(a) A population is a group of individuals of the same species living in the same area or interbreeding and sharing genetic information.
(b) Population density is the number of individuals of a species per unit area/ space at a given time. Various processes affecting population density are:
(i) Natality: It refers to the number of births during a given period in the population that are added to the initial density.
(ii) Mortality: It is the number of deaths in the population during a given period.
(iii) Immigration: It is the number of individuals of the same species that have come into the habitat from elsewhere during the time period.
(iv) Emigration: It is the number of individuals of the population who left the habitat and gone elsewhere during the time period.
If N is the population density at time t, then its density at time t + 1 is
Nt+1 = Nt + [(B + I) – (D + E)]
where, B = natality, D = mortality, E = emigration and I = immigration.