Population is a group of individuals of a species in a well-defined geographical area that share or compete for similar resources and potentially interbreed. Populations possess certain attributes that individuals do not. Three important characteristics are sex ratio, age distribution, and population growth.

Sex ratio is an attribute where a population has a proportion of males and females, e.g., 60% females and 40% males. An individual is either male or female, but only a population can have a sex ratio.

Age distribution refers to the composition of individuals of different ages in a population. When the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. The shape of the pyramids reflects the growth status of the population – whether it is growing, stable, or declining.

Population growth indicates that the size of a population is not a static parameter. It keeps changing with time, depending on various factors including food availability, predation pressure and adverse weather. Changes in population density give us an idea of whether the population is flourishing or declining. These fluctuations are due to changes in four basic processes.