Four processes of population growth

Population size is not fixed. It changes with food availability, predation pressure, bad weather and other factors, and these changes show whether a population is flourishing or declining.

Density in a habitat changes through four basic processes. Natality and immigration add to it; mortality and emigration take away from it.

Natality: the number of births in the population during a given period, added to the starting density.

Mortality: the number of deaths in the population during that period.

Immigration: the number of individuals of the same species that move into the habitat from elsewhere during the period.

Emigration: the number of individuals that leave the habitat for somewhere else during the period.

If N is the density at time t, the density one step later is Nt+1 = Nt + [(B + I) − (D + E)].

Density rises when births plus immigrants (B + I) exceed deaths plus emigrants (D + E).

Normally births and deaths matter most. Immigration becomes important in special cases, such as when a new habitat is first being colonised.