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Predator-prey cycles

The predator-prey cycle demonstrates how populations of predators and prey interact and influence each other's growth. As predator numbers increase, prey populations decline, leading to a decrease in predators. This allows prey populations to recover, restarting the cycle. The snowshoe hare and Canadian lynx exemplify this ecological relationship through observed population fluctuations over time. Created by Sal Khan.

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Video transcript

- [Voiceover] What I wanna do in this video is think about how different populations that share the same ecosystem can interact with each other and actually provide a feedback loop on each other. And there's many cases of this, but the most cited general example is the case when one population wants to eat another population. And so you have the predator population that likes to eat the prey. So you have the predator and prey interactions. I'm doing the prey in I guess a somewhat bloody color, I guess 'cause, well, they're going to be eaten. So let's just think about how these populations could interact. Let me draw a little chart here that you're probably familiar with by now where we show how a population can change over time. So the time, the horizontal axis is time. The vertical axis is population. Population. And so let's just, in our starting point, let's say that our prey is starting out at a relatively high point. Let's say we're right there in time, and let's say for whatever reason, our predator population is relatively low. So what do we think is going to happen here? Well, at this point, with a low density of predators, it's gonna be much easier for them for find a meal, and it's gonna be much easier for the prey to get caught. So since it's more easy, it's easier for the predators to find a meal, you can imagine their population starting to increase. But what's going to happen is their population is increasing. Well, it's gonna be more likely that they're gonna, they prey is gonna get caught. There's gonna be more of their hunters around, more of their predators around. So that population is going to start decreasing all the way to a point where if the population of the prey gets low enough, the predators are gonna have, they're gonna start having trouble finding food again, and so that their population might start to decrease, and as their population decreases, what's gonna happen to the prey? Well, then, there's gonna be less predators around, so they might be able to, their population might start to increase. And so I think you see what's happening. The predator and prey, they can kind of form this cyclic interaction with each other. And what I've just drawn, this is often known as the predator-prey cycle. And I just reasoned through that you can imagine a world where you can have the cycle between predator and prey populations. But you can also run computer simulations that will show this, and even observational data out in the field also shows this. One of the often cited examples is interactions between, between the snowshoe hare, which would be the prey in this situation, and the Canadian lynx, which would be the predator, the predator in this situation. And you see a very similar cycle to what I just drew, kind of just reasoning through it, and this, right here, is actual data. You see the passage of time here, and this is a long passage of time. We're starting in the early 1800's going all the way to the early-mid 1900's. So it's roughly 100 years of data that we're showing, and in the vertical axis, you have thousands of animals and we're plotting both the population of snowshoe hares and Canadian lynx in a certain area on this chart. And as you see, when the prey population is high, when the prey population, sorry, when the predator population is high, when we have a lot of the Canadian lynx around, that we see a lower, a lower population of the prey, of the hare. But then as, since you have a low population of the food in this situation, the predator population starts to decrease. So let me draw an arrow here. The predator population starts to decrease and, let me do that same blue color. And so the predator population decreases, and as that predator population decreases, well then the prey population increases 'cause there's less folks around to hunt them. So the prey population increases, and you see that the other way around. When the prey population is really is, well maybe we'll show it right over here, and this is real data. That's why it's not always super clean. But when the prey population is really, really high and the predator population is relatively low, well, then, the predators say, "Hey, it's really "easy for us to find meals right now." That's kind of that starting point in that, when I was just reasoning through it. And so their population starts to. Oh, oops, what did I do? There, there. Let me make sure. So their population starts to increase, and as the predator population increases, well the prey population, the prey population is going to decrease. So this is real data showing the snowshoe hare, the prey, and the Canadian lynx, the predator, on over many, many decades to show this predator-prey cycle.