The correct answer is Both 1 and 2. Key Points The size of a population for any species is not a static parameter. It keeps changing with time, depending on various factors including food availability, predation pressure, and adverse weather. In fact, it is these changes in population density that give us some idea of what is happening to the population – whether it is flourishing or declining. Whatever might be the ultimate reasons, 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. Natality refers to the number of births during a given period in the population that are added to the initial density. Hence, Statement 1 is correct. Mortality is the number of deaths in the population during a given period. Hence, Statement 2 is correct. Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.