Generally any individual of any species does not live in isolation but in a group in a well-defined geographical area. These individuals live in a group share or compete for similar resources, interbreed and thus constitute a population. The individuals of a species form a community by living in a group. A population has certain attributes that an individual does not have. For example:
(1) An individual may have birth and death but a population has berth rates and death rates. These rates in the population are called per capita birth rate or death rate respectively. Hence the rate is expressed as change in numbers (increase or decrease with respect to the members of the population).
(2) Another attribute characteristic of a population is sex ratio, that is, the ratio of males and females. An individual is either a male or female but the population has sex ratio. A population at any given time is made up of individuals of different ages. If the age distribution for the population is plotted, the resulting structure is formed called an age pyramid.
(3) The size of the population states about its status (position) in the habitat. Sometimes, despite the small size of the population, the number of individuals is high. For example in Bhaaratpur wetland areas the mumber of Siberian cranes may be in lakhs in a year. Their numbers are expressed as population density.