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The population has show exponential growth since the industrial revolution and all countries will eventually move through the demographic transition. If the growth rate is positive the population will grow exponentially and the doubling time can be calculated using the Rule of 70. The crude birth rate and crude death rate can be used to calculate the natural rate of increase. The demographic transition model and age-structure diagrams can be used to predict future populations.
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Transcript provided by YouTube:
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.
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This post was previously published on YouTube.