-
In our series on biology we
spent many weeks together,
-
talking about the physiology
of animals and plants,
-
and how cells work
together to make tissues,
-
to make organs, to make organ systems.
-
To make us the hunks of meat
and vegetables that we are.
-
Understanding the whole organism.
-
It's important to know what's
going on at all those levels.
-
The same is true for ecology.
-
Only, instead of zooming in and out on
different levels within a living thing,
-
we can zoom in and out on the earth.
-
Depending on the power
of the magnification,
-
we can understand a whole range
of things about our planet.
-
For instance, we can look
at groups within a species,
-
and how they live together
in one geographic area.
-
That's population ecology.
-
There's also community ecology,
-
where you look at groups of
different organisms living together,
-
and figure out how they
influence each other.
-
Then, the most zoomed out
we get is ecosystem ecology.
-
The study of how all living
and non-living things,
-
interact within an entire ecosystem.
-
Let's start by zooming in
with population ecology.
-
The study of groups within a species,
-
that interact mostly with each other.
-
To understand why these populations,
-
are different in one time and
place than they are in another.
-
How, you may be asking yourself, is
that in any way useful to anyone ever?
-
Well, it's actually super
useful to everybody always.
-
Let's look, for instance, at
the outbreak of West Nile Virus,
-
that struck Dallas, Texas
in the summer of 2012.
-
In Dallas County, twelve
people died from the virus,
-
as of the filming of this.
-
Nearly three hundred
people have been infected.
-
In 2011 the whole state of Texas,
-
reported only twenty seven cases
of West Nile and only two deaths.
-
That seems kind of significant.
-
So, what's up?
-
Turns out that this is a
population ecology problem.
-
West Nile is a mosquito born illness,
-
and the population of
mosquitoes in Dallas in 2012,
-
busted through brick walls
like the Kool-aid man,
-
spreading West Nile like crazy.
-
Why did this outbreak happen in
2012 and not the year before?
-
And why did it happen in
Texas and not in New Jersey?
-
The answer, is population ecology.
-
(fast lively music)
-
Before we start solving any
disease outbreak mysteries,
-
we got to understand the
fundamentals of population ecology.
-
For starters, a population is
just a group of individuals,
-
of one species who interact regularly.
-
How often organisms interact
have a lot to do with geography.
-
You're going to have a lot more face
time with the folks you live near,
-
than those who live farther away.
-
As a result, individuals
who are closer to you,
-
will be the ones that you compete
with for food and living space,
-
mates, all that stuff.
-
In order to understand why
populations are different,
-
from time to time and place to place,
-
a population ecologist needs to know
a few things about a population.
-
Like, it's density.
-
In this instance, how
many mosquitoes there are,
-
in the greater Dallas area that might
come into contact with each other.
-
A population's density changes
due to a number of factors,
-
all of which are pretty intuitive.
-
It increases when new
individuals are either,
-
born or immigrate, that is, move in.
-
It decreases because of
deaths or emigration,
-
or individuals moving out.
-
Simple enough, but as
a population ecologist,
-
you also need to know about
the geographic arrangement,
-
of the individuals within the population.
-
This is their dispersion.
-
Like, are the mosquitoes
all clumped together?
-
Are they evenly spaced
throughout the county?
-
Is there some kind of random spacing?
-
The answers to these
questions give scientists,
-
a snapshot of a population
at any given moment.
-
To figure out a puzzle like
the West Nile outbreak,
-
which involves studying how a
population has changed over time,
-
you have to investigate one of
population ecology's central principles.
-
Population growth.
-
There are all kinds of factors
that drive population growth,
-
and they can vary radically
from one organism to the next.
-
Things like fecundity.
-
How many offspring an individual
can have in a lifetime,
-
make a huge difference in
the size of a population.
-
For instance, why do mosquito
populations seem to grow so quickly,
-
while, the endangered black
rhino may never recover,
-
from a single act of poaching?
-
For starters, mosquitoes can
have two thousand offspring,
-
in their two week lifetime.
-
While the rhino can have
like five in forty years.
-
Still, a population doesn't
usually or even ever,
-
grow to its full potential and it
can't keep growing indefinitely.
-
To understand how fast or slow,
-
and high or low a
population actually grows,
-
you need to focus on what's
keeping growth in check.
-
These factors are appropriately called,
-
limiting factors.
