Intro Biology 1102
Spring 2002
Population Growth - An Introduction
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
    Populations can also change size if organisms move in (immigration) or leave (emigration)

Putting It All Together
    We can write a simple equation to show population growth as:

            Change in Population Size = (Births + Immigration) - (Deaths + Emigration)

Expressing Population Changes as a Percentage
    Suppose we had a population of 100,000 individuals.  Suppose in one year there were 1000 births, and 500 deaths.
    What percentage of the population were births?
                    1000/100,000 = 0.01, or in percentage terms, this is 1% of the population.

    What percentage of the population were deaths?
                    500/100,000 = 0.005, or in percentage terms, this is 0.5% of the population.

    Assume immigration equals emigration.  If so, then they cancel out of our population equation.   We'll come back to
this assumption later.

                    Now, subtract deaths from births but express as a percentage:
                     1000-500/100,000 = 500/100,000 = 0.005, or 0.5% net growth

    Thus, this population would be growing by 0.5% this first year.  That means that after one year, there will be 500 more
individuals than the previous year.  So, after one year, the population would be 100,500 individuals.

The Net Reproductive Rate
    The net reproductive rate (r) is the percentage growth after accounting for births and deaths.  In the example above, the population reproductive rate is 0.5%/yr.

    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:
                        1,000,000 x 0.05 (which is 0.5%) = 50,000

    This means that now 50,000 new individuals are added in one year!!  The net reproductive rate is the same as before, but because
the population is so much bigger, many more individuals are added.

Exponential Growth
    If a population grows by a constant percentage per year, this eventually adds up to what we call exponential growth.  In other words, the larger the population grows, the faster it grows!!  A curve of exponential growth is an upward sweeping growth curve. 

 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) changes.

Some Population Statistics for Humans
    At the end of the 1700s, Robert Malthus, a priest, wrote one of the most influential essays in the world - He was pondering why there was so much suffering among humans, and came to the conclusion that human population growth tended to always outstrip food supply.  The Core Principles of Malthus are:

        1.Food is necessary for human existence.
        2.Human population tends to grow faster than the power in the earth to produce subsistence, and that
        3.The effects of these two unequal powers must be kept equal.
        4.Since humans tend not to limit their population size voluntarily, population reduction tends to be accomplished through the
            "positive" checks of famine, disease, poverty and war.

    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?
    The current population of the earth is about 6.2 Billion people!  This is more humans alive than at any time in human history.
    To see how fast the world population is growing, click on this to see a clock of human population growth.
    For interesting facts on worldwide population growth, and the factors affecting it, click here.  Recommended web site!!
    For even more interesting facts, click here.  The Population Reference Bureau has some of the best information on human population growth of anywhere!

What is Current Net Reproductive Rate of Humans Worldwide?
    The current percentage increase in the human population (as of 2000) is about 1.3%, or 0.013 per year.
    If we multiply this, as shown above, by the current population, we get the increase in humans per year:

                                        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 Century?
    Remember, only two things affect population growth: births and deaths.  So, have these changed?
            Birth rates: have been constant for many years at about 22 babies/1000 people/year
            Death rates: have declined dramatically due to more food, less disease, more social structure
                    Death Rates in 1900:     20/1000/year   Net Reproductive Rate 1900: 22-20/1000 = 0.002 or 0.2%
                    Death Rates in 2000:    9/1000/year     Net Reproductive Rate 2000: 22-9/1000 = 0.013 or 0.13%

             Due to decline in death rates, r for humans has risen nearly 6 fold!!!

Some Representative Growth Rates for Countries Around the World
        Consider this statistic: 90% of all world population growth occurs in developing countries!!
        Go to this website to see net reproductive rates for all countries:

        Net Reproductive Rates
        World                                            1.3%
        More Developed Countries                0.1%
        Less Developed Countries                1.6%
        Africa                                            2.9%  !!
        Liberia                                            3.1%  !!!!!
        Canada                                            0.3%
        United States                                  0.6% (much of it immigration, about 1/3!)
        Mexico                                            1.9%
        Europe                                            -0.1% (population is declining!!)
        England                                            0.1
        France                                             0.4
        Latvia                                             -0.6%

    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?
    Demographics!!  If you have more young people, then you have more opportunity to make babies!!  Developing countries have more young people than developed ones.  Why?  In developed countries, couples wait longer to have babies, and, they tend to have fewer per couple.  In undeveloped countries, children are produced sooner, and couples have larger families than in developed countries.
    In Mexico, 50% of the population is age 15 or younger!
    In the United States, only about 25% of the population is this young.

What Can Be Done to Control Population Growth?
    There are two simple ways to lower population growth: increase the number of deaths, or, decrease the number of births.  I think for most of us, we would opt for the latter solution.  How to do that?
    1. Family Planning - have babies at a later age, use contraception (birth control), limit number of babies per family
    2. Education - the best correlate of lowering the number of babies per family is the educational status of the females
            The more education the females have, the more control they have over their reproductive lives
    3. Better social security - in developing countries, large families are a form of social security.  If poverty can be reduced,
            then the need for large families is lowered (hard to do though!)

The Future - How Large Will the Population Become?
    In your lifetime, the population could approach or exceed 14 Billion people.  Can we feed that many?  Not likely.  Is there enough water for that many?  Probably not.  Enough habitat?  Probably not.
    So, what will the population stabilize at?  Best guesses are between 7-10 Billion people.  Will the world still be a great place with that many people?  Unlikely.
    There is a great need to reduce population growth, starting now!!  You can help a great deal.

    1. Have only 2 children per family.  That way, population growth is reduced to essentially zero.
    2. Wait till a later age to have children.

Return to Bio 1102 Main Page