Open & Closed Populations: Characteristics & Differences in Growth Patterns

Instructor: Megan Gilbert

Megan has a master's degree in nursing and is a board certified Women's Health Nurse Practitioner. Her area of clinical focus is the impact of infectious disease on pregnancy. She has experience teaching college allied health classes. She is also a certified EMT and holds a certificate of added qualification in electronic fetal monitoring.

A population of a species can be described as either an open population or a closed population. Using examples, this lesson looks at the differences between these two types of populations. It then goes further to look at differences in their growth patterns.

Population

You spot a few ants on a hot summer day making their way toward the perfect watermelon you brought home from the store. You track their path and find that they are living behind your refrigerator, living off your forgotten crumbs. You wonder where they came from? How big of a problem do you have? Are new members joining the community? Or is this an isolated population of ants destined to only interact with other ants from inside your kitchen?

A population is a group of members of the same species that all live in the same geographical area and are capable of interbreeding. In this lesson, we'll explore the difference between an open population and a closed population.

Closed Population

A closed population is a group this is not able to exchange members with different populations over time. The population can grow only through new individuals being born and will only lose members through death. This situation is commonly seen on islands as a population may be established during a storm or by a different influence but no additional members will be added over time.

When we look at short periods of time, a population is more likely to be closed. For example, an island is less likely to experience a storm event where more turtles are added during a single year than over 100 years. Or a river is more likely to stay at it's full height and prevent animals from crossing it during a normal year than over a longer period where occasional droughts may occur.

Think about out ant example in the intro. If the ants had hitched a ride on your clothing initially, but no new ants were able to enter the house, this would be a closed population. Because they used a fairly rare method of entering the home, and they had no way to exit your home, the members of the population would be fairly constant (with the exception of birth and death).

Growth Patterns

Closed populations are easier to project growth rates for as the population can only grow through birth and decrease through death. In a closed population, growth rate will be determined by the rate the organism can reproduce at and the starting number of organisms.

Additionally, the population will reduced by the death rate.

Population growth can be impacted by:

  1. space
  2. genetics
  3. age of individuals
  4. resources

Growth patterns in closed populations often follow a simple exponential curve at first, but the situation can be complicated by the starting population or the availability of resources where the population is located. For example, the group of ants will be able to reproduce much faster than a group of humans also living in the home.

Open Population

An open population is a population that is able to gain and lose outside members different populations over time. This means that the population is not geographically isolated. The longer the period of time considered, the more likely it becomes that the population will become open. This is due to the normal changes in an environmental system.

Over time, we expect that rivers will experience times of dryness, mountain passes will open and close, and bridges (man-made and natural) will be constructed and also be demolished. All of these things will impact the ability of new members to join an existing population.

If the ants in your kitchen had a way to have new ants come from the outside and join their community, and some members of the community were able to escape to the outside and join other communities of ants, it would be considered an open population.

Growth Patterns

Growth patterns in open populations follow the same general principles of closed populations, where the population growth is based on the rate of reproduction and the starting population. However, it will also be impacted by not only the death rate, but also the rate that individuals leave and join the population.

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