoot the carrying capacity in a lab you can also do it in real life as I mentioned to you you might have some kind of flood or fire or you could have global warming which is a slightly longer process but still results in extinction or ultimately you could have a disease that kills all the females you're only left with males and that means that males can't reproduce without females so the population becomes extinct now as always I like to finish off my section with a terminology recap please use these words to study from for flashcards but also these are the words that you should be using when you are describing growth in graphs first of all we get two major kinds of graph we get the exponential graph which is the graph where we see slow growth followed by a rapid burst of growth and it is continuing to grow it has not reached its peak yet that is Then followed by the logistical growth curve which is when exponential growth has occurred but now it's being followed by a period of stability and the growth is slowing down and becoming flat like a plateau now what creates that Plateau what creates a logistical graph shape is the carrying capacity which is the maximum amount of individuals a ecosystem can support So as you move towards the carrying capacity the graph will become flatter and who makes this carrying capacity is something known as environmental resistance which is essentially the environment resisting the growth of the population and the environmental resistance is things like space Water Shelter mates soil if you're a plant and light if you're a plant these are all the things that if we calculate them all together it will give us a number of how many organisms we can support now there are three major growth points or major phases in graphs of populations we get the lag which is the initial stage it's slow there's not a lot of breeding Partners we get the acceleration point which is where there's Financial growth lots of babies lots of healthy children and very low death and then it can be followed by a deceleration point and it leads into what we call a plateau or a flattening out where the population has met the carrying capacity and it is now stable now please note that this final video for this section is focusing on the graphs in general I will be doing also a predator prey cycle which is a very unique growth curve because there's lots of fluctuations in it so keep your eyes peeled for that video If you like this video don't forget to give it a thumbs up and make sure you're subscribed and I will see you all again soon bye