Population Ecology

Hi. In this lesson, we'll be talking about population ecology. Ah, population is all the organisms of the same species that inhabit a certain area. Now population Ecologist. They're really gonna be interested in looking at the abundance and distribution of population and how that changes over time, some useful metrics. They'll look at our population size, which is abbreviated with capital letter end, which just represent our, which is just looking at the number of individuals in a population. So it's just a simple tally, and they'll also look at population density, which is the number of individuals per area or in some cases per volume, like, for example, looking at organisms in the ocean. Now you can see here a nice graphic that shows us the population by country. Now, this can actually give us an idea of population density. For example, you might notice India here is quite dark, meaning that it's going to have a pretty high population. Now Russia here is a lighter color. It's also a much bigger country, meaning that the population density in India is definitely going to be a lot higher than in Russia. Now, of course, population Ecologists are going Thio want uhh slightly more specific information than the density in the whole country of Russia. But you get the idea now they're also gonna wanna look at range. This is the geographic distribution of a species, and it's going to be due to biotic and a biotic. Factors in the biotic factors will be like the other organisms present in an area, and you know whether they can act as food, perhaps, or whether they'll compete with another species. Now, some of the a biotic factors or things like climate and, uh, physical barriers like mountain ranges or bodies of water like the ocean. And here you can see an example of range. We're looking at the distribution of the common juniper. Now, population dynamics look at the size and age composition of populations, and they're interested in the processes that drive them. So one of the things they're gonna be interested in is immigration and emigration. Immigration is the influx of new individuals from another population, so individuals from some other population will enter our population, as you can see here, and immigration is the movement of individuals away from a population, so that's going to be reducing our population size. Now, immigration and emigration, combined with births and deaths, will basically give you a good idea of the, um, sort of size and stability of a population now. Populations that are separated by space. You know, some type of space, but interact in some way are called meta populations. So, basically, if you have, let's say two populations that live on either side of a lake population one over here and population to over here. And occasionally they swim out into the middle and, you know, do a synchronized swimming routine or something, you know, just to entertain the local folks. Well, then you have a meta population because thes two populations are separate, uh, separated by space. But they do interact in some capacity. When they do, they're synchronized, dancer, swim or whatever it is Now, immigration, immigration can actually link populations into a meta population. So if you have those two populations again separated by space and maybe they don't really see each other, they don't do those synchronized swims anymore. But occasionally, Cem people from population one move over to population two or vice versa. Then you still have a meta population. Now the thing is populations in meta popular populations in a meta population that is will regularly go extinct. But individuals can colonize new territory. So in a meta population, you know, you might have, let's say, three populations in the meta population. Even if you know, let's say population to goes extinct. You might have some individuals, uh, start a new population and, you know, return to a state. We have three populations in your meta population, still or, for example, you know, if you think about it in much greater scale. You know, if you have 100 populations in your meta population, even if one of those goes extinct, the meta population, it's still going to survive. Right with that, let's go ahead and flip the page.

dispersion is the distribution of individuals in a population throughout an area. We're gonna look at three types of dispersion here. The first is random and in random dispersion. The position of one individual is independent from another in the population. Usually this is due to random factors like wind dispersal. So if you can imagine, uh, dandelion blowing its seeds off into the wind, you might end up with a dispersion of those seeds like you see here totally random. Now clumped is going to be when the organisms group together. And usually this is due to social factors. And you can see, here we have these groups of organisms clumping together in the population. Last type we're gonna look at is uniforms, and in uniformed, the individuals are more or less evenly spaced throughout the area. It's not going to be, you know, as perfect as this image here, but they're more or less evenly space, and it's usually due to competition. For resource is, uh In fact, you can especially see this if the animals, uh, ex rather express territoriality. So they have a territory this bounded space that they're going to defend. So if each of these organisms is defending its territory. Then they're going thio have a distribution? Um, that spaces them out because they're going to have thio make sure their territories don't overlap. For example, no one way that population Ecologists estimate population size because, as you can imagine, it's impractical and kind of impossible to just go and try to count every single organism. So one of the methods they use is the mark recapture method. And basically what you do is you capture some organisms and you mark them like you can see with this snail. Here has this little sticker giving, giving it a number, and then these marked organisms will be released into the wild again. Ecologists will later capture another group from the population, and they'll look at the recaptured group for marked organisms. And usually they'll iterating this a few times. So you can see here is like the initial batch of organisms they catch. And here's the Rick are great. Recapture here, right, because they've recaptured a few, and you know, they'll usually iterated this a few times and then use statistical models to estimate the population size. You guys don't need to worry about the math, just the general idea of capturing a group of organisms marking them and then capturing new groups and seeing how many in the new group are marked with that, let's go ahead and flip the page.

