Hi. It’s Mr. Andersen and this environmental
science video 13. It is on human population dynamics. Imagine you have $5000 in the bank
at 5 percent interest rate. How long would it take you to double your money to $10000?
Well you would have to go year by year. So it would be 5000 times 5 percent. Which give
you $250 dollars in the first year. We then have to take 5 percent of that. And five percent
of that. It is a really hard problem. Unless you understand the rule of 70 which is really
simple. To figure out the time it takes to double your money all you do is take the number
70 and you divide it by that percent, so divided it by 5 and it is going to take you approximately
14 years to double your money. Super easy to do. Let’s say we have a population of
100 individuals, a growth rate of 7 percent, how long is it going to take that population
to double to 200 in size? All you do is take 70 divided by that growth rate, 70 divided
by 7 gives you 10 years. How long is it going to take you to double it again from 200 to
400? It is going to take another 10 years? And so you start to see this exponential growth
of the population over time. So in the last video we talked about the important characteristics
of a population. The size, which can be increased and decreased. We then talked about other
characteristics. Density distribution, age structure, sex ratio. So in this video we
are going to talk about how this all applies to the human population. We will start with
density and distribution. We will then talk about the size and how that size has changed
over time. It is very easy to calculate growth rate and now you know how to calculate the
doubling time using the rule of 70. We will talk specifically about birth rates, using
the fertility rate and death rates using the mortality rate. And then how industrialization,
so the development of a country can lead to what is called the demographic transition.
And then finally we will finish up with the age structure diagrams. And these two things
can be used as tools to predict the future population. And so if we start with human
density and distribution it is all over the place. So if we look here in Montana, it is
less than 2 people per kilometer. If we look in the US there are going to be way more people
on the coasts. Way more people in the northeast. Way more people in Western Europe and may
more people in Southeast Asia. In certain areas it is going to be more than 500 per
square kilometer. So there is unequal density and distribution. You can also see some neat
patterns where there is desert. Obviously there is not going to be a lot of people.
We could look at the history of humans which has maintained stability over a long period
of time but recently has shown exponential growth. Now what lead to that is going to
be industrialization. So if you have access to constant food, constant sanitation, good
medicine, we see an increase in the population. Now there are some blips along the way. There
is going to be the bubonic plague where a third of the people in Europe died. But you
can see over time we are seeing massive increases. What happens next? We will talk more about
that in the next video. And so if we look at the basic level, what is increasing a population?
Births. What is decreasing it? Deaths. And so we have what is called the rate of natural
increase. And so the equation is simple. The rate of natural increase equals the crude
birth rate minus the crude death rate divided by 10. And the reason we are dividing by 10
is these values are generally expressed per thousand peoples. So in 2013 the crude birth
rate in Afghanistan was 34 people being born for every 1000 people. Crude death rate is
only 8. So if we want to figure out the rate of natural increase we just plug those values
in. 34 minus 8 is 26 divided by 10 is 2.6 percent. And so that means this population
is going to be increasing over time. Now you could try one with the United States. If I
give you the birth rate and the death rate could you calculate that rate of natural increase?
I hope so. I will put the answer in the description down below. Now once we have that value, how
much it is increasing is very easy to figure out the doubling time. So how do we do that?
Remember that rule of 70? You simply take 70 divided by the increase percent, which
is going to be 2.6. And then it is going to give us approximately 27 years. So what does
that mean? The population in Afghanistan right now is 30 million. So in 27 years it will
be 60 million. And so if it is 2013 when these values were given out, it is going to be 2040
when the population goes to 60 million. Now is that super accurate? It may change over
time, and I am not really dealing with immigration and emigration, but it is a simple powerful
tool that you can use. Now what is changing the birth rate? We call that, demographers
call that a total fertility rate. That is going to be the number of births per woman
during her reproductive period of time. And so if we look at what it is in Afghanistan,
it is 4.9. So this imaginary woman who is around today will in general have 5 children.
If we look at what it is in the US that number is going to be 1.9. So why do we see such
a huge difference between Afghanistan and the US? Well let’s talk globally for a second.
