Describing populations | Ecology and natural systems | High school biology | Khan Academy

As you might imagine from the title in this video, we're going to do a little bit of describing populations.

So the first question is: what is a population? You can view it as a group of individuals from the same species living in the same general area. So now let's think about how we can start to describe it well, or describe a population.

Well, the first thing we can do is think about the density of a population. When we're thinking about density, in general, it's how much of something you have in a certain area or a certain volume. So when we're talking about density of a population, we're thinking about... well, let's just take a certain area. Let's say take an area like that. If you don't have a lot of that population in that area, then that would be relatively low density. But then if you have a lot of it in that area, that is a higher density, in general.

General density is just the amount of that population, the amount of that species, the number of individuals, I should say, of that species you have in a given unit of area, in that general geography, in that general area. So this would be a high density; this would be a low density if we're looking at, say, the land from above.

The next way that we can describe populations—and there's many ways to do this—is dispersion. You can view dispersion as thinking about how that population is actually distributed within an area. So one way to think about dispersion is clumped dispersion.

That's where a lot of the individuals of that population like to clump together. One reason they might want to do that is for protection, for warmth, for finding food, for avoiding predators. One example you could think of is a school of fish. They like to go together; it can confuse predators. There's more eyes that are collectively looking for food or to avoid becoming someone else's food. So this right over here is clumped dispersion—clumped.

Now, at the other extreme of dispersion, you could have just random dispersion, where there's really no rhyme or reason for what you might see. Sometimes they're close together; sometimes, the members of a population are further apart. So that is random. Unless there's a good reason for folks to clump or be organized in some way, you're oftentimes going to see a random dispersion.

If you think about how many plants spread their seeds, it's random. It literally goes in the wind or some animals eat the fruit and then drop it randomly someplace. So that would give you that random dispersion.

Then one way to think about it in between would be a uniform dispersion, where the members of the population are reasonably uniform in how they are separated. They're roughly the same distance apart. You might say, "Why would you ever have a uniform distribution?" It's very few times it's exactly uniform, but it might be closer to uniform.

Well, a uniform distribution maximizes how much space each member of the population might get, where they just have just enough space, but maybe they don't have too much space. For example, this character over here has a ton of space, while these folks over here don't have too much space. There might be reasons for individual survival where it makes sense, or the population's survival to just spread out just enough, or to spread out in a uniform way.

Now, the last dimension of describing populations—and as I mentioned, there's many other ways to describe them—is the growth of a population. So what would cause a population to grow or shrink? At a very basic level, if your birth rate is larger than your death rate, well, that's going to lead to growth. Your population is going to increase.

If, for example, 100 individuals are born per year and 90 individuals, on average, die per year, well, your population is going to grow by 10 per year. But it's not just about birth and death. You can actually have individuals migrating into or out of a population.

So similarly, if your immigration in a given period of time is greater than your emigration... Immigration is individuals coming into a population; emigration is individuals leaving a population. Well, that too is going to lead to growth.