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In our series on biology we
spent many weeks together,

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talking about the physiology
of animals and plants,

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and how cells work
together to make tissues,

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to make organs, to make organ systems.

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To make us the hunks of meat
and vegetables that we are.

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Understanding the whole organism.

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It's important to know what's
going on at all those levels.

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The same is true for ecology.

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Only, instead of zooming in and out on
different levels within a living thing,

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we can zoom in and out on the earth.

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Depending on the power
of the magnification,

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we can understand a whole range
of things about our planet.

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For instance, we can look
at groups within a species,

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and how they live together
in one geographic area.

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That's population ecology.

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There's also community ecology,

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where you look at groups of
different organisms living together,

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and figure out how they
influence each other.

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Then, the most zoomed out
we get is ecosystem ecology.

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The study of how all living
and non-living things,

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interact within an entire ecosystem.

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Let's start by zooming in
with population ecology.

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The study of groups within a species,

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that interact mostly with each other.

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To understand why these populations,

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are different in one time and
place than they are in another.

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How, you may be asking yourself, is
that in any way useful to anyone ever?

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Well, it's actually super
useful to everybody always.

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Let's look, for instance, at
the outbreak of West Nile Virus,

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that struck Dallas, Texas
in the summer of 2012.

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In Dallas County, twelve
people died from the virus,

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as of the filming of this.

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Nearly three hundred
people have been infected.

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In 2011 the whole state of Texas,

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reported only twenty seven cases
of West Nile and only two deaths.

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That seems kind of significant.

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So, what's up?

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Turns out that this is a
population ecology problem.

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West Nile is a mosquito born illness,

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and the population of
mosquitoes in Dallas in 2012,

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busted through brick walls
like the Kool-aid man,

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spreading West Nile like crazy.

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Why did this outbreak happen in
2012 and not the year before?

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And why did it happen in
Texas and not in New Jersey?

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The answer, is population ecology.

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(fast lively music)

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Before we start solving any
disease outbreak mysteries,

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we got to understand the
fundamentals of population ecology.

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For starters, a population is
just a group of individuals,

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of one species who interact regularly.

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How often organisms interact
have a lot to do with geography.

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You're going to have a lot more face
time with the folks you live near,

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than those who live farther away.

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As a result, individuals
who are closer to you,

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will be the ones that you compete
with for food and living space,

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mates, all that stuff.

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In order to understand why
populations are different,

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from time to time and place to place,

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a population ecologist needs to know
a few things about a population.

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Like, it's density.

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In this instance, how
many mosquitoes there are,

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in the greater Dallas area that might
come into contact with each other.

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A population's density changes
due to a number of factors,

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all of which are pretty intuitive.

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It increases when new
individuals are either,

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born or immigrate, that is, move in.

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It decreases because of
deaths or emigration,

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or individuals moving out.

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Simple enough, but as
a population ecologist,

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you also need to know about
the geographic arrangement,

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of the individuals within the population.

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This is their dispersion.

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Like, are the mosquitoes
all clumped together?

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Are they evenly spaced
throughout the county?

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Is there some kind of random spacing?

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The answers to these
questions give scientists,

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a snapshot of a population
at any given moment.

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To figure out a puzzle like
the West Nile outbreak,

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which involves studying how a
population has changed over time,

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you have to investigate one of
population ecology's central principles.

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Population growth.

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There are all kinds of factors
that drive population growth,

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and they can vary radically
from one organism to the next.

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Things like fecundity.

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How many offspring an individual
can have in a lifetime,

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make a huge difference in
the size of a population.

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For instance, why do mosquito
populations seem to grow so quickly,

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while, the endangered black
rhino may never recover,

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from a single act of poaching?

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For starters, mosquitoes can
have two thousand offspring,

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in their two week lifetime.

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While the rhino can have
like five in forty years.

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Still, a population doesn't
usually or even ever,

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grow to its full potential and it
can't keep growing indefinitely.

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To understand how fast or slow,

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and high or low a
population actually grows,

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you need to focus on what's
keeping growth in check.

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These factors are appropriately called,

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limiting factors.

