Biology Unit 4: Human Populations (AQA)

The growth rate of a population is shown by the equation below:

growth rate = (birth rate - death rate) + (immigration rate - emigration rate)

This can also be shown as a percentage change using this equation:

annual percent change = ((Population at end of year - population at start of year)/ population at start of year) x 100

If the population decreases the population change will be negative.

Stage one (Pre industrial): Birth rate and death rates are high. Infant mortality is high and life expectancies are short due to poor sanitation and medical care. Parents choose to have many children because they are useful and cheap.

Stage two (Developing): Improving health care and farming etc, cause the death rate to decrease and life expectancy to increase. The population increases because the birth rate remains high.

Stage three (Industrial): The death rate is low and birth rate decreases as parents decide to have fewer children. The population continues to increase, but at a slow rate.

Stage four (Post industrial): Birth rate and death rate are low, so the population is stable. In some countries, birth rate may fall beneath the death rate, this is sometimes described as stage five.

When interpreting population pyramids, it may help to seperate the chart into three sections, pre - reproductive (0 - 14) reproductive (15 - 44) and post - reproductive (over 45).

A pyramid with a wide base indicates a population with a fast growth rate.

The steeper the sides of the pyramid, the longer the life expectancy.

Survivorship curves are graphs that plot the age of a group of people who were born at around the same time against the number of people in that group who survived to that age.

Using a survivorship curve, you can calculate the life expectancy by reading the age at which 50% of the cohort have survived.