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Population regulation

There are two types of population regulation: density-dependent and density-independent. Density-dependent factors, like competition for resources, predation, disease, and waste accumulation, limit population growth based on population density. Density-independent factors, such as natural disasters, affect populations regardless of density. Understanding these factors helps us consider human population growth and potential carrying capacity. Created by Sal Khan.

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Video transcript

- [Voiceover] What I want to do in this video, is think a little bit more about how populations can be regulated. And broadly speaking, we can think of the regulation of populations it 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. Independent, independent regulation. So first let's think about density dependent regulation. Let me draw a little chart here, 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 exponentially, it wouldn't take long for them to cover the surface of the earth, and and then fill the universe, if in some way 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, use a different 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. The rabbits were first introduced in order to have a little bit of hunting, but then they reproduced 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, how cute, but it was a huge problem. (stammering) The were eating all of the farmable, they were eating crops. They were eating the grass that other types of livestock would graze on. So it was a huge infestation of rabbits. And so you can imagine one competition for resources is just the grass itself. In this picture you can see 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 water, there might only be so much water to support organisms of a certain kind. 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 rain forest, 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 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 dense the population can get, or 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 right 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 at kinda this very unstable situation, and then something happens and 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 you were 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 predators who can say, hey, we can start, it's way easier for us to get our lunch. Now the other thing, 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 even in modern times today 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'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 population, and the waste is just everywhere, it could poison the water, it might poison sources of food, 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. Lemmings 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 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 be not 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 their 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 meteorite coming 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 at 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, but 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 capacity and the different density dependent factors, you might be thinking well, what about human beings? We are for sure a species, 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's 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 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 that carrying capacity, and the term for that carrying capacity, in the case of human beings, that Thomas Malthus set up, and there's a whole video on this, is the Malthusian limit. And he hypothesized that once you crossed 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 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 get rid of waste and sewage and all of that, so it's an interesting philosophical question to say, is there ever gonna 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.