How Birth, Immigration, Emigration & Death Affect Populations

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  • 0:02 Populations are Dynamic
  • 1:12 Population Factors
  • 2:29 Calculating Population Changes
  • 5:00 Resources Are Limiting Factors
  • 7:23 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Populations change based on the births, deaths, immigrations, and emigrations that occur within the group. These additions and subtractions all affect population size, but do so in different ways.

Populations are Dynamic

The term 'population' can be somewhat confusing because it completely depends on the context you put it in. For example, the human population on Earth is quite large - over 7 billion people. But the U.S. population is much smaller, around 300 million people. You can see why it's very important to specify which group of people (which population) you're talking about.

Populations of animals, plants, and other organisms are no different. In ecology, a population is a group of individuals of the same species that live in a given area. As we saw, both a country and the entire planet can constitute a 'given area', so the distinction really does matter.

Because populations are made of individual organisms they are constantly changing. Some populations may change rapidly, while others stay somewhat constant over time. Population size can be affected in numerous ways: new individuals are born, old individuals die, some individuals leave, while others find their way into the group from the outside. These additions and subtractions all influence populations because they change the total number of individuals.

Population Factors

There are two main ways that individuals are added to the population. The first is through births of new individuals. The way we measure additions of this type is with the natality rate (also called the birth rate), which is the number of births per 1000 individuals per unit of time, usually per year.

The second way individuals are added to the population is through immigration. This is the permanent arrival of new individuals into the population. These individuals are of the same species as the rest of the population, and they increase the size of the population as they join the group.

On the flip side, we have two main ways that individuals leave a population and reduce its size. First is through deaths of individuals. We measure this with the mortality rate (also called the death rate), which is the number of deaths per 1000 individuals per unit of time. Again, this time period is usually a year.

Second, individuals may leave through emigration. This is the permanent movement of individuals out of a population. These may be juveniles who are heading out on their own, or it may be adults leaving the group for some other reason such as overcrowding or searching for new areas of food and shelter.

Calculating Population Changes

Immigration and emigration are fairly easy to calculate - it's simply the number of individuals permanently moving into or out of a population. For the natality and mortality rates, it's a bit different.

To get either the natality or mortality rate, we take the number of births or deaths over the period of time we're interested in, divide that number by the estimated population size at the beginning of the time period, and then multiply that number by 1000. So for example, if we had 50 births over a year (the time period we're interested in), and the original population size at the beginning of that year was 2000 individuals, we would divide 50 by 2000, then multiply by 1000. When we do this we get 25 births per 1000 individuals per year, the natality rate for the population. If instead of births we had 50 deaths for the same population, we would have a mortality rate of 25 deaths per 1000 individuals per year.

When you put all of these factors together you can paint a picture of how the population is either growing or declining. Basically, your population growth will be the births - deaths + immigration - emigration. For example, if you have a population of bears living on an island, we can assume there is no immigration or emigration because it is unlikely that new bears will be swimming out to the island to join the population or that bears currently in the population will be swimming back to the mainland. So if the population consists of 1500 bears initially, and there are 17 births and 8 deaths in one year, we can calculate how this population is changing.

Using our rate formulas, the natality rate for this bear population is 17 births/1500 total individuals * 1000 = 11.33 births per 1000 individuals per year. Our death rate is calculated the same way: 8 deaths/1500 total individuals * 1000 = 5.33 deaths per 1000 individuals per year. In this case, the bear population will be growing quite quickly because the birth rate is about twice that of the death rate! If it were the other way around, the population would be in decline because more individuals are dying than are being born to replace them in the population.

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