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How do immigration and emigration affect an animal population?

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Line graph showing world population growth from 1700 to 2000 with a steep rise after 1950, followed by three projection scenarios for 2000 to 2100: high, medium, and low, reaching between 7 and 14 billion. — population Graph of the world's estimated human population from 1700 until 2000, with population projections extending until 2100.

In the study of population biology, immigration and emigration are key components of demographic change that determine the size and composition of an animal population.1

When a population is not closed—meaning individuals can enter or leave—the population size at the end of a specific interval is calculated by adding births and in-migrants (immigration) and subtracting deaths and out-migrants (emigration). Consequently, immigration increases population size, while emigration decreases it. These movements can offset or exacerbate the effects of natural increase (births minus deaths) on population growth.12

Beyond mere numbers, these processes influence the population's composition, including its age, sex, and geographic distribution. Furthermore, the movement of individuals between populations facilitates gene flow, which introduces new genetic material and increases variability within the receiving population. While migration effects on total growth rates are often smaller than those of fertility and mortality, they remain essential for understanding how species exploit fluctuating resources and adapt to their environments.1324