-
Say, you're a mosquito in Dallas in 2011,
-
the year before the outbreak.
-
Back then, the growth rate
wasn't what it was in 2012,
-
so something was keeping you down.
-
To figure out what your
limiting factors were,
-
you first have to narrow down
what you need as a mosquito,
-
to live and reproduce successfully.
-
First, you got to find your food.
-
Now, you mosquitoes, you
eat all kinds of things.
-
But in order to reproduce,
assuming you're a female,
-
you need a blood meal.
-
You have to find a vertebrate
and suck some of its blood out.
-
Presumably there's no shortage of
vertebrates walking around Dallas,
-
for you to suck blood out of.
-
I have good friends who
are vertebrates in Dallas.
-
You might even be able to
suck some of their blood.
-
Next, temperature.
-
Because you mosquitoes are
ectothermic, it has to be warm,
-
in order for you to be active.
-
Now, Texas is pretty warm and the winter
of 2011, 2012 was especially balmy.
-
In fact, the summer of
2012 was exceptionally hot,
-
which helps speed up
the mosquito life cycle.
-
That's one limiting factor
that's been removed,
-
for Dallas area mosquitoes.
-
Moving on to mates.
-
If you're a female mosquito, you
need to find a nice male mosquito,
-
with a job and preferably his own car,
-
because Dallas is a pretty
big city, to mate with.
-
This isn't actually all that hard
because the way that mosquitoes do it.
-
Males just gather into a mosquito cloud,
-
at dusk every night during mating season,
-
and all the female has to do
is find her local dude cloud,
-
and fly into it in
order to get mated with.
-
Easy cheese.
-
Finally, space.
-
And, aha!
-
Because here we have
another important clue.
-
Mosquitoes need to lay their
eggs in stagnant water,
-
if there's anything mosquito larva hate,
-
it's a rainstorm flushing out
the little puddle of water,
-
they've been living in.
-
Since Dallas saw a pretty severe
drought in the summer of 2012,
-
there were lots of pockets of
stagnant, nasty mosquito water,
-
sitting around acting
as nurseries for many,
-
many West Nile infected mosquitoes.
-
When we look at this evidence,
-
we find at least two limiting factors,
-
for Dallas' mosquito population growth,
-
that were removed in 2011.
-
The constraints of temperature and space.
-
It was plenty hot and there were lots,
-
of egg-laying locations so
the bugs were free to go nuts.
-
Population ecologists group
limiting factors like these,
-
into two different categories.
-
Density dependent and density independent.
-
They do it this way
because we need to know,
-
whether a population's growth
rate is being controlled,
-
by how many individuals are in it,
-
or whether it's being
controlled by something else.
-
The reason these limitations matter,
-
is because they affect what's
known as the carrying capacity,
-
of the mosquitoes' habitat.
-
That's the number of individuals
that a habitat can sustain,
-
with the resources that it has available.
-
So, density dependent limitations
are factors that inhibit growth,
-
because of the environmental
stress caused by a population size.
-
For example, there may
simply not be enough,
-
food, water, and space
to accommodate everyone.
-
Or maybe because there
are so many individuals,
-
a nearby predator population explodes,
-
which helps keep the population in check.
-
Things like disease can also be
a density dependent limitation.
-
Lots of individuals
living in close quarters,
-
can make infections spread like crazy.
-
Now, I don't think that
the Dallas mosquitoes,
-
are going to run out of vertebrates
to dine on any time soon,
-
but let's say hypothetically, that the
explosion of local mosquito populations,
-
caused a similar explosion,
-
in the number of Mexican free tailed bats,
-
the official flying mammal
of the state of Texas.
-
They eat mosquitoes.
-
That would be a limiting factor
that was density dependent.
-
More mosquitoes leads to more bats,
-
which leads to fewer mosquitoes.
-
It's pretty simple.
-
When density dependent
limitations start to kick in,
-
and start to limit a population's growth,
-
that means that the habitat's
carrying capacity has been reached.
-
The other type of limiting factor,
the density independent ones,
-
have nothing to do with how
many individuals there are,
-
or how dense the population is.
-
A lot of times, these
limitations are described,
-
in terms of some catastrophe.
-
A volcanic eruption, a
monsoon, a Chernobyl.
-
In any case, some crucial aspect
of the population's lifestyle,
-
changes enough that it
makes it harder to get by.