demography is the statistical study of populations and how they change over time. A demographic is a section of the population group based on some commonality. Now, age structure is the number of individuals of each age that are alive. You can see a generic representation of this here and an actual chart showing Libya's age structure in 2011. Now a generation is a term that refers to the average age between a mother's first offspring and her daughter's first offspring. Essentially, it's a way to kind of gauge, like the turnover rate, the rate at which your offspring can, on average, start having their own offspring. Now the life a life table is, ah, statistic group of statistical data for population that will relate to their life expectancy and death rate, or how long they're expected to live and through the rate at which members of the population are dying. Now, an age classes a group of individuals of a specific age, and here you can see a variety of age classes in this life table for the U. S. Population in 2003. Here are some of thes statistical measures you don't really need to worry about any of this, just giving you an example of what it might look like now. Often, researchers will use cohorts these air groups of individuals that share a characteristic and are usually studied over a period of time So the researcher will return to these individuals throughout time. Thio continue their study with that, let's go ahead and turn the page.

survivorship is the average ratio of offspring that survived to a certain age compared to those that don't and a survivorship curve is a graphical representation of the number of individuals of a specific species species that survived thio each age and you'll see basically three stereotypical trends. What we refer to as a type one curve, which you see in humans where there's a really low mortality rate early in life, and then later in life, it becomes much higher. So basically in humans, you know, thanks to modern medicine and all that, we have relatively low mortality rate early in life. But as we get older, you know it Z the mortality rate increases. Now. Type two is a constant mortality rate. This is gonna be a straight line, basically, and you'll see this in songbirds, For example, Type three has a really high mortality rate early in life. That then peters out and becomes much more mild, and you'll see this in frogs now a reproductive table. It's just a table of fertility and reproductive rates in the population. Fecundity is the reproductive rate of an organism in a population and age specific fecundity is the average number of female offspring produced by each age class, thes air, all just ways of looking at population changes. Now here you can actually see the birth rates, so the fecundity of people in different countries on Earth. So we're looking at humans here, and you can see that the various colors correspond to a certain number of Children. And again, this is by country. So you know, you can see that, for example, the birth rate in Australia. I don't know why I'm picking on them. Just cause birth rate in Australia is lower, for example, than the birth rate in France now with net reproductive rate is a kind of a weird measure. It's the average number of female offspring that a mother will have as she passes through life, conforming to age specific fecundity and average mortality rates. Basically, it's just saying, What's the average number of female offspring of mothers gonna have given what we know about the populations? Average age specific fecundity which, if you recall, it's the average number of female offspring produced by each age class. So basically, you know, according Thio, the averages on female offspring for this population and this populations mortality rate. What's the average number of female offspring of mothers gonna produce? So with that, let's go ahead and turn the page.

per capita just means per individual in a population, So the per capita birth rate is the number of offspring produced by the average member of population per unit time. Because it's a rate, remember, so the mortality rate is going to be the average number of deaths per unit time in a population. Well, look at these two factors to see how population sizes fluctuate. Now, if the birth rate is equal to the death rate, your population size won't change. However, if your birth rate is higher than your death rate, your population is going to increase in size. Whereas if your death rate is higher than your birth rate, your population is going to decrease in size now. The difference between the birth rate in the death rate or the mortality rate, I should say, is the per capita rate of increase our now when R equals zero like we saw at that midpoint, you have zero population growth, and again, that's because the birth and mortality rates are equal. When you have an are, that's greater than one, meaning your birth rate exceeds your mortality rate. You get exponential population growth and this is a J shaped curve. You can see it right down here. This is what exponential growth looks like. And basically, uh, this is a density, independent type of growth, meaning that it does not depend on the number of individuals in the population. And if the birth rate isas highest possible and the death rate isas low is possible, you can have an intrinsic rate of increase or our max the maximum rate of increase. So exponential growth does occur in nature sometimes, but it can't occur indefinitely. Basically, resource is our finite space is finite. Everything is finite in nature. And that's why even though you might have a period of exponential growth for a population, eventually that's gonna burn itself out. And what we will often see is logistic population growth. So basically, there's a new upper limit to, uh, the population size that an area can sustain. And this is ah, limit due to the number of resource is available, how much space is available and because individuals they're going to compete for those resource is and that space, and there's going to be, in fact, other species competing for that. Those resource is in that space. Additionally, and this doesn't always apply, but the amount of waste produced by the individuals can lead Thio limits on population size. For example, you know, if you take a look at yeast which produce alcohol, you know, they they produce ethanol which is waste to them as a byproduct of their metabolism and eventually that waste can kill them. So you know it. Z not always relevant, but it can be relevant. So carrying capacity is just this maximum population size. It's the limit on the population that the area can sustain and we represent it with letter K. So here you can see that in this particular chart, the carrying capacity is reached due to limit on the food supply. However, it could be other factors as well, you know, in actual real life scenarios. Now, here we can see the characteristic s shaped curve of the logistic growth curve and couple of things that I want you to take note of is that the population size will increase and our will go up and then our will go down. So here are is increasing, and then here are is decreasing. Actually, in the second part of the curve are is approaching zero, and one thing that I want you to take note of is that our is going to be at its greatest when n equals one half K. So if this is our carrying capacity, the half way marker there is going to represent the greatest our value, right, let's turn the page.

