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How do births and deaths change the size of a closed 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, a closed population is defined as a group of individuals that is geographically isolated, meaning no immigration or emigration occurs. Because there is no exchange of individuals with other populations, the size of a closed population changes exclusively through two fundamental processes: births (natality) and deaths (mortality).123

The change in population size over a specific interval can be expressed by a simple equation: the population at the end of the interval equals the population at the beginning, plus the number of births, minus the number of deaths.1

Ecologists often use life tables to evaluate how these birth and death rates influence the overall growth rate of a population. The difference between the birth rate and the death rate is referred to as the natural increase. When the birth rate exceeds the death rate, the population grows; conversely, if the death rate is higher, the population size decreases. If the rates are equal, the population remains stable.345