Human populations

Human population has been kept in check by factors such as food availability, disease, predators and climate. However two major events have led to an explosion in the human population:

 > The development of agriculture

 > The development of manufacturing & trade that created the industrial revolution

War, disease & famine have caused temporary reversals in the upward trend.

Demographic Transition = the change from a society with short life expectancy & high birth rates to a society with long life expectancy and low birth rates.

In more economically developed countries, there tends to be a long life expectancy and low birth rates. This leads to levelling-off of the population and the sigmoid population growth curve is re-established.

Three common types of population:

 > Stable population - the birth rate & death rate are in balance, so no increase or decrease in population size

 > Increasing population - High birth rate (represented by wide base to pyramid) and fewer older people giving a narrower apex to the pyramid. Typical of LEDCs.

 > Decreasing population - lower birth rate (narrow base) and lower mortality rate (more elderly people). More common in MEDCs.

Survival curves - plots percentage of a population still alive at different ages (or following a particular event). 

 Average life expectancy = the age at which 50% of the individuals in a particular population are still alive. Can be calculated from a survival curve.