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How do immigration and emigration affect the size of an open population?

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Common wildebeest — population ecology A herd of common wildebeest (*Connochaetes taurinus*) migrating across a dusty savanna in Africa. The animal is a keystone species (i.e., a species with a disproportionately large effect on its biological community) in plains and acacia savanna ecosystems from southeastern Africa to central Kenya.

In population ecology, an open population is one that is not geographically isolated and experiences exchanges with other populations of the same species. Unlike a closed population, which changes only through births and deaths, the size of an open population is directly influenced by migration.12

Immigration and emigration affect population size in the following ways:

Immigration: The movement of individuals into a particular location, which acts as a source of population gain.3
Emigration: The departure of individuals from a location, which acts as a source of population loss.34

To determine the size of an open population at the end of a specific time interval, ecologists use the following relationship: the initial population size plus births and immigrants, minus deaths and emigrants. Consequently, population growth rates are calculated by subtracting the combined rates of mortality and emigration from the combined rates of fertility and immigration. These movements are essential for understanding how species distribute themselves across environments and how they exploit fluctuating resources.2356