Why do you suppose some organisms display higher birth rates and others lower birth rates

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:http://www.prb.org/Content/NavigationMenu/Other_reports/2000-2002/sheet1.html

        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.

How does birth rate affect the growth of a population?

What is birth rate in environmental science?

Which of the following does not contribute to limits on population growth?

What factors influence human population dynamics?