Calculating population growth rates

            How can you tell if a population is growing?  You have to consider four factors: how many organisms are born (natality), how many die (mortality), how many come into the population from somewhere else (immigration) and how many leave the population for somewhere else (emigration)

Natality and immigration are always positive for the population since they increase the population.  Mortality and emigration are always negative for the poulation since they decrease it.  You can calculate the rate a population is growing or shrinking by using the following formula:

        Growth rate= Natality - Mortality + Immigration – Emigration

Since natality, mortality, immigration and emigration are usually give with a time attached to them, the formula will give you the growth rate per unit of time.  For example, in a population of lemmings, 300 are born each year, 175 die, 100 come to the population from across the valley and 250 leave for greener pastures.  What is the growth rate of the population?

            Growth rate = 300 – 175 + 100 – 250 = - 25 lemmings/year.

            Because the growth rate is negative, this means the population is shrinking.  You can figure out what the populations will be in the future if you know what is at some point in time and the growth rate.  For example, to calculate the size of the lemming population above 8 years from now, you need to know the population is currently 325 lemmings and the following formula:

          Future population size = current population + (# time units  •  growth rate)

So our lemming population 8 years from now = 325 + (8  • -25) = 325 – 200 = 125 lemmings.

You can calculate what the population was some time in the past using:

          Past population size = current population - (# time units  •  growth rate)

Our lemming population 10 years ago = 325 – (10  • -25) =325 –(-250)= 325 + 250 =575 lemmings