human populations

Human population size and growth rate
Most of our history has been kept in check by food availability, disease, predators and climate. Two recent events have lead to an explosion in human population:

- The development of agriculture
- The development of manufacturing and trade that created the industrial revolution.

Factors affecting the growth and size of human populations
The basic factors that affect the growth rate of human populations are birth rate and death rate. 

Individual populations are further affected by migration, which occurs when individuals move from one population to another. There are two types:
- Immigration, where individuals join a population from outside.
- Emigration, where individuals leave a population.

Population growth = (births + immigration) - (deaths + emigration)

Percentage population   = population change during the period    x100
growth rate                        population at the start of the period

Economic conditions - countries with a low per capita income tend to have higher birth rates. 

cultural and religious backgrounds - some countries encourage larger families and some religions are opposed to birth control.

Social pressures and conditions - in some countries a large family improves social standing.

Birth control - the extent to which contraception and abortion are used markedly influences the birth rate

Political factors - governments influence brith rates through education and taxaion policies. 

Birth rate =     number of births per year             x100

                 total population in the same year  

Age profile - the greater the proportion of elderly people in a population, the higher the death rate is likely to be.

Life expectancy at birth - the residents of economically developed countries live longer than those of economically less developed countries. 

Food supply - an adequate and balanced diet reduces death rate

safe drinking water and effective sanitation - reduce death rate by reducing the risk of contracting water-borne diseases such as cholera. 

Medical care - accss to healthcare and education reduces the death rate. 

Natural disasters - the more prone a region is to a drought, famine of disease the higher its death. 

War - deaths during wars produce an immediate drop in population and a longer term fall as a result of fewer fertile adults. 

Death rate =     number of deaths per year         x100
                      total population in the same year 

Age population pyramid

There are three typical types of population pyramids:

- Stable population - where the brith rate and the deah rate are in balance and so there is no increase of decrease in the population size.  

- Increasing populatio - where there is a high birth rate, giving a wider base to the population pyramid (compared to a stable population) and fewer older people, giving a narrower apex to the pyramid). This type of population is typical of economically less developed countries.  

- Decreasing population - where there is a lower birth rate (narrower base of the population pyramid) and a lwoer mortality rate leading to more elderly people (wider apex to pyramid). This type of population occurs in certain economically more developed countries, such as Japan. 

A survival curve plots the number of people alive as a function of time. Typically it plots te percentage of a population still alive at different ages but it can also be used to plot the percentage of a population still alice following a partiular event, such as as a medical operation or the onset of a disease.  

The average life expectancy is the age at which 50 per cent of the individuals in a particular population are still alive. It follows that life expectancy can be calculated from a survival.