Play our final KS3 Geography quiz on population. Changes in populations are due to many factors - migration, improving or worsening standards of living, wars, natural disasters and so on. An example of the latter was the destruction of the town of Pompeii by the eruption of Mount Vesuvius in AD79 - the town was completely depopulated.
In more recent times, in places like Africa and the Balkan states, certain groups of people have tried to completely exterminate other populations by killing them. This is called genocide and is usually caused by racial or religious prejudices.
Populations fluctuate (change up and down) naturally. In times of famine, populations will decline as the most vulnerable people starve to death.
The immune systems of babies are not as well developed as older children and adults so if there are diseases around, infant mortality will be high - fewer babies will survive and if the rates of infant survival is less than the death rate amongst the rest of the population overall, the population will fall. In LEDCs, women tend to have more children than women in MEDCs, and they start having children at much younger ages too. The birth rate is often higher than the death rate and so the population shows a natural increase