The density of a population in a given habitat during a given period,
fluctuates due to changes in four basic processes, two of which
(natality and immigration) contribute to an increase in population
density and two (mortality and emigration) to a decrease.
(i) Natality refers to the number of births during a given period in the population that are added to the initial density.
(ii) Mortality is the number of deaths in the population during a given period.
(iii)
Immigration is the number of individuals of the same species that have
come into the habitat from elsewhere during the time period under
consideration.
(iv) Emigration is the number of individuals of the
population who left the habitat and gone elsewhere during the time
period under consideration.