So the TFR, total fertility rate, back in 1950 globally around the world was 4.9. So
it is closer to what Afghanistan is today. And you can see that it is decreased over
time. So why do we see this decrease over time? Well in developed countries women are
going to have access to birth control. They are also going to have access to education
and employment. Therefore they do not have to get married and start having kids right
away for stability. So they are going to have less kids over time. Now a value that demographers
will use is called the replacement level fertility. What does that value have to be to keep our
population stable? And so you might think it is going to be 2. Because we have the mom
and she has to replace herself and the dad. So you would think it would be 2. But do to
mortality rate it has to be a little bit higher. And in developing countries it has to be even
a little bit higher than that to have a stable population. So if we look at those fertility
rates there is going to be unequal distribution. So in the United States the value is going
to be between 1 and 2. But in Sub-Saharan Africa we are going to see really large values.
That is because a lot of these are developing countries. Now we have only talked about the
fertility rate and we have not talked about the mortality rate. So if we look at the average
life span in different countries, in a lot of Sub-Saharan Africa that is going to be
really low. Like in Afghanistan it is going to be the average person only living between
45 and 50. Now disease, war can contribute to that. One of the best measures of how developed
a country is is the infant mortality rate. So in the US that value is going to be between
5 and 9 per 1000 people. But look at this. In some parts of Africa that number is going
to be approaching 100. In other words 1 of 10 infants that are born are going to die.
And so what I have been alluding to is this demographic transition. What happens as a
country is industrialized? What happens as it goes from a preindustrial to postindustrial
country? And a really good model is called the demographic transition. So births are
going to be in green. Deaths are going to be in red. And so if we look at the death
rate what happens to the death rate during industrialization? Well it will jump up and
down but eventually what happens is it drops way down and it remains stable. So once we
have access to constant food, sanitation, medicine we are going to decrease that death
rate profoundly. If you look at what happens to the birth rate over time you will see that
it also decreases. But it is shifted to the right a little bit. And so if we look back
at the this model of size being increased by births and decreased by deaths, what do
you think is going to happen preindustrial? Well if these birth and death rates are essentially
the same, now they are moving up and down, what is going to happen to our population
which I have graphed over here on the right. It is going to remain stable. As we move into
this transitional period watch what happens. The first thing that happens is the death
rate will drop off during industrialization. But it takes awhile for people to realize
that and start having less children. And so it takes awhile for the birth rate to drop
off. And so if we have a population where the birth rate is higher than the death rate,
think about this down here, if more births are coming in then deaths are leaving we are
going to see a massive exponential increase in the population. What happens as these two
approach each other, during the industrial phase, as the birth rate eventually drops
to the death rate you could imagine that it is going to stabilize again. And then what
is happening postindustrial, eventually we see the population dropping off. So we can
see this is a lot of countries. Again it is just a model. So it is not going to be perfect.
But we can look at countries who have gone through this and allow us to make predictions
about what is going to happen to countries in the future. So in certain countries we
can see the demographic transition occur. So this is Sweden. What we are looking at
here is going to be the birth rates in blue and then the death rates in red. So if we
look way back in time in the 1700s there is a huge increase in death rates, so this was
the small pox outbreak. But what happens over time is the death rate drops off first and
then the birth rate drops off second. And then you can see they reach each other at
the same point. So what happened to the population of Sweden during this demographic transition?
It increased and then it became stable. A good way to look at where a population is
and where it is headed is using something called an age structure diagram. And so on
an age structure diagram, we will put males on one side, females on the other. And then
the percent they make up of the population. And so if you are a female watching this and
you are 18 right now, we could find you right here. So you would be right here. So in this
country you would make up around 7 percent of the population. Now if we see an age structure
diagram like this where we do not have very many people that are very old, and we have
lots of young people, this is going to be a population that is just increasing, just
going through that demographic transition. If however we get an age structure diagram
that looks like this which is pretty much straight on the sides, this is going to be
a more stable population. Now this would be what the United States looks like and you
can even see the baby boomers, this large group of babies that were born after World
War II. And they are just going to work their way up through the age structure diagram.
If we look at this demographic transition, where is this going to be? Right there. But
if we look at a country like Japan, at the bottom we are not going to have many youth
at all. And so where is that going to be? We are going to call that postindustrial.
Now there are going to be advantages and disadvantages of each of these. But we will talk about those
in the next video. And so did you learn the following. That the human population, characteristics
are density, distribution, sex ratio, age structure, which we can see in an age structure
diagram. The size is the most important thing. We can use the growth rate to figure out the
doubling time. But what are the other two things that contribute to that? Birth rate
and mortality rate or fertility rate and mortality rate. Remember industrialization is going
to bring about the demographic transition. And we can use these two tools to predict
the future. We will talk more about what the future holds for humans, but I hope that was
helpful.
