Identify the factor responsible for increasing the population density.

The correct answer is Option (3) → Natality

The density of a population changes through four basic processes. Natality and immigration add individuals, so they increase the density. Mortality and emigration remove individuals, so they decrease it. Natality is the number of births during a given period that are added to the initial density.

(1) Emigration — Incorrect. This is the number of individuals of the population who leave the habitat. It lowers the density.

(2) Mortality — Incorrect. This is the number of deaths in the population. It lowers the density.

(3) Natality — Correct. Births add new individuals, so the density goes up.

(4) Infertility — Incorrect. It is not one of the four basic processes. It only reduces the number of births, so it cannot raise the density.

NCERT Extract:
Population Growth – “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. (i) Natality refers to the number of births during a given period in the population that are added to the initial density.”