An important characteristic of any population is its growth rate. The Population Growth Rate (PGR) tells us how fast a given population is growing. There are important characteristics we must know about a population, or at least be able to estimate.
Natality: the birth rate of a population; or the number of individuals born in a given time period.
Mortality: the death rate of a population; or the number of deaths that occur during a given time period.
Population Movement
The number of individual organisms moving into or out of a population is another important characteristic.
Emigration: individuals moving away from a population
Immigration: individuals moving into a population
In most instances, emigration and immigration are about equal to each other.
Some populations tend to remain approximately the same size from year to year. Other populations vary in size depending on conditions within their habitats. To better understand why populations grow in different ways we can look at 2 mathematical models for population growth: exponential growth & logistic growth.