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3.1 Population Dynamics. Crude Birth Rate (b). The number of births per thousand individuals in a population in a year. Crude Death Rate (d). The number of deaths per thousand individuals in a population in a year. Natural increase rate (r).

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crude birth rate b
Crude Birth Rate (b)

The number of births per thousand individuals in a population in a year.

crude death rate d
Crude Death Rate (d)

The number of deaths per thousand individuals in a population in a year.

natural increase rate r
Natural increase rate (r)

Natural Increase rate (%)= crude birth rate – crude death rate


Immigration and emigration is ignored!

  • birth rate expressed as number of births per 1000 per year (currently 14 in the U.S.);
  • death rate expressed as the number of deaths per 1000 per year (currently 8 in the U.S.);
  • So the rate of natural increase is 6 per thousand (0.006 or 0.6%).

Although the value of r is affected by both birth rate and death rate, the recent history of the human population has been affected more by declines in death rates than by increases in birth rates.


case studies mexico since 1930
Case studies - Mexico since 1930.
  • The graph shows birth and death rates in The introduction of public health measures, such as:
    • better nutrition
    • greater access to medical care
    • improved sanitation
    • more widespread immunization
  • has produced a rapid decline in death rates, but until recently there was no corresponding decline in birth rates. In 2010, r is 1.4%. (Data from the Population Reference Bureau.)

Although death rates declined in all age groups, the reduction among infants and children had — and will continue to have — the greatest impact on population growth. This is because they will soon be having children of their own.

This situation, resulting in a rapid rate of population growth, is characteristic of many of the poorer regions of the world.


the demographic transition
The Demographic Transition

Slowly declining birth rates following an earlier sharp decline in death rates are today characteristic of most of the less-developed regions of the world.

The shift from high birth and death rates to low birth as well as death rates is called the demographic transition.

This graph (based on data from the Population Reference Bureau) shows that the demographic transition began much earlier in Sweden than in Mexico and was, in fact, completed by the end of the nineteenth century. The spike in deaths in the interval between 1901 and 1926 was caused by the worldwide influenza pandemic of 1918–1919.

The birth rate in Sweden is now (2010) 12/1000; the death rate 10/1000, giving a rate of natural increase (r) of 0.2%.


case study sri lanka
Case Study – Sri Lanka

Prior to World War II, advances in public health has been largely limited to affluent, industrialized countries. But since then, improvements in public health have been made in many of the poorer countries of the world — always with dramatic effect on death rates.

  • In 1945, the death rate in Sri Lanka (then called Ceylon) was 22/1000.
  • In 1946, a large-scale program of mosquito control — using DDT — was started.
  • By eliminating its vector, the incidence of malaria dropped sharply.
  • After 9 years, the death rate dropped to 10/1000, and by 2010 was 7.
  • But a compensating decline in birth rates has come more slowly (19/1000 in 2010).
  • So by 2010 the population was increasing at an annual rate of 1.2% (12/1000/year).
  • At this rate the population would double in 57.5 years.
  • Let's see why.


what is exponential growth
What is exponential growth?
  • The prediction that Sri Lanka will double its population in 57.5 years is based on:

the assumption that r will remain unchanged (which is surely false)

the mathematics of exponential growth.

  • The product of growth grows itself. So the growth of populations is a problem in "compound interest". At the end of each year (or whatever period you choose to use), the base against which the rate is applied has grown. Whatever figures you pick, as long as r is positive, a plot of population as time elapses will produce an exponential growth curve like this one.


the population of the world
The Population of the World
  • The solid line in this graph shows estimates of the size of the world's population over the last two millennia. The estimates from 1800 to 1991 are based on more accurate data than those before.
  • The dotted line shows what would happen if exponential growth continued to the year 2100.
  • As you can see, the world's population has been growing exponentially (except during the years of the black death). How long will it continue to do so? (Since the graph was drawn, the world's population has reached 6.9 billion; that is, in 2010 we are still on course.) But can it continue indefinitely? Surely not.


the implications of exponential growth in human populations
The implications of exponential growth in Human Populations
  • Think of the population density dependent factors from the Ecology section:
  • Living space – as more people need more land to live there becomes less land available for food production / nature;
  • Look at this web site: http://tranquileye.com/clock/
  • Disease – epidemics will become more common as people have higher population densities;
  • Waste management – more people make more waste – think sewage and trash;
water water everywhere but not a drop to drink
Water, Water everywhere but not a drop to drink………..

Click on the image to go to this very good website – or click here

For another summary article on the water crisis from the BBC – click here


peak everything
“Peak everything”

Watch these YouTube videos on exponential growth :



Take a look at this quite scary article about Peak oil followed by the YouTube video:



Want to see more from Chris Martenson then check out:-


  • The potential for reproduction exhibited in a population:
  • Fertility Rate - the no. of births per 1000 women of child bearing age.
  • Total Fertility – the average number of children a woman has in her life-time.
doubling time
Doubling time
  • The number of years it would take a population to double its size at its current growth rate.
  • A natural increase of 1% will allow a population to double in 70 years.
  • Other doubling times can be calculated proportionally i.e:

Doubling time = 70

natural increase rate

some practice questions
Some practice questions……….
  • Mexico – 1970 –

Birth rate (b)= 40/1000; Death rate (d)= 10/1000

Natural increase rate (r) =

Doubling time =

Birth rate (b) – Death rate (d)




% Increase rate

70 / 3%

23 years

now try sweden
Now try Sweden……….
  • The birth rate in Sweden is now (2010) 12/1000; the death rate 10/1000, giving a rate of natural increase (r) of

Doubling time of


350 years

……..or try Sri Lanka……….

