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.