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Human population growth
Video transcript
Presenter:If being alive on
Earth was some kind of contest, humans, I think, would win it hands down. As 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. 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 seven
billion 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. (cheerful music) 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 got a lot of choices
at your grocery store. You could buy five packs
of ramen for a dollar, or you could buy some fancy
ravioli made by Italian nuns out of organic pasta
for, like, $20 a pound. They're both noodles. They're both food. But with the ramen, you get more, whereas with the handmade stuff, it tastes better, 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 versus K Selection Theory. The R versus 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 a 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, our 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 they 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, as most animals aren't very strongly K-selected or R-selected. It's actually 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. 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, usually only have 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-ishly, for the past few centuries, our population growth curve
has been looking suspiciously like that of an R-selected species. An exponential growth, even
for our 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, long time ago. These obstacles, you will recall, are limiting factors, and we 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 mechanized, 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 vaccination. 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 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 1500's, but they weren't widely
used until the 1800's, 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 have 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 and air
conditioning and warm clothes and 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. It's just, it's just a downer. And a lot of smart
scientists and 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 is sure what it 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 one billion to one trillion, which is 1,000 billion, so that seems a little extreme, but the averages of
these estimates are from 10 to 15 billion folks. And we did a lot of
obvious things to survive, food, clean water, non-renewable resources like
metals and fossil fuels, but everything that we
consume requires space, whether it's space to
grow or space to mine or produce or put our waste. 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 12-by-12-meter patch of ground, about half the size of a tennis court. So it could be that you could fit 1,000 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 and soil and clean water, that reduces the amount available to all kinds of other organisms. This is why biologists
say that we are currently living through one of the
biggest extinction events in recent geological history. We're out competing 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. In these days, it's declined to about 1.1%, and it's still falling. Families in most industrialized countries are getting smaller and smaller. But why? Well, part of that has to do with women. As women in developed
nations get more education, they're 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, like, sexually able to, that animal is going to
have fewer offspring. Also, if you gave women more
choices and more education, they might be liable to choose a second career in astrophysics 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 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 workforce, 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, and 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 worked 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 any time 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.