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Main content
Current time:0:00Total duration:10:33
AP.BIO:
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Video transcript

what I want to do in this video is think a little bit more about how population's can be regulated and broadly speaking we can think of the regulation of populations in two different categories there's the regulation dependent on density so density density dependent regulation density dependent and then there's the type of regulation that isn't dependent on density so we could call that density independent regulation in the pendent independent regulation so first let's think about density dependent regulation and let me draw a little chart here for it to help us visualize that so let's say that that axis is the population I'll say P for population and let's say this axis is time so T for time in previous videos we talked about a population and I like to use the example of rabbits how it could grow exponentially so if it's just growing a certain percent every month that population will grow exponentially but we can't expect that that will just happen forever if rabbits just kept growing on growing exponentially it wouldn't take long for them to just cover the surface of the earth and then have it fill the universe if in some ways they weren't limited by anything but we know that they are limited by things and so the environment only has a certain amount of carrying capacity and we'll think a little bit about this carrying capacity in a second and what's determining the carrying capacity and so as the density of the rabbits in a certain area get higher and higher and higher well then the density dependent we use a different color color the density dependent factors start to play the density dependent limiting factors and what could be some of these density dependent limiting factors that keep the population from going dramatically above the carrying capacity well the most obvious one could be competition for resources competition competition for resources and the one that might come to mind most clearly is food resources so this is actually a picture of Australia in the mid-1800s and they had a bunny population problem that the rabbits were first introduced in order to have a little bit of hunting but then they they reproduce like rabbits and it was estimated that at some point you had over a billion rabbits that had populated the country and that was you might say oh how cute but it was a huge problem they were they were eating all of the the great they were eating they were eating the the farmable they're eating crops they were they were eating the grass that you that other types of livestock would graze on so it was a huge infestation of rabbits and so you could imagine one competition for resources is just the grass itself in this picture you can see that it's that the land is barren maybe this happened because the rabbits ate all of the vegetation here so competition for resources one type of resource could be food another type of resource could be could be water there might only be so much water to support organisms of a certain kind and we're only you know here we often talk about animals but it could be plants or it could be bacteria it could be all sorts of organisms that we're talking about and if we're talking about plants we could think about light you could say well what limits having an infinite number of plants in a certain area well water will limit the nutrients in the soil will limit but also access to light you've seen pictures of a dense canopy in a rainforest and the plants are trying to seek out whatever gap in the canopy they can find so that they can get some access to that to that to that light now there's other examples and this wouldn't apply as much to say plants but the idea of shelter this might apply to humans or to other types of animals that maybe need shelter in order to hide or a place to reproduce or whatever else so at some point if the population density gets too high in a certain region then these things are going to limit how how dense the population can get are frankly just what the population actually is and so that would lead once again we talked about this in a previous video to this logistic curve over here where we just we don't we just start approaching the carrying capacity and it is possible that you could even go above the carrying capacity and then you're kind of this very unstable situation and then something happens you go below it then you go above it and then below it and then something like that but what are other density dependent factors that we could think about well another thing is if if you are a predator when say the rabbits become this dense it's much easier to start to pick them off it's much easier to get your lunch and so predatory factors or we could say predation predation once a population gets large enough and dense enough it might be the predator the Predators who can say hey we can start it's way easier for us to get our lunch now the other thing that it might be a little less obvious but when you have a high density population and there's examples of this in medieval times in Europe and and even in modern times a day with with human populations but this happens with all organisms is that when you become a dense population there's more interaction there's more contact there's more sharing of resources like water and so disease and parasites becomes an issue so let me write this down disease disease and parasites can spread much easier and they could they're much more likely to start limiting the population the thing that always comes to my mind is the plague in medieval times where it was very easy to spread from one human to the next or frankly from rats to humans and whatever else now the other thing and this is maybe somewhat related to everything else we've talked about is waste accumulation so let me write this right over here waste if you have a really high density of population and and the waste is just everywhere it could poison the water it might poison sources of food it was it might help the spread of disease and parasites and once again all of these things help define what the carrying capacity how dense can a population get in a certain region now you might say well maybe they don't have to stay in a region maybe they can go and explore other places and that's possible and that's been the story for many different types of species let me they are famous for when their population gets dense in a certain area groups of them start just running to start exploring other areas sometimes running in directions that are not that good for them so all of these are density dependent factors and a lot of these as we as we just talked about you could think of them as biotic factors they're related to other living things around the density independent factors tend to be a biotic they tend to not be related to living things so the most common density independent factor is natural disasters so natural natural disaster we have a picture here of a forest fire the deer population here might not be in any way close to there carrying capacity but despite that the forest fire maybe might kill off a lot of the deer other natural disasters you could have a flood you could have a tsunami you could have a a meteorite coming from from outer space that happened to the dinosaurs to just knock out huge populations and so density independent factors you could have the population growing and it's and it's just some random point it just there's some density independent factor there's a forest fire there's a flood or something else and then maybe the population grows from there and eventually gets closer to its carrying capacity who knows when the density independent factors once again it's not related to where we are on this curve it could happen at any time and to some degree they feel a little bit more random now with all of this talk about carrying capacities and and and the different you know density dependent factors you might be think well what about human beings we we are foreshore species and we we and and so the same ideas apply to us and so is there a natural carrying capacity for the environments that we are in and there is a famous philosopher scientist Thomas Malthus and I have a whole video on him but he hypothesized that humanity had a very serious problem because we were our populations were growing exponentially so this is population this is time and so he said look there's just a natural carrying capacity for human beings and as human beings just kept growing exponentially we would hit we would hit that that carrying capacity and the term for that carrying capacity in the case of human beings that Thomas Malthus that up and there's a whole video on this is the Malthusian limit and he hypothesized that once we got once we cross it or approached it there would be all sorts of crises that once you're at this carrying capacity there might not be enough food and then there might be a famine or the we we go across it and then disease spreads a lot more and so he was just applying these ideas of density dependent factors to human populations and said hey this is not going to be pleasant for Humanity now what's been interesting is that humanity has found ways repeatedly of pushing up the carrying capacity for us as a species we've been able to do it frankly through technology frankly finding ways to grow food in denser and denser ways ways to stave off disease ways to have to get rid of waste and and sewage and all of that so it's an interesting philosophical question to say is there ever going to be a point where human being just hits this Malthusian where human society hits this Malthusian limit or are we always going to be able to fend it off by just better and better technology or maybe even just regulation of the population itself so that we don't you know where we just have whatever birth control or family planning or whatever it might be so that we are less likely to hit some eventual limit
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