An individual never lives in isolation. It prefers to stay in association with other species or individuals of its own kind. Such a group of individuals belonging to the same species living in a given geographical area that share or compete for similar resources and interbreed only among themselves is known as population.
Populations of various organisms tend to cope with the changing environment for their survival. Evolutionary changes through natural selection take place at the population level and so population ecology is an important field of ecology. Interestingly, a population has certain characteristics or attributes such as birth and death rates, sex ratio, age distribution and population density, which are statistically measured. However, these attributes cannot be calculated for individual organisms.
Birth and death rates are expressed in terms of per capita, that is, by dividing the total number of births or deaths by the total population that exists in a given time period. Apart from birth and death rates, another important attribute of a population is sex ratio. Moreover, at any point in time, this population consists of individuals who may belong to pre-reproductive, reproductive and post-reproductive age groups. A graphical representation of the various age groups is known as the age pyramid. In the human population, the age pyramid shows the age distribution of males and females in the same diagram. In fact, the shape of the age pyramid reflects the growth status of the population and indicates whether the population is growing, stable or declining.
The size of the population is usually taken into consideration for studies on ecological processes in the population such as the effect of pesticides or pollution, impact of predators or competitors with other species. Although the total number of individuals of a species provides the most appropriate measure of its population density, in some cases, measuring population density in terms of numbers is either meaningless or difficult to determine. Thus, population attributes provide important information about the organisms in a particular habitat and they provide the basis for further ecological studies and research.