-
These factors don't have
to be super dramatic.
-
Going back to mosquitoes, say, in
2013 there's a huge thunderstorm.
-
A really gully washer in Dallas
every day for three months.
-
That's going to disturb the
clutches of mosquito eggs,
-
hanging out in the stagnant water.
-
So the number born that year
would be substantially smaller.
-
By the same token, if the
temperature swung the other way,
-
and it was unseasonably cold all summer,
-
the bugs' growth rate would drop.
-
Now, the truth is, there are a
billion and a half situations,
-
both big and small that
could lead to a population,
-
either reaching its carrying capacity,
-
or collapsing because of external factors.
-
It's a population ecologist's job to
figure out what those factors are.
-
That is what math is for.
-
Our friend math says that
any population of anything,
-
anything, will grow exponentially,
-
unless there's some reason that it can't.
-
Exponential growth means
that the population grows,
-
at a rate proportional to
the size of the population.
-
Here at the beginning of 2012,
-
we might only have had a
thousand mosquitoes in Dallas,
-
but then after, say, one
month we got three thousand.
-
Now, with three times as
many reproducing mosquitoes,
-
the population grew three times as fast,
-
as when there were a thousand.
-
Then there are nine thousand,
-
at which point it's growing
three times as fast,
-
as when there were three thousand.
-
And on and on into infinity.
-
And in this scenario,
the mosquitoes are all,
-
carrying capacity my chitin-covered butt!
-
There's no stopping us!
-
But you know what doesn't really happen?
-
I mean, it can happen for a while.
-
Humans have been on an
exponential growth curve,
-
since the Industrial
Revolution, for example.
-
Eventually something always knocks
the population size back down.
-
That thing might be a
density dependent factor,
-
like food scarcity or an epidemic.
-
Or a density independent one,
-
like an asteroid that takes
out the whole continent.
-
Regardless, this exponential
growth curve can't go up forever.
-
When those factors come into play,
-
a population experiences
only logistic growth.
-
This means that the population is limited,
-
to the carrying capacity
of its habitat, which,
-
when you think about it,
ain't too much to ask.
-
See how this graph flattens up at the top?
-
The factor that creates that
plateau is almost always,
-
a density dependent limitation.
-
As you add mosquitoes, eventually
the rate of population growth,
-
is going to slow down because
they run out of food or space.
-
When we get to where
that number levels off,
-
that number is the carrying capacity,
-
of the mosquito population
in that particular habitat.
-
Now, let's apply all of these ideas,
-
using a simple equation
that will allow us,
-
to calculate the population
growth of anything we feel like.
-
I know it's math, but wake
up because this is important.
-
The city of Dallas is depending on you!
-
So, let's calculate the growth
of Dallas' mosquito population,
-
over a span of two weeks.
-
All we have to do to get the
rate of growth, that's R,
-
is take the number of births.
-
Births minus the number of deaths.
-
Then divide that all by the
initial population size.
-
Which we generally just call N.
-
So, let's say we start
with an initial population,
-
of a hundred mosquitoes.
-
Each of those mosquitoes
lives an average of two weeks.
-
Our deaths, over a span of two
weeks, will be one hundred.
-
Half of these mosquitoes are going
to be female, so fifty of them.
-
They can produce about two
thousand babies in their lifetime,
-
so that's times two thousand.
-
Ugh!
-
Fifty mommy mosquitoes times
two thousand babies per mommy.
-
You get births equaling one hundred
thousand little baby mosquitoes.
-
Once we plug in all the
numbers into this equation,
-
even though this is
totally a hypothetical,
-
we will see the true scope
of Dallas' mosquito problem.
-
Blink, in two weeks the population
had a hundred thousand babies,
-
and only a hundred of them died.
-
This is a population growth
rate, if you do the math,
-
of nine hundred and ninety nine.
-
This means, that for
every mosquito out there,
-
at the beginning of two weeks,
-
there will be ninety hundred
and ninety nine more,
-
at the end of two weeks.
-
That is a ninety nine thousand,
eight hundred percent increase.
-
By Thor's hammer!
-
Again, these are hypothetical numbers,
-
but it gives you a sense
of how a population,
-
can just go out of control,
-
when all the factors we
talk about go in its favor.
-
You guys haven't even seen
trouble until you see,
-
what the graph of human
population looks like,
-
over the last couple millennia.