A population is a group of individuals of the same species, residing in a particular geographical area at a particular time and functioning as a unit. For example, all tigers living in a biosphere reserve at a particular time constitute the population of tigers.

The three important characteristics of a population are:

(i) Birth rate or Natality Rate: It is expressed as the number of births per thousand individuals of a population per year.

If in a pond there are 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year.

(ii) Death rate or Mortality rate: It is expressed as the number of deaths per thousand individuals of a populations during a given period.

If 4 individuals in a laboratory population of 40 fruit flies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruit fly per week.

(iv) Sex ratio: It is expressed as the number of females per 1000 males of a population in given time.