There are five main characteristics of a population and these include population size, population density, population distribution, age structure and reproductive base. These characteristics of a population can be found in population ecology, which is a combination of the study of how organisms interact with each other and their environment focusing on the group of individuals of the same species, most often humans in population ecology, who live in a given area.
The population size is the number of individuals in the gene pool. The population density is the number of individuals living in a given area. The population distribution is the pattern of distribution. The pattern of distribution can either be clumped, random or uniformed. The age structure is prereproductive, reproductive and post-reproductive. The reproductive base is filled with the individuals that are in the reproductive stage and the prereproductive stage.
Births and immigration increase the population size while deaths and emigration decrease the population size. There are some populations that have zero population growth, or ZPG, which means that they do not have a net increase or a decrease in their population size. In time, as long as the birth rate for a population is slightly above the death rate, then the population will grow exponentially with a J-curve.