Population regulation | Ecology | Khan Academy

What I want to do in this video is think a little bit more about how populations can be regulated. Broadly speaking, we can think of the regulation of populations in two different categories: there's the regulation dependent on density - so, density-dependent regulation - and then there's a type of regulation that isn't dependent on density, so we could call that density-independent regulation.

So first, let's think about density-dependent regulation, and let me draw a little chart here for to help us visualize that. 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 and how it could grow exponentially. 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 and growing exponentially, it wouldn't take long for them to cover the surface of the Earth 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. We'll think a little bit about this carrying capacity in a second and what's determining the carrying capacity.

As the density of the rabbits in a certain area gets higher and higher and higher, then the density-dependent factors start to play the density-dependent limiting factors. 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 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. It was estimated that at some point, you had over a billion rabbits that had populated the country. You might say, "Oh, how cute," but it was a huge problem. They were eating all of the crops; they were eating the grass that other types of livestock would graze on.

So it was a huge infestation of rabbits. You can imagine one competition for resources is just the grass itself. In this picture, you can see 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 water; there might only be so much water to support organisms of a certain kind.

We're often talking 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 light. Now, there are other examples, and this wouldn't apply as much to say plants, but the idea of shelter applies to humans or to other types of animals that maybe need shelter in order to hide or have 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. That would lead, once again—like we talked about this in a previous video—to this logistic curve right over here where we just start approaching the carrying capacity.

It is possible that you could even go above the carrying capacity, and then you're in this very unstable situation. 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 you are a predator—when say the rabbits become this dense, it’s much easier to start to pick them off, and it's much easier to get your lunch. So predator factors, or we could say 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 that might be a little less obvious is that when you have a high-density population—and there are examples of this in medieval times in Europe, and even in modern times today with human populations—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 become an issue.

Let me write this down: 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. Let me write this right over here: waste. If you have a really high density of 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.

Once again, all of these things help define what the carrying capacity is—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! 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 just start running to explore 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 abiotic; they tend to not be related to living things. The most common density-independent factor is natural disasters.

So, natural disasters—here we have a picture 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 might kill off a lot of the deer. Other natural disasters could include a flood, a tsunami, or a meteorite coming from outer space that happened to the dinosaurs—just knocking out huge populations.

So density-independent factors could have the population growing, and at just some random point, 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, are not related to where we are on this curve; they 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 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. Is there a natural carrying capacity for the environments that we are in? And there is a famous philosopher, scientist, Thomas Malthus. 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. He said, "Look, there's just a natural carrying capacity for human beings." As human beings just kept growing exponentially, we would hit that carrying capacity. The term for that carrying capacity in the case of human beings that Thomas Malthus hypothesized is the Malthusian limit.

He hypothesized that once we hit 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 could go across it, and then disease spreads a lot more. 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 through technology and 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 going to be a point where a human being just 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?