Predator prey cycle | Ecology | Khan Academy

- [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.