- [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.