If being alive on Earth were
some kind of contest, humans,
I think, would win it, hands down.
As a population of organisms,
we're the Michael Phelps of being
alive, only we have like
250,000 times more gold medals.
Last week, we talked about
exponential growth, when a population
grows at a rate proportional
to the size of the population,
even as that size of the
population keeps increasing.
Well, since around the year
1650, the human population
has been undergoing
probably the longest period of
exponential growth of any
large animal in history, ever.
In 1650, there were about 500
million people on the planet.
By 1850, the population
had doubled to one billion,
and it doubled again
just 80 years after that,
and doubled again just
45 years after that.
We are now well past
7 billion now, and counting.
So, think about this:
Today there are 80-year olds
who have watched the population
of their species on Earth triple.
So, why is this happening? And...how?
And how long can it go on?
Because it's kind of uncomfortable.
Let's say you're shopping
for dinner, and bear with me,
we're going to relate it
back to ecology in a second,
but you have a lot of
choices at your grocery store.
You could buy 5 packs of ramen
for a dollar or you could buy
some fancy ravioli made by
Italian nuns out of organic pasta
for like $20 dollars a pound.
They're both noodles.
They're both food. But, you know,
with the ramen you get more,
whereas with the handmade
stuff it tastes better.
It's higher quality.
What do you do?
It's a perennial problem in
nature, and in our lives,
satisfying the two competing
impulses: do I have more,
or do I have the best?
Quantity or quality? Tough choice.
Although we're not really
aware of it, all organisms
make a similar choice
through how they reproduce.
In ecology, we size up who
chooses quantity over quality
by something called the
R vs. K Selection Theory.
The R vs. K Selection Theory
says that some organisms
will reproduce in a way that
aims for huge, exponential growth,
while others are just content
to hit the number of individuals
that their habitat
can support, that is,
the carrying capacity, and then
stay around that level.
Species that reproduce in a way
that leads to very fast growth
are called R-selected species,
because "R" is the maximum growth
rate of a population when
you're talking math talk,
as we learned last week.
Very strongly R-selected
animals make a lot of babies
in their lifetime and
just hope that they make it.
If some of the babies get
eaten or something, no biggie:
there are others
where those came from.
On the other hand, K-selected
species only make a few babies
in their lifetime, and
invest in them very heavily.
"K" in math language
is carrying capacity,
since K-selected species usually
end up living at population
densities closer to their carrying
capacity than r-selected ones.
Of course, things aren't
so cut and dry in nature.
Most animals aren't very
strongly K-selected or r-selected.
It's actually, you know,
a spectrum: Some organisms,
usually smallish ones,
reproducing more on the r side,
and others, usually
larger ones, on the K side.
Most species are
somewhere in the middle.
So, the reason I'm telling you
this is to drive home how bananas
it is that humans have gotten
to the population size we have:
Because we tend to reproduce way on
the K-selected side of the spectrum.
We're pretty big mammals
who usually have only a few kids
during our lifetimes, and those
kids are a total pain in the butt
to raise, but we put a ton
of resources into them anyway.
So even though humans
reproduce K-selected-ish-ly,
for the past few centuries,
our population growth curve
has been looking suspiciously
like that of a R-selected species.
And exponential growth,
even for R-selected species,
usually does not
go on for 350 years.
So how did this all happen?
Well, the short answer is,
humans figured out how to raise
our carrying capacity,
so far, indefinitely.
And we did this by eliminating a
bunch of obstacles that would have
made our numbers level off at
a carrying capacity a long time ago.
These obstacles, you will
recall, are limiting factors,
and we've managed to blast them to
pieces in a few different ways.
First, we've upped our
ability to feed ourselves.
Our crazy-rapid population
growth started in Europe around
the 17th century, because that's
when agriculture was becoming
echanized, and fancy new farming
practices like the domestication
of animals and crops,
were increasing food production.
From Europe, those agricultural
practices and their
accompanying population explosion
spread to the New World
and to much of the rest of the
world by the mid-19th century.
Another game-changer
for the human population
came in the form
of medical advances.
Anton van Leeuwenhoek,
Father of Microbiology all around
really smart guy, was the first
modern scientist to propose
the germ theory of disease in
1700, and even though it took
about a century and a half for
people to take it seriously,
it revolutionized human health,
leading to things like vaccinations.
Suddenly people stopped dying
of stupid, avoidable stuff
as they had been for thousands
of years, which meant that
everybody lived longer:
childhood survival rates improved,
and those kids went on to make their
own babies and get very, very old.
And we also increased
our carrying capacity
by not being so disgusting.
We figured out that
you can't just sit around
in your own poop and
live to tell the tale.
So sewage systems became a thing.
In Europe at least, it started
around the 1500s, but they weren't
widely used until the 1800s,
and we all benefited from that.
And finally, we've
gotten a lot better
at living comfortably
in inhospitable places.
That is to say, people had been
living in deserts and tundra
for thousands of years,
but in the 20th century,
we expanded the human habitat to
pretty much everywhere in the world,
thanks to heating, air
conditioning, warm clothes,
airplanes and trucks
that bring food everywhere
from Svalbard, Norway
to New South Wales.