the traits and characteristics in a life table that will affect an organism's ability to survive and reproduce. But that organisms life history. Now this are these traits will result from natural selection toe optimize fitness, and this is going to require trade offs between reproduction and survival. So we're gonna look at some different reproductive strategies right now. In the first two, I want to talk about our Cemal parody and Itera parody. Now, symbol parody is like, what, for example, salmon do where they have many offspring once right before they die. And you can see this strategy of Cemal parody in this chart behind me here, I don't want you Thio, you know, worry too much about what's going on this chart. It's actually like an economics analysis. Eso It's kind of beyond what you know, the scope of what we're trying to dio. All I want you to see is that in this chart, the reproductive effort the organism puts forth is going to maximize the offspring it produces and minimize the offspring for gone. Those You know it, uh, those that would come at a cost of speak. Now, with Itera parody, we have a reproductive strategy, where an organism has offspring multiple times throughout its life. For example, what humans dio And here again, we're going to see that in this strategy, the organism is going to maximize the number of offspring produces and minimize its costs to have, you know, an optimal reproductive effort. Now, another way that people look at reproductive strategies is R K selection theory, and this is basically looking at the traits of an organism. Is a trade off between quantity of offspring and quality of offspring. So with K selection, which again is what you see in humans or, for example, whales right behind me, the organism will invest more heavily and fewer offspring. That way, each offspring is a higher probability of surviving to adulthood, so it's a greater investment in fewer things. But you you know you have a higher chance of getting those offspring to adulthood. Now, these species tend to live close to the carrying capacity of their environments, and they tend to live in mawr predictable environment. So these aren't gonna be environments that have crazy fluctuations and all that. Now the offspring of these organisms require extensive parental care and usually take longer to mature to adulthood, you know, think about how long a human parent raises their offspring, you know, years and years and years, and it takes humans a long time to reach adulthood. Now these organisms, generally speaking or larger and also tend to live longer. But these air just trends. Of course, there's always exceptions now with our selection. You know, like you see with this mouse right here, the organism will produce many offspring, but there's a low probability of surviving to adulthood, so these species basically invest in greater numbers of offspring and tend not to invest is heavily in each individual offspring. Uh, the species tend to live in less stable environments, which is why the ability to reproduce quickly is important. And in fact, you know mice, for example, can have offspring multiple times in a year. So these species have high fecundity, and a short generation time doesn't take long for one of these, a little kind of yucky to be honest. Little yucky mice grow up to be a new adult mouse, and, uh, uh, sorry. So the offspring mature quickly, and the offspring reach adulthood and can then have their own offsprings of short generation time. Uh, so they also do that quickly. Now, organisms that use our selection just a trend again tend to be smaller and have shorter life spans. So here, with that survivorship curve we were looking at before, hopefully you realize that humans are going to be case organisms that use case selection in these frogs or organisms that use our selection. So with that, let's go ahead and flip the page.

