|Population Growth - Basic Information
All populations change in size with time
- if births exceed deaths, the population grows
- if deaths exceed births, the population shrinks
- only when births equal deaths does the population stay the same
Other Population Growth Factors
Putting It All Together
Change in Population Size = (Births + Immigration) - (Deaths + Emigration)
Expressing Population Changes as a Percentage
What percentage of the population were
Assume immigration equals emigration.
If so, then they cancel out of our population equation. We'll
come back to
Now, subtract deaths from births but express as a percentage:
Thus, this population would be growing
by 0.5% this first year. That means that after one year, there will
be 500 more
The Net Reproductive Rate
Net reproductive rate (r) is calculated as: r = (births-deaths)/population size or to get in percentage terms, just multiply by 100.
Suppose we came back many years later, the net reproductive
rate was still the same, but now the population had grown to 1,000,000.
How many new individuals would be added each year now? Simply multiply
the population by the reproductive rate:
This means that now 50,000 new individuals
are added in one year!! The net reproductive rate is the same as
before, but because
See figure to right - the curve sweeping upwards
is the exponential growth curve.
|In this case, if nothing else is done, the population
size approaches infinity. But the earth's resources are limited,
and such a curve is a physical impossibility. Instead, things become
limiting: food, habitats and shelter, disease, etc. In that case
population tends to reach an upper limit, known as the carrying
capacity (k) for that environment. Then, you get the yellow
curve in the figure above, known as the logistic
model. Here, as the population approaches a theoretical upper
limit, the net reproductive rate decreases. In exponential growth,
it stays constant. The logistic curve is the more realistic, even
though it is still an abstraction (most populations don't behave so nicely
in the real environment - they tend to bounce around, and r tends to change
through time in ways that are unpredictable, due to stochastic (unpredictable)
Some Population Statistics for Humans
1.Food is necessary for human existence.
Malthus wrote that human population growth tended to be exponential (see above graph), whereas agricultural growth tended to be arithmetic, that is, linear (see graph below).
|Note how after 9 generations, the exponential curve (human
population growth) outstrips food supply (arithmetic growth).
Darwin used this information to help develop his theory of natural selection by assuming that this situation occurs for all living organisms, not just humans!!
So, does it work? Consider that in a typical day, 35,000 humans starve to death around the world. Most in developing countries.
What is the Current Population of the Earth?
What is Current Net Reproductive Rate of Humans Worldwide?
0.013 x 6.2 B = 80,600,000 new people per year, or 80.6 million new humans each year!!
That is the equivalent of 2.5 California's per year, or 1 new Germany per year. It is 1.6 million people per week (one New Mexico per week), or 221,000 people per day (one Charlotte, NC added per day!).
An astounding growth rate, even though the net reproductive rate is actually quite small. But growth is not evenly distributed around the world. Certain countries are growing faster than others, while some are actually losing growth (deaths and emigration exceed births plus immigration - Albania is an example).
Why the Increase in Human Population Growth Rates This
Due to decline in death rates, r for humans has risen nearly 6 fold!!!
Some Representative Growth Rates
for Countries Around the World
You can determine the population doubling times for the world and countries by dividing 69.3 by the growth rate. For example, if the world growth rate is 1.3%, then the time it takes to double the population is:
69.3/1.3 = 53 years
Thus, if things don't change, the world population could rise to 12.4 Billion in the year 2055!! When I was born, the population was about 2 Billion in 1952. It is now 50 years later, and the population is 6.2 Billion. That is nearly a tripling!! Why? The world population growth rate was much higher in the past 50 years than it currently is. When I was born, the population growth rate was over 2% per year, and the doubling time was down to 42 years!!
Why Do Growth Rates Differ Between Countries?
What Can Be Done to Control Population Growth?
The Future - How Large Will the Population Become?
1. Have only 2 children per family.
That way, population growth is reduced to essentially zero.
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