If ‘Nt’ is the population density at time t, ‘B’ is natality, ‘D’ is mortality, ‘I’ is immigration, ‘E’ is emigration then then the density of the population at time t +1 is

Natality and immigration contribute to an increase in the population size while mortality and emigration contribute to a decrease in the population size.If ‘Nt’ is the population density at time t, ‘B’ is natality, ‘D’ is mortality, ‘I’ is immigration, ‘E’ is emigration then then the density of the population at time t +1 is  Nt + [(B + I) – (D + E)].