  • The birth rate in Sri Lanka is now (2010) 19/1000; the death rate 7/1000, giving a rate of natural increase (r) of

Doubling time of



age sex pyramids
Age / Sex Pyramids
  • Aside from the total size, the most important demographic characteristic of a population is its age and sex structure, or the proportion of people at each age, by sex. The age-sex structure determines potential for future growth of specific age groups, as well as the total population. For these reasons, the age structure has significant government policy implications. A population of young people needs a sufficient number of schools and, later, enough jobs to accommodate them. Countries with a large proportion of older people must develop retirement systems and medical facilities to serve them. Therefore, as a population ages, needs change from childcare and schools to jobs, housing, and medical care. Read on at………….


  • China had an extreme youth bulge until the 1960s, when it sharply curbed partly as an effect of the one-child policy.
  • Compare this with the population pyramid of the USA which was bulging until the 1960s and has steadily slimmed since.
  • Take a look at gender balance in both………


demographic transition models
Demographic Transition Models
  • Stable pyramid - A population pyramid showing an unchanging pattern of fertility and mortality.
  • Expansive pyramid - A population pyramid showing a broad base, indicating a high proportion of children, a rapid rate of population growth, and a low proportion of older people. This wide base indicates a large number of children. A steady upwards narrowing shows that more people die at each higher age band. This type of pyramid indicates a population in which there is a high birth rate, a high death rate and a short life expectancy. This is the typical pattern for less economically developed countries, due to little access to and incentive to use birth control, negative environmental factors (for example, lack of clean water) and poor access to health care.
  • Stationary pyramid - A population pyramid typical of countries with low fertility and low mortality, very similar to a constrictive pyramid.
  • Constrictive pyramid - A population pyramid showing lower numbers or percentages of younger people. The country will have a greying population which means that people are generally older, as the country has long life expectancy, a low death rate, but also a low birth rate. This pyramid has been occurring more frequently, especially when immigrants are factored out, and is often a typical pattern for a very developed country, a high over-all education and easy access and incentive to use birth control, good health care and a low number to no negative environmental factors.



The demographic transition (DT) is a model used to represent the transition from high birth and death rates to low birth and death rates as a country develops from a pre-industrial to an industrialized economic system. The theory is based on an interpretation of demographic history developed in 1929 by the American demographer Warren Thompson (1887–1973).[1] Thompson observed changes, or transitions, in birth and death rates in industrialized societies over the previous 200 years. Most developed countries are in stage 3 or 4 of the model; the majority of developing countries have reached stage 2 or stage 3. The major (relative) exceptions are some poor countries, mainly in sub-Saharan Africa and some Middle Eastern countries, which are poor or affected by government policy or civil strife, notably Pakistan, Palestinian Territories, Yemen and Afghanistan.[2]

Read More: http://en.wikipedia.org/wiki/Demographic_transition

questions on demographic transition model
Questions on Demographic Transition Model:
  • Look back at the ‘Birth rate/death rate’ graphs for Mexico, Sweden and Sri Lanka. Identify which stage in the Demographic Transition Model is each country in the following years:
  • Look at this great animation of Canada’s Age sex population graphs from 1901 to the present day. Try to identify about when Canada goes through each of the stages in the DTM. http://www12.statcan.ca/english/census01/products/analytic/companion/age/cda01pymd.cfm
predicting populations
Predicting Populations
  • Look at this great website- explore!


  • Find out :

Who are Malthus (1766-1834) and Boserup (1965)?

Compare their predictions for what will happen to populations and why. Which do you think is most likely?


Looking Ahead

  • Exponential growth cannot continue indefinitely. If the current world value for r (1.2%) remains unchanged, the world population would grow from its current 6.9 billion to 9.5 billion over the next 40 years (2050).
  • Could the earth's resources sustain such a population?
  • If not, how large a human population can live decently on this planet?
  • Some demographers (students of population) say we have already exceeded the number. Others say the earth can hold billions more.
  • Whatever the case, there are grounds for some optimism about future population growth.
  • The world value for r peaked around 1990 and has declined since. This is a reflection of the decline in total fertility rates (TFRs) in undeveloped countries, presumably as the various factors involved in the demographic transition take hold, e.g.,
    • improved standard of living
    • increased confidence that your children will survive to maturity
    • improved status of women
    • increased use of birth control measures

Total Fertility Rate (TFR)

The total fertility rate is the average number of children that each woman will have during her lifetime. The TFR is an average because, of course, some women will have more, some fewer, and some no children at all.

Theoretically, when the TFR = 2, each pair of parents just replaces itself.

Actually it takes a TFR of 2.1 or 2.2 to replace each generation — this number is called the replacement rate — because some children will die before they grow up to have their own two children. In countries with low life expectancies, the replacement rate is even higher (2.2–3).



The projection of future TFRs in the graph on the previous slide (from the Population Reference Bureau) predicts that the less developed countries of the world will reach replacement fertility around the year 2020. In fact, they will probably reach it sooner because by 2010 the world TFR has dropped to 2.5. Even so, will the world reach zero population growth (ZPG) then?

  • This graph (based on data from the UN Long-Range World Population Projections, 1991) gives 5 estimates of the growth of the world population from now until 2150, assuming that TFRs decline from the 1991 value of 3.4 to the values shown.
  • A value of 2.06 will produce a stable population of about 11.5 billion.
  • A value 5% below that (1.96) will cause the population to drop back to close to its present value (6.1 billion) while
  • a value of only 5% above (2.17) would produce a population of over 20 billion and still rising.


to conclude
To conclude:

The several agencies that try to predict future population seem to be moving closer to a consensus that:

  • the world population will continue to grow until after the middle of this century
  • reaching a peak of some 9.5 billion (up from today's 6.9 billion) and then
  • perhaps declining in the waning years of this century.