A population is a group of individual organisms of the same kind that are limited to some particular space. The most familiar example is the human population, but there are also populations of animals and plants everywhere on Earth. In fact, scientists regard a population as a biological unit that has both structure and function. The parts of a population are its individual members. The functions of a population are similar to those of other biological units: growth, development, and self-maintenance in a changing environment. Individuals enter a population by birth and by moving in, that is, by immigration. Individuals leave a population by death and by moving out, that is, by emigration. If the environment of a population remains the same, loss and replacement of members are in balance. The population will be able to survive in that particular environment. If the environment changes, however, loss or addition of members increases or decreases the size of the population.