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Say, you're a mosquito in Dallas in 2011,

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the year before the outbreak.

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Back then, the growth rate
wasn't what it was in 2012,

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so something was keeping you down.

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To figure out what your
limiting factors were,

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you first have to narrow down
what you need as a mosquito,

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to live and reproduce successfully.

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First, you got to find your food.

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Now, you mosquitoes, you
eat all kinds of things.

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But in order to reproduce,
assuming you're a female,

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you need a blood meal.

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You have to find a vertebrate
and suck some of its blood out.

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Presumably there's no shortage of
vertebrates walking around Dallas,

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for you to suck blood out of.

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I have good friends who
are vertebrates in Dallas.

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You might even be able to
suck some of their blood.

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Next, temperature.

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Because you mosquitoes are
ectothermic, it has to be warm,

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in order for you to be active.

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Now, Texas is pretty warm and the winter
of 2011, 2012 was especially balmy.

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In fact, the summer of
2012 was exceptionally hot,

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which helps speed up
the mosquito life cycle.

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That's one limiting factor
that's been removed,

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for Dallas area mosquitoes.

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Moving on to mates.

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If you're a female mosquito, you
need to find a nice male mosquito,

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with a job and preferably his own car,

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because Dallas is a pretty
big city, to mate with.

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This isn't actually all that hard
because the way that mosquitoes do it.

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Males just gather into a mosquito cloud,

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at dusk every night during mating season,

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and all the female has to do
is find her local dude cloud,

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and fly into it in
order to get mated with.

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Easy cheese.

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Finally, space.

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And, aha!

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Because here we have
another important clue.

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Mosquitoes need to lay their
eggs in stagnant water,

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if there's anything mosquito larva hate,

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it's a rainstorm flushing out
the little puddle of water,

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they've been living in.

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Since Dallas saw a pretty severe
drought in the summer of 2012,

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there were lots of pockets of
stagnant, nasty mosquito water,

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sitting around acting
as nurseries for many,

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many West Nile infected mosquitoes.

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When we look at this evidence,

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we find at least two limiting factors,

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for Dallas' mosquito population growth,

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that were removed in 2011.

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The constraints of temperature and space.

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It was plenty hot and there were lots,

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of egg-laying locations so
the bugs were free to go nuts.

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Population ecologists group
limiting factors like these,

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into two different categories.

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Density dependent and density independent.

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They do it this way
because we need to know,

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whether a population's growth
rate is being controlled,

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by how many individuals are in it,

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or whether it's being
controlled by something else.

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The reason these limitations matter,

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is because they affect what's
known as the carrying capacity,

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of the mosquitoes' habitat.

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That's the number of individuals
that a habitat can sustain,

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with the resources that it has available.

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So, density dependent limitations
are factors that inhibit growth,

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because of the environmental
stress caused by a population size.

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For example, there may
simply not be enough,

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food, water, and space
to accommodate everyone.

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Or maybe because there
are so many individuals,

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a nearby predator population explodes,

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which helps keep the population in check.

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Things like disease can also be
a density dependent limitation.

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Lots of individuals
living in close quarters,

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can make infections spread like crazy.

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Now, I don't think that
the Dallas mosquitoes,

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are going to run out of vertebrates
to dine on any time soon,

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but let's say hypothetically, that the
explosion of local mosquito populations,

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caused a similar explosion,

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in the number of Mexican free tailed bats,

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the official flying mammal
of the state of Texas.

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They eat mosquitoes.

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That would be a limiting factor
that was density dependent.

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More mosquitoes leads to more bats,

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which leads to fewer mosquitoes.

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It's pretty simple.

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When density dependent
limitations start to kick in,

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and start to limit a population's growth,

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that means that the habitat's
carrying capacity has been reached.

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The other type of limiting factor,
the density independent ones,

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have nothing to do with how
many individuals there are,

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or how dense the population is.

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A lot of times, these
limitations are described,

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in terms of some catastrophe.

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A volcanic eruption, a
monsoon, a Chernobyl.

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In any case, some crucial aspect
of the population's lifestyle,

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changes enough that it
makes it harder to get by.

