Population growth strategies: r selected and K selected species

- [Narrator] What we're
going to do in this video

is talk about different
population growth strategies

for different species.

And think about if we can come up

with a broad categorization

or if there's a broad
categorization already

out there for us.

So, we see that there are
species like elephants

that are long lived,

in the wild.

An African elephant can
live roughly as long

as a human being can 50, 60,
70, in some cases, 80 years.

You also have things like killer whales

that are also long lived.

You have things like human beings that

live a pretty good long time compared

to other things in nature.

You have things like gorillas
that are also reasonably long

lived, live many, many, many decades.

Now there's also other
patterns that you see

on these large mammals.

They tend to be large.

Especially, if you think about compared

to very small things
in the animal kingdom.

They tend to not have a lot of children

every reproductive event,
I guess you could say

at every time they give birth.

So few few children,

children

at a time

And it takes them a while to
give birth to those children.

And then they take care
of them for a while.

Now, the other end of the
spectrum, you have some

of the things that we have here.

You have amphibians like
frogs, you have insects.

You could imagine small
fish, bacteria, mosquitoes

that are more short-lived.

Short

lived.

They tend to be smaller.

And they tend to have a
lot of children at a time.

So lots of children

at a time.

Just an example, a frog
or what do you see here

is this is a spiders
coming out of an egg sack.

Frogs or spiders,

they can lay in many cases,
thousands of eggs at a time.

And they're not really dependent
on any one of those eggs,

necessarily surviving.

Many of them will be
killed off by predators

but as long as a few survive
then the next generation,

you can imagine, they get to reproduction

and then they'll lay thousands of eggs.

And they're shorter lived.

A frog could live a few years.

Some insects might live
anywhere from a few days

to a few years.

You have things like
bacteria that can have

an even shorter life span.

So, when biologists or
ecologists look at this, they say

it looks like there is
a spectrum over here.

You have these large long-lived animals

that have a few children at a time.

And really the limiting
factor on their population

is how dense their population is.

At some point, these
animals are going to compete

for resources with each other.

And that's true of
things like human beings.

And the other end, you have these things

like spiders or frogs,
instead might be limited

on their reproduction
by environmental factors

like how moist or dry it
is, how hot or cold it is.

And so generally speaking
folks have attempted

to classify these types of species.

They call them K selected.

I'll talk about where the
K comes from in a second.

And they talk about these
types of species as R selected.

And where these letters come from,

is there, there's a general idea that

if we draw a little axis here, time

and then draw up axis here, population

on the vertical axis, most species that

when there's not a lot of
them, and they're not limited

by resources they will grow
exponentially like this.

Now, when you go into the math of it,

that rate of growth is usually
denoted by the letter R,

and you could see where this
R selected is coming from.

And what that R is how
large it is is determined

by a species biotic potential.

And biotic potential just
means in a given environment,

how quickly can they reproduce?

What percentage of those
are able to get to maturity

so that they can reproduce,
et cetera, et cetera.

And so you could imagine the higher the R,

the faster this
exponential growth goes up.

Now, at some point you're going
to be resource constrained.

And some there's some carrying capacity.

And so let's say this is
the carrying capacity here,

and let's call that K.

And the reason why we use
K instead of C is K comes

from a word for capacity in German.

So, at some point you're
gonna have so many things

that you're not gonna
have enough resources

to support more.

And so your population would flatten out

something like this.

And so, generally speaking
biologists have categorized

these left categories of
species as K selected.

The amount that you have,
the amount of population

is going to be determined by
the resources that are there,

the carrying capacity in the environment,

while things like frogs
and mosquitoes and bacteria

and spiders, you view more as R selected.

They're gonna grow exponentially
at their biotic potential,

but that biotic potential
is gonna be determined

by how hot or cold or wet or dry

or other environmental factors.

They're not gonna get
to such a high density,

that they're really
competing with each other.

Now, as you can imagine
with most categorizations,

nature is not that clean
that things are always going

to be on the left side, or always gonna

be on the right side.

For example, things like
turtles, lay a lot of eggs

but they live a very long time.

It's also worth noting that
there's no value judgment

that one strategy is
better than the other.

They've both been successful.

The fact that these species exist today,

show that they have worked well.

It is worth noting that
invasive species tend

to be more R selected.

They will just grow and grow and grow,

and gonna be limited by the
environment and not as much

by each other.

While K selected species
are often the ones

that are most effected
by invasive species.

Now, the last thing I wanna highlight is,

what we just talked about as
population growth strategies.

And that can sometimes be
confused with niche strategies

where are there certain
species that are generalists,

that can occupy many different niches

while there's certain species
that are more specialists.

Here, we're just talking
about generally speaking,

how do species tend to grow in population?

While when we talk about niche strategies,

we're gonna talk about the
types of niches they feel

in order to get their resources.

And we'll talk more about
all of that in future videos.