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Determining Change in Population Size: Formula & Examples

Instructor: Lauren Posey

Lauren has taught intermediate reading in an English Language Institute, and she has her Master's degree in Linguistics.

Populations are always fluctuating. In this lesson, you will explore several examples to learn the formula needed to calculate the change in size of a population.

Aspects of Population

Think for a minute about your hometown. How many people do you know who were born there? How many people moved from another place? How many people have left? All of these are pieces of information that must be considered when determining the change in population size. The population of an area is made up of everyone living there, regardless of whether they were born there or not.

Formula for Population Change

When you want to calculate the change in a population, you use the following formula:

N = (B + I) - (D + E)

  • N is the change in population.
  • B is the number of births.
  • I is the number of immigrants, or people who have moved into the area.
  • D is the number of deaths.
  • E is the number of emigrants, or the number of people who have moved out of the area.

Essentially, you want to take the number of people lost to the population in some way and subtract it from the number of people the population has gained in some way. Change in population size must be calculated as a factor of time. That is, you will always be looking at how the population has changed in the past year, or five years, or ten years. Without a finite time period to look at, it is impossible to calculate the change in population.

The formula for population change is the same regardless of what population you are measuring
White tailed deer

Example One

Let's take a look at an example population. Say you want to know the change in population of your town over the past two years. Your town currently has 20,000 people, but you don't know how many people it had two years ago. However, town records show the number of births, deaths, immigrations, and emigrations. Those numbers are listed below:

1. There were 2,000 births

2. There were 700 new residents who immigrated.

3. There were 1,500 deaths.

4. There were 800 people who moved away.

Taking this information and plugging it into the formula gives you this:

N = (2,000 + 700) - (1,500 + 800)

Now that you have the information and the formula, all that's left is to solve the problem.

N = 2,700 - 2,300

N = 400

Over the last two years, the population of your town has increased by 400 people. That means that two years ago, the population was 19,600.

Example Two

Now let's look at a non-human population. The formula works exactly the same way, no matter what type of population you are looking at. In a forest, the current population of white-tailed deer is 600. Wildlife biologists have kept track of the births, deaths, and so on for the last five years. That information looks like this:

1. There were 300 new deer born.

2. 55 deer moved in from another nearby forest.

3. There were 800 deaths, both natural and from hunters or predators.

4. No deer moved away from the herd.

Putting all of this together gives us the following formula:

N = (300 + 55) - (800 + 0)

Just as with the human population, now that the information is all together, you just have to solve the equation.

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