WEBVTT

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- [Voiceover] What I
wanna do in this video

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is think about how different populations

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that share the same ecosystem can interact

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with each other and actually provide

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a feedback loop on each other.

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And there's many cases of this,

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but the most cited general example

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is the case when one population

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wants to eat another population.

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And so you have the predator population

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that likes to eat the prey.

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So you have the predator
and prey interactions.

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I'm doing the prey in I guess
a somewhat bloody color,

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I guess 'cause, well,
they're going to be eaten.

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So let's just think about how these

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populations could interact.

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Let me draw a little chart
here that you're probably

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familiar with by now where we show

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how a population can change over time.

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So the time, the horizontal axis is time.

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The vertical axis is population.

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Population.

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And so let's just, in our starting point,

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let's say that our prey is starting out

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at a relatively high point.

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Let's say we're right there in time,

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and let's say for whatever reason,

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our predator population is relatively low.

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So what do we think is
going to happen here?

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Well, at this point, with
a low density of predators,

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it's gonna be much easier
for them for find a meal,

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and it's gonna be much easier
for the prey to get caught.

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So since it's more easy,
it's easier for the predators

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to find a meal, you can
imagine their population

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starting to increase.

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But what's going to happen is

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their population is increasing.

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Well, it's gonna be more
likely that they're gonna,

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they prey is gonna get caught.

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There's gonna be more
of their hunters around,

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more of their predators around.

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So that population is
going to start decreasing

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all the way to a point
where if the population

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of the prey gets low enough, the predators

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are gonna have, they're gonna start having

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trouble finding food again,
and so that their population

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might start to decrease,
and as their population

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decreases, what's gonna
happen to the prey?

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Well, then, there's gonna
be less predators around,

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so they might be able to, their population

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might start to increase.

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And so I think you see what's happening.

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The predator and prey,
they can kind of form

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this cyclic interaction with each other.

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And what I've just drawn,
this is often known

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as the predator-prey cycle.

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And I just reasoned through
that you can imagine

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a world where you can
have the cycle between

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predator and prey populations.

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But you can also run computer simulations

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that will show this,
and even observational

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data out in the field also shows this.

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One of the often cited examples

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is interactions between,
between the snowshoe hare,

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which would be the prey in this situation,

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and the Canadian lynx,
which would be the predator,

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the predator in this situation.

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And you see a very similar
cycle to what I just drew,

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kind of just reasoning through it,

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and this, right here, is actual data.

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You see the passage of time here,

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and this is a long passage of time.

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We're starting in the early 1800's

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going all the way to the early-mid 1900's.

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So it's roughly 100 years
of data that we're showing,

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and in the vertical axis,
you have thousands of animals

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and we're plotting both the population

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of snowshoe hares and Canadian lynx

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in a certain area on this chart.

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And as you see, when the prey population

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is high, when the prey population,

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sorry, when the predator
population is high,

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when we have a lot of
the Canadian lynx around,

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that we see a lower, a lower population

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of the prey, of the hare.

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But then as, since you
have a low population

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of the food in this situation,

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the predator population
starts to decrease.

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So let me draw an arrow here.

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The predator population starts to decrease

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and, let me do that same blue color.

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And so the predator population decreases,

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and as that predator population decreases,

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well then the prey population increases

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'cause there's less folks
around to hunt them.

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So the prey population increases,

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and you see that the other way around.

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When the prey population
is really is, well maybe

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we'll show it right over
here, and this is real data.

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That's why it's not always super clean.

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But when the prey population
is really, really high

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and the predator population
is relatively low,

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well, then, the predators
say, "Hey, it's really

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"easy for us to find meals right now."

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That's kind of that
starting point in that,

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when I was just reasoning through it.

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And so their population starts to.

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Oh, oops, what did I do?

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There, there.

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Let me make sure.

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So their population starts to increase,

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and as the predator population increases,

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well the prey population,
the prey population

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is going to decrease.

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So this is real data
showing the snowshoe hare,

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the prey, and the Canadian
lynx, the predator,

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on over many, many decades to show

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this predator-prey cycle.