Population density
The number of organisms of a species living in a unit area of a chosen habitat is called the population density.
For example, the population density of Sri Lanka in 2014 was 329.12 km−2 — that many people for every square kilometre.
The size of a natural population changes continually. Four main factors affect population density:
Add to the population
Births — the number of new organisms born within the population
Immigration — the number of organisms joining the population from outside
Take from the population
Deaths — the number of organisms of the population that die
Emigration — the number of organisms leaving the population
Figure to be added
Why density, not just number
A thousand elephants in a vast national park and a thousand elephants in a small patch of forest are very different situations: in the second, food runs short and the animals raid nearby farms. Dividing the number by the area gives a fair comparison. Sri Lanka's 329 people per square kilometre makes it one of the more crowded countries in Asia. The four factors are a simple balance sheet: the population grows when births and immigration together exceed deaths and emigration, and shrinks when they fall short.
Births and arrivals in, deaths and departures out
Population density is the number per unit area; it is set by births, deaths, immigration and emigration.