N is the population density at time t, then its density at time t +1 is Nt+1 = Nt + [(Natality + Immigration) – (Mortality + Emigration )the  population density will increase if the number of births plus the number of immigrants [(Natality + Immigration)  is more than the number of deaths plus the number of emigrants (Mortality + Emigration )otherwise it will decrease. Under normal conditions, births and deaths are the most important factors influencing population density, the other two factors assuming importance only under special conditions. For instance, if a new habitat is just being colonised, immigration may contribute more significantly to population growth than birth rates.