factors that affect population growth rates can be said to be either density, independent or density dependent. Now density independent factors will affect population growth rate regardless of the density. This includes stuff like severe weather or natural disasters like this forest fire you see here. Now density dependent factors will change growth rates based on changes in the population density. There are many factors to consider here, including things like competition with other organisms, which could be for resource is or mates or space. Also, diseases spread more easily in denser populations and denser populations can also attract and better sustain predators. Some organisms will also experience intrinsic factors, which are internal changes that will affect its ability to reproduce. Some organisms will actually experience intrinsic factors, uh, due to, uh, high densities in their population. So density dependent intrinsic factors and in some cases thes will actually suppress reproduction so that the organism doesn't add to an already oversaturated population. Now, toxic waste can also accumulate to the point of affecting population growth. For example, yeast which produce alcohol will grow and grow and grow and grow, and if they're in a closed environment, they'll actually grow until they kill themselves with the ethanol they produce. Now here is a graph that just I want you to just think of. This is a generic representation of density dependent factors. So let's just think of the y axis as n and think of. I'm sorry. Think of the X axis is and and the Y axis is are So this is our population growth rate and this is our population size. And all I want you to get out of this is that our growth rate goes down as our size goes up. So that would be illustrating a density dependent factor. So as our population size increases, its growth rate plummets. All right, moving on. Some populations actually experienced regular fluctuations, and these air sometimes referred to as population cycles very famous example of this, although not totally accurate. Um, but for our purposes, it's it's the perfect example. Uh, is the snowshoe hare population and links population in Canada, and these populations actually cycle based on interactions between the two species? The reason I say it's not totally correct is because it's been found out. You know that there's other factors at play here, but we can just make a reductive ist, you know, view of this. It's fine for our purposes. So basically the links this cat you see here, it eats the hairs. As hair populations grow, the lynxes are able thio exploit the availability of their food source and their own population will grow. So as the links food food source becomes more abundant, the lynx population increases. And as the lynx population increases, it means they're going to be eating more and more hairs, which can lead to over predation. And a crash in the hair population with scarce hairs means there's less food available for the lynxes, which will lead to a decrease in their population with a smaller links population. That means there's less predation, which allows the hair population to grow again. And that's why we see this cycle you see here the hair population we use red, so it's easier to see the hair population. This blue line is going to peak and crash just ahead of the lynx population in green, and that's because of the pattern just described so that as the hare population goes up, the lynx population will then go up when links population gets too high and they start overeating. The rabbits, the rabbit population, her hair population is going to go down again. And that's going to cause the lynx population to go down because their food source is gone. So hopefully you can see how these two populations interacting leads to this cycle. With that, let's flip the page.

over the last century, the human population size and growth rate has exploded. We went from a world where there was never more than two billion humans ever to now a world where there's nearly eight billion people on the planet, and that is rising. In fact, if you're curious, here are some estimates from the U. N As to population increases in the coming century. Now it's important to remember that, just like there's a carrying capacity in nature, there's a carrying capacity for humans, and there is a limit to global carrying capacity for the human population. Although estimates for what this is very and additionally that could change based on technological advances. However, the point is we need to be conscious about our ecological footprint, which is the amount of land needed to sustain an organism or populations. Use of resource is so if we think about it in terms of an individual, there's gonna be a certain amount of land that person needs. Thio provide them with the necessary nutrients, right food and whatnot. Also, that individual is going to generate waste. There needs to be a certain amount of space for that waste to go so that that person doesn't kill themselves with their own waste. I mean, these are just two examples, but the point is we need to think about are use of this earth and how sustainable our lifestyles are because this is unsustainable, as we saw with our, uh, our logistic growth model. Previously, there is a carrying capacity. We just haven't hit it yet. Now, I don't mean to fear monger anything, but those are important things to think about. However, moving on, I'm gonna talk about demographic transitions. This is maybe getting a little more into sociological territory. But this is a transition from high birth rate and high death rates, toe low birth rates and low death rates. So, essentially, this is why you see, you know what are often referred to as developing nations have many more young people than older people because their birth rates are high and their death rates are high. In more developed nations, we have lower birth and death rates. And so our age pyramids, instead of looking like that, look mawr like this because there's just a higher survival for a large range of ages. Now the fertility rate is the average number of surviving Children each woman has in her lifetime. And this factors into what we call the replacement rate, which is the fertility rate required for women to give birth to enough Children to sustain the population. And this is something we're thinking about in developed nations now because, as we can see, for example, in places like Japan, their population is decreasing because their replacement rate is, well, not high enough. They're not sustaining their population. Their fertility rate is too low. Generally speaking, I should say replacement rate is about 2.1, because if you think about it, you have. And this is for developed nations. I should say this is like us. Uh, you know, if you think about it, a woman would have to give birth to 2.1 Children. Basically, that's saying that most women need to give birth to two Children, which will replace their them Selves. And you know, there, partner. And you know this 0.1 is basically because there's, uh you know, ah, chance that not all of these people are going to make it to the age where they can reproduce. You know, they're they're not gonna make it all the way through that generation time. So that's why it's 2.1. But basically you could almost just think of it as two to replace each parent. All right with that. Let's call it a day. It's all I have for this lesson. See you guys next time.