So for all those reasons and
more, humans have been able
to avoid that old party
pooper, Carrying Capacity.
Which is good because I don't
like it when people die.
Its just...it's just a downer.
And a lot of smart scientists,
mathematicians and economists
have argued that each person
born in the past 350 years
has not only represented another
mouth to feed but also two hands
to work to raise the
human carrying capacity,
just enough for themselves
and a teensy bit more.
So then as our population grows,
our carrying capacity
grows right along with it,
like some really steep escalator
going up and the ceiling
just above our heads.
And if it stayed there, we'd all
get squished, but it keeps moving.
But of course this
can't go on forever.
The human population does
have a carrying capacity,
it's just that nobody's
sure what that is.
Back in 1679, it was Leeuwenhoek
himself who was the first
to publicly hazard a guess about
the earth's carrying capacity
for humans, guessing it to be
around 13.4 billion people.
Since then, estimates have ranged
from 1 billion to 1 trillion,
which is 1,000 billion.
So that seems a little
extreme, but the averages
of these estimates are
from 10-15 billion folks.
Now, we need a lot of obvious things
to survive: food, clean water,
nonrenewable resources like
metals and fossil fuels.
But everything that we consume
requires space, whether it's
space to grow, to mine, to
produce, or to put our waste.
So a lot of ecologists make
their estimates of how many people
this planet can handle based
on an ecological footprint,
a calculation of how much
land and how many resources
each person on the
planet requires to live.
That footprint is
very different depending
on where you live and
what your habits are.
People in India use a lot fewer
resources, and therefore, space,
than Americans, for example.
Meat eaters require a lot
more acreage than vegetarians.
In fact, if everybody on
the planet ate as much meat
as the wealthiest people in
the world do, current food harvests
could feed less than half of
the present world's population.
So, despite the fact that the
earth is a very big place,
space is a real
limiting factor for us.
And as our population grows,
there will probably be more
conflict over how
our space is used.
For instance, if there really were
a trillion people on the planet,
everybody would have to
live, grow food on and poop
on a 12m x 12m patch of ground,
about half the size of tennis court.
So it could be that you
could fit a thousand billion people
on Earth, but I can
guarantee that those people
would have a hard time
getting along with each other.
But how about we stop thinking
about ourselves just for a moment.
As we take up more space, we also
leave less space for other species.
And as we use resources, like
trees, soil, and clean water, that
reduces the amount available
to all kinds of other organisms.
This is why biologists say that
we are living through one of
the biggest extinction events
in recent geological history:
We're outcompeting other species
for the very basics of life.
And eventually, or in the
case of oil and water, already,
we're starting to compete
with ourselves as a species.
So, serious stuff here.
But here's a little glimmer of hope:
Unlike some other animals,
a lot of our actions are based
on a little thing called
culture, and human culture
has brought about some huge
changes in the last 50 years.
The fact is, even though the
human population continues to grow,
the rate of population growth
actually peaked around 1962,
and has been declining ever since.
At its peak, the human
population was growing at about
2.2% per year, and these days, it's
declined to around 1.1% per year,
and it's still falling.
Families in most
industrialized countries are
getting smaller
and smaller. But why?
Well, part of it has
to do with women.
As women in developed
nations get more education,
they are having babies
later in life.
And when an animal doesn't reproduce
to its fullest potential,
meaning it doesn't start having
babies as soon as it's sexually
able to, that animal is going
to have fewer offspring.
Also, if you give women more
choices and more education,
they might be liable to
choose a second career
in astrophysicists rather
than becoming a mother.
Another reason for the falling
population growth rate
has to do with the way
that we live our lives.
Back in the early the 20th
century, more of the world
worked on farms and maybe
ate their own food.
Kids were a real asset
to a farm back then.
It's a good example of that idea
about more hands doing more work
to increase the carrying capacity
of the human population.
Yeah, kids were extra mouths to
feed, but they were also
a really important work force,
and you could just feed the kids
the stuff you were producing.
That's what we call a
positive feedback loop:
as the population grows,
the workforce gets bigger,
and the place, as a result,
supports more of us.
But these days, that's not
happening so much anymore.
More and more people
are living in cities,
where you don't need kids
to help with the crops,
so fewer people are
having them, because
a) they cost a lot
of money to raise,
b) they're not bringing in money
like they were back on the farm,
c) a lot of people have
access to good birth control
so they don't have
as many "oops children."
All these factors together are
forming a negative feedback loop:
The effects of reproduction
in this case work
to slow down the
rate of reproduction.
But just because our population's
growth rate is decreasing doesn't
mean that this juggernaut of humanity
is going to stop anytime soon.
In addition to reminding
us that the rules of ecology
apply to us just like
any other organism,
human population is
important to think about
because we kind of need
to do something about it.
And I think pretty
much every other species
on the planet would
agree with me on that.
Thanks for watching this episode
of Crash Course Ecology and
thanks to all the people who
helped us put it together.
If you want to review
anything from this episode
there's a table of
contents over there.
And if you have any questions or
ideas or comments, we're on Facebook
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down in the comments below.
We'll see you next time.