In science, a population refers to a group of organisms of the same species living together in the same place and time. The concept of population is not exclusive to the field of biology, as it is also used in statistical science. Scientists study populations by examining how individuals in that population interact with each other and how the population as a whole interacts with its environment. Population biology is the study of population characteristics and the factors that affect their size and distribution. The characteristics which are studied include the migratory patterns, the population density, the demographics (the birth and death rates, sex ratio and age distribution), the population genetics, the life history traits, and the group dynamics (the interactions within and between populations). Population ecology is the study of how these populations interact with the environment. Most populations are not stable, fluctuating in size over time in response to changes in the abiotic and biotic factors, which act as limiting factors against indefinite exponential growth of populations. Scientists use demographic parameters, such as size, density, and distribution, to describe a population.