this is the preview video for chapter 27 on population growth and regulation we'll start by just thinking about populations and how population size can change we'll then look at how that's regulated we'll talk about life history strategies that different species employ we'll look at how organisms are distributed within their own populations and then finish up by thinking about the human population specifically so there are lots of different things that can change a population size so some of the obvious things are individuals can uh be born individuals um can pass um and you also have individuals either moving in or out of a particular population because remember we usually Define population as all the members of a species who are living in a particular area and you get to Define that area so sometimes it's very narrow and sometimes it's very broad if we start to go into it a little bit more deeply those terms that you might see in addition to births and deaths you also have immigration which is individuals coming in to a particular population and immigration when individuals are leaving we'll spend a little bit of time um thinking about the size changes in very simple mathematical terms so if you have a growth rate we're looking at the birth rate minus the death rate so obviously if you have more births than deaths this this growth rate R is going to be positive and if you have more deaths than births you would have a negative growth rate we can then think about the population growth specifically Big G the number of individuals that are added to a population and so to do this you need not only the rate but you need to know what the population size was to begin with and the population growth is just the growth rate times the initial population size when we think about um how this affects things numerically if you have a constant positive growth rate more births than deaths you are going to have what's called exponential growth and if you graphed it out this would this would be a j shaped curve um we'll also talk about what the biotic potential is the maximum right that a particular population can grow and this is going to be influenced by things like the age that the organism confirms reproduce how frequently they reproduce how many offspring they U make each time how long they are actually of reproductive age and also the death of individuals and so if you have a species with a low biotic potential it might take much longer to reach the same population SI size as a species with high biotic potential when we think about population growth being regulated we'll talk about environmental resistance these are things that limit population growth and they can either be from living or non-living sources so things like predation uh competition for limited resources natural events freezing weather wildfires other storms droughts those would all be environmental resistance we'll also introduce the phenomena of boom and bust Cycles where sometimes there's a very rapid population growth that's followed by a very rapid and sudden die off and this often repeats um uh cyclically and we'll look at some scenarios where that happens we'll also talk about the carrying capacity this is usually abbreviated uppercase K um and this is the based on nutrients and energy and space and it's the maximum uh population that a particular environment can support and usually where you have populations increase to hit that carrying capacity we call the logistic population growth and often looks like an s-shaped curved and so if a species is introduced to an environment where it can grow but there's nobody there no competition it often grows very rapidly at first and then as it starts to hit the maximum for the nutrin energy in the space it will start to level off and again hit that carrying capacity and make that s-shaped curve when we think about some of the limits on population density we'll split the different factors into things that are either density independent and so these are going to limit population size regardless of how dense the population already is so things like the climate and the weather um would be um big uh factors here and there are also density dependent factors and they become more intense as the population density goes up so things like Predator prey interactions um competition those would all be density independent or density dependent um and things we'll be talking about things like parasites um those are often density dependent and sometimes these factors overlap with each other as well when we talk about competition um we'll be looking at both intraspecific competition So within the same species or interspecific competition between different species and these are going to limit um competition or uh population size as they're often competing for similar resources the next section looks at life history strategies and basically an R selected species is a species that um reproduces rapidly and often that's because it is existing in a unpredictable environment and it's unlikely to hit its carrying capacity before something happens to the population so um these individuals often have um uh a short rap uh rapid uh time to reach maturity a short lifespan they produce offspring that are very small um they don't provide a lot of Parental care but often there are a lot of Offspring and this would be something like a mosquito K selected species live in much more stable environments and they mature much more slowly and they often have a long lifespan produce a small number of offspring that they put a lot of energy um into supporting so an elephant would be a classic example of a k selected species we'll also look at survivorship tables which track organisms they were born at the same time throughout their lives and record how many are surviving in each particular year and so you can imagine um that as they get older that cohort is going to have fewer and fewer individuals represented when you graph these numbers for different species a couple patterns emerge some species are um constant loss um populations and so they lose individuals at a very um kind of stable rate so Robins are the example here and this would be a straight line on a survivorship curve there are some species that um make a lot of Offspring and a lot of them die very early this would be something like a dandelion and so um over time there are only a small number of individuals that make it to old age and then we have the opposite L late loss populations where most of the individuals that are born end up Surviving and then you lose most of those individuals in a very old age when we think about how populations are distributed um spatially we see a couple patterns we'll be talking about uniform distribution um random distribution and clumped distribution and some of the advantage and disadvantages of all of these different patterns we'll finish up by looking at how the human population is changing and so if you think about humans on this planet going back you know only um you know 10 or so thousand years we had very small populations for a very long time and then as we started to have both technological advances and agricultural advances our population started to go up and lately in the um uh Industrial Revolution where people started Living in more urban areas and with medical advances like antibiotics we have had a very rapid increase in the human population size and so um we are now faced with a situation where we have to consider how uh we and our offspring are potentially threatening um the stability of our biosphere based on all of the resources that we're consuming with such a high population size we'll talk about a couple terms related to population size one of the concepts is just um replacement level fertility this is abbreviated rlf and it's basically saying that if you have two adults they will produce on average two children and when we think about um replacement level fertility um and the variations we see patterns emerging in more developed countries versus less developed countries and so we'll talk about how some different um countries and societies change from um pre-industrial levels where maybe you had um a birth um and death rate that was pretty even when they start to get to a transitional stage you might still have a high birth rate um but now the death rate is declining that means you're going to have um an increase in the population in the industrial stage all of a sudden now maybe because more people are working your birth rate might go down but you also have a pretty low death rate and then in the post-industrial stage where a lot of countries might be now all of a sudden for various reasons you might have a low birth rate um and also a a low death rate and again we'll look at all of these implications when you start to graph some of these um some really clear patterns emerge if you have a population that is rapidly growing um you'll get a a curve that looks like this that's slanted on the sides if if you have a um population that's relatively uh stable maybe growing slowly it'll have relatively straight sides and if a population is declining you'll see these um start to um uh turn in diagonally at the bottom and have a more narrow bottom than at the top and that's just showing that you don't have as many individuals that are very young um to replace the population we'll finish up by thinking specifically about the United States and um not only our Uh current growth and our projected growth but what sort of ecological footprint we have on the planet and um the take-home message is that uh we use a lot of resources in the US so when we have a large population size on top of that that has some pretty big implications for the future of our planet all right that's it for the preview of chapter 27