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These factors don't have
to be super dramatic.

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Going back to mosquitoes, say, in
2013 there's a huge thunderstorm.

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A really gully washer in Dallas
every day for three months.

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That's going to disturb the
clutches of mosquito eggs,

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hanging out in the stagnant water.

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So the number born that year
would be substantially smaller.

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By the same token, if the
temperature swung the other way,

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and it was unseasonably cold all summer,

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the bugs' growth rate would drop.

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Now, the truth is, there are a
billion and a half situations,

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both big and small that
could lead to a population,

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either reaching its carrying capacity,

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or collapsing because of external factors.

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It's a population ecologist's job to
figure out what those factors are.

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That is what math is for.

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Our friend math says that
any population of anything,

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anything, will grow exponentially,

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unless there's some reason that it can't.

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Exponential growth means
that the population grows,

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at a rate proportional to
the size of the population.

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Here at the beginning of 2012,

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we might only have had a
thousand mosquitoes in Dallas,

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but then after, say, one
month we got three thousand.

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Now, with three times as
many reproducing mosquitoes,

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the population grew three times as fast,

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as when there were a thousand.

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Then there are nine thousand,

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at which point it's growing
three times as fast,

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as when there were three thousand.

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And on and on into infinity.

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And in this scenario,
the mosquitoes are all,

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carrying capacity my chitin-covered butt!

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There's no stopping us!

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But you know what doesn't really happen?

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I mean, it can happen for a while.

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Humans have been on an
exponential growth curve,

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since the Industrial
Revolution, for example.

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Eventually something always knocks
the population size back down.

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That thing might be a
density dependent factor,

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like food scarcity or an epidemic.

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Or a density independent one,

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like an asteroid that takes
out the whole continent.

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Regardless, this exponential
growth curve can't go up forever.

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When those factors come into play,

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a population experiences
only logistic growth.

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This means that the population is limited,

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to the carrying capacity
of its habitat, which,

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when you think about it,
ain't too much to ask.

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See how this graph flattens up at the top?

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The factor that creates that
plateau is almost always,

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a density dependent limitation.

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As you add mosquitoes, eventually
the rate of population growth,

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is going to slow down because
they run out of food or space.

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When we get to where
that number levels off,

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that number is the carrying capacity,

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of the mosquito population
in that particular habitat.

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Now, let's apply all of these ideas,

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using a simple equation
that will allow us,

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to calculate the population
growth of anything we feel like.

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I know it's math, but wake
up because this is important.

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The city of Dallas is depending on you!

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So, let's calculate the growth
of Dallas' mosquito population,

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over a span of two weeks.

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All we have to do to get the
rate of growth, that's R,

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is take the number of births.

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Births minus the number of deaths.

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Then divide that all by the
initial population size.

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Which we generally just call N.

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So, let's say we start
with an initial population,

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of a hundred mosquitoes.

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Each of those mosquitoes
lives an average of two weeks.

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Our deaths, over a span of two
weeks, will be one hundred.

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Half of these mosquitoes are going
to be female, so fifty of them.

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They can produce about two
thousand babies in their lifetime,

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so that's times two thousand.

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Ugh!

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Fifty mommy mosquitoes times
two thousand babies per mommy.

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You get births equaling one hundred
thousand little baby mosquitoes.

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Once we plug in all the
numbers into this equation,

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even though this is
totally a hypothetical,

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we will see the true scope
of Dallas' mosquito problem.

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Blink, in two weeks the population
had a hundred thousand babies,

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and only a hundred of them died.

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This is a population growth
rate, if you do the math,

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of nine hundred and ninety nine.

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This means, that for
every mosquito out there,

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at the beginning of two weeks,

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there will be ninety hundred
and ninety nine more,

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at the end of two weeks.

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That is a ninety nine thousand,
eight hundred percent increase.

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By Thor's hammer!

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Again, these are hypothetical numbers,

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but it gives you a sense
of how a population,

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can just go out of control,

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when all the factors we
talk about go in its favor.

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You guys haven't even seen
trouble until you see,

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what the graph of human
population looks like,

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over the last couple millennia.