Predator-prey population cycles | Ecology and natural systems | High school biology | Khan Academy

What I want to 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. There are many cases of this, but the most cited general example is the case when one population wants to eat another population. So, you have the predator population that likes to eat the prey. You have predator and prey interactions. I'm doing the prey in, I guess, a somewhat bloody color because, 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. The horizontal axis is time, and the vertical axis is population.

Let's just, in our starting point, 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. What do we think is going to happen here? Well, at this point, with a low density of predators, it's going to be much easier for them to find a meal, and it's going to be much easier for the prey to get caught.

Since it's easier for the predators to find a meal, you could imagine their population starting to increase. But what's going to happen is, as their population is increasing, it's going to be more likely that the prey is going to get caught. There's going to be more of their hunters around, more of the 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 going to start having trouble finding food again, and their population might start to decrease. As their population decreases, what's going to happen to the prey? Well, then there's going to be less predators around, so their population might start to increase.

I think you see what's happening. The predator and prey can kind of form this cyclic interaction with each other. What I've just drawn is often known as the predator-prey cycle. Now, to be clear, this predator-prey cycle is a simplified model; it's only taking into account the population of the prey and the population of the predators. It doesn't take into account many other factors that would affect their populations: abiotic non-living and biotic living factors, things like climate, resource abundance, or interaction with other species.

For example, a drought could reduce the amount of food available for the prey species, and that could cause the prey population to decline, regardless of what's happening with the predators. You could imagine a world where you could have the cycle between predator and prey populations, but you can also run computer simulations that will show this.

Even observational data out in the field also shows this. One of the often cited examples is interactions between the snowshoe hare, which would be the prey in this situation, and the Canadian lynx, which would be the predator in this situation. You see a very similar cycle to what I just drew, kind of just reasoning through it.

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 1800s, going all the way to the early mid-1900s, so it's roughly a hundred years of data that we're showing. 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.

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, we see a lower population of the prey, of the hare. But then, since you have a low population of the food in this situation, the predator population starts to decrease.

Let me draw an arrow here: the predator population starts to decrease. Do that same blue color. So the predator population decreases, and as that predator population decreases, well, then the prey population increases because there are fewer folks around to hunt them. The prey population increases, and you see that the other way around.

When the prey population is really high and the predator population is relatively low, then the predators say, “Hey, it's really easy for us to find meals right now!” That was kind of that starting point in my reasoning through it. So their population starts to increase, and as the predator population increases, the prey population is going to decrease.

This is real data showing the snowshoe hare, the prey, and the Canadian lynx, the predator, over many decades to show this predator-prey cycle.

Now, I want to reiterate a point that I made earlier in this video: that even though we see this clear pattern between the snowshoe hare and lynx populations, remember that it's not just the number of lynx affecting the hare population, and it's not just the number of hares affecting the lynx population.

There could be many other factors that affect both of these species, including other species. It could be non-living factors, their environment. For example, there has been research that indicates that the patterns in the snowshoe hare population are also influenced by stress and food availability, as you would guess, on top of predation by the lynx.