The size of a population for any species is not a static parameter. It keeps charging in time, depending on various factors including food availability, predation pressure, and adverse weather. The density of population in a given area flourishes or declines due to the changes in these four processes – Natality: number of births during a given period of time in a population that are added to initial density. Mortality: the number of deaths in a population during a given period of time. Immigration: the number of individuals of the same species has come into the habitat from elsewhere during the time period under consideration. Emigration: the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration. Natality and Immigration contribute to the increase in population and density and Mortality and Emigration to a decrease in population.