good morning welcome to unit 7 ecology concept 3 notes we are going to be talking about a branch of ecology called population ecology which is specifically looking at the relationships between organisms in their environment on a population level so just a reminder from concept one intro to ecology a population is just a group of organisms of the same species living in the same place so we would be looking at all of the penguins you know in this one area of the arctic and we would be looking at how they interact with each other and then how they interact with other organisms of different species and then how they interact with their environment so we're just looking at the population level here so when we're looking at a population there's a couple of different ways that we describe populations now you can describe uh you can quantify a population of organisms by just the number there are like there's a hundred penguins or there's 5 000 chickens or whatever there may be but a more helpful number to use and to quantify a population is a population density so this measures the number of individual organisms living in a defined space and you could use any unit to measure this so if there are you know a hundred chickens living in a hundred square feet of a coop you could you would divide a hundred chickens by a hundred square feet and say that there are one chicken per square foot in the coop um if you're looking at how many deers are in a certain acreage of a farm you know if there are a hundred deer and there are 20 acres you would do 100 divided by 20 and say there are five deer per acre so as long as you keep track of your units as organisms per the spatial unit you're good on how you do that math it's a pretty simple math equation if you will but so when we're looking at population density we can refer to populations as being a high population density or low so notice the defined space here is the same but the there's a lot of organisms in this given space and then not as many organisms in this given space so that's why we refer to population density and there are a couple things that affect population density so birth which is also referred to as natality and immigration or organisms moving in to a given space both of those will create a higher population density because they're going to add organisms to our given space death or mortality and immigration or immigration or i always say immigration those cause low population density because they're going to be removing organisms from the defined space um i think i for coming in and e for exiting to remember which are which so those are two are those are some of the different factors that can create a higher population density or a lower so those are the larger overarching factors now another way we can kind of um look and describe populations is not just by their density but we can also describe populations by their survivorship and a survivorship curve is a graphic representation of mortality patterns if you remember from the last slide mortality means death so it's going to look at the number of individuals in a population that can be expected to survive aka not die to any specific age okay let me explain more what that means we kind of have three types of survivorship patterns that populations tend or species tend to follow so one is a type one survivorship and that's based on this graph so look at we're looking at age so as we're going to the right we're getting older and then as we're going up or um like how likely you are to survive so type or one organisms they live early in life they live long lives and then they die late at life these are called late loss species and in order for this to happen they often have a heavy investment of parental care you know so they're these species you're gonna have less offspring because you're gonna put a lot of care into keeping them alive for a long time and an example of this is humans you know humans tend to have less children so that they can put a lot in their children and get them to an age where they can survive and be independent on their own that would be an example of a type 1 survivorship type 2 you see is constant loss so mortality if you're going to die or survive it's unaffected by your age um so some bird some rodents you're just kind of having a medium amount of kids your whole life you're doing a medium amount investment into their survival um and that's like the same amount are kind of going to die at birth versus diet middle age versus diet late age it's just kind of more random type three are early loss survivorship so these are species that produce a ton of offspring right away all at once knowing that a ton of them are going to die and just hoping that some will go on to live and so these are species that don't really care like a lot for their organs once they're born it's just kind of like good luck hope you some of you survive and this is how fish and mosquitoes and some populations like that live so this is a way that we can kind of represent from a visual standpoint that idea of mortality um another way we can kind of describe populations with is how are they are dispersed within their defined space so dispersion is just looking at the spatial distribution of organisms in a population remember when we were looking at that high population density or low population density kind of looking at how they're spaced out within their given space and there's kind of three patterns we see one is just random so if each of these dots represents you know a different organism they're just kind of randomly scattered in this defined space there's no real pattern uniform or even is where they really are spaced out pretty evenly among the resources in their defined space and then clumped is where they group up in like little groups of organisms throughout the different space so these are different ways we could describe how organisms are dispersed in a population another way we describe populations is by how they grow and they tend to follow two patterns either exponential growth or logistic exponential looks like this um it reminds me of that quote from mean girls the limit does not exist there's no limit the population is growing without limit and this is how the human population is right now we are just creating more resources as we need them so that we can keep growing and growing and growing now logistic growth is what we see in nature and i remember this because the logistic has an s in it and it kind of looks like an s so you see a population growing quickly you see that exponential growth at first but then it levels off this is how most natural populations are and this is a natural thing that happens because of limited resources creating competition so organisms compete for those resources and some die off and that leveling off point is called the carrying capacity it's a theoretical maximum population that a given environment could support based on the resources it has and it's theoretical because this level can change as resources increase this carrying capacity can be raised as resources decrease it could be lowered and so that's what we mean by carrying capacity now what limits the growth of a population what makes there be a carrying capacity at all and why don't we have one per se these are limiting factors so these are any aspect of the environment that can limit the size a population can reach and we categorize limiting factors as biotic which means living or abiotic which means non-living so examples a living biotic factor could be competition you're competing with other living things it could be predation being hunted down by another organism it could be like a bacterial disease that comes and sweeps through that sort of thing whereas abiotic would be like a climate thing like a natural disaster or flooding or something happens that can limit a population's growth we can also classify limiting factors as density dependent or density independent so this is where going back to understanding population density from the very beginning of our notes is important so a density dependent limiting factor depends on the density so it's going to have a bigger impact on a population that's more dense and a lesser impact on a population that is less dense these so these are factors that are triggered by an increase in population size and thus crowding so for example competition predation parasitism disease all these biotic things are density dependent so think about this way in terms of disease think about you know right now as i'm recording this we're living through the covid pandemic and one of the main things to help prevent coved is not being in densely crowded areas because the more dense a population is the higher the density the more likely that that disease can spread and affect more people whereas if we socially distance and we're not near each other and we're spread out the disease has to travel from person to person and so it can't do that it would have a lesser impact in a smaller space so covet has had a much bigger impact in new york city where it is extremely high population density as opposed to picking south carolina which is a really small town in the middle of upstate south carolina so that's where i did my student teachings that's why i brought it up so that's what we mean by density dependent and depending on the density whereas density independent is the opposite its impact is independent of the density it's going to regulate population growth regardless of whether it's new york city or pick in south carolina it doesn't matter all species in an ecosystem are affected equally by these density independent limiting factors so it's going to have an equal impact in new york city as it would in pickens so for example weather changes pollution natural disasters etc so whether uh you know an earthquake hits new york city or it hits pickens it doesn't matter it's not it's not the earthquake doesn't get bigger in size when it sees more living things as opposed to when it sees less it's just gonna affect it and that's what we mean by a density independent limiting factor and that's your overview of population ecology