Factors That Influence a Population Growth Curve

Instructor: Adrianne Baron

Adrianne has a master's degree in cancer biology and has taught high school and college biology.

There are multiple factors in the world that impact our population as well as the populations of every organism on the face of the earth. We will go over what a population is and the various factors that can affect a population.

Population Growth

Have you ever thought about how many students are in your school or seen the signs when you enter your city that show how many people live there? This information tells the size of the population, all members of the same species in a specific area. The area for a particular population could be your school, city, country or world.

Your school's population changes over time just as any other population. Changes in population size are referred to as population growth. The population in your school changes based on how many students enroll in and transfer to your school versus how many graduate and transfer out of your school.

Other populations change the same way. The population growth can increase or decrease based on individuals born in and immigrating, meaning move into, the area, versus individuals dying and emigrating, meaning move out, the area.

If more students come into your school than leave, then your school population increases just like a city's population increases if more people are coming in than going out. If the opposite occurs, then the population growth will decrease.

Factors that Affect a Population Growth Curve

Over your lifetime, you have probably gained or lost friends due to your family or your friends' families moving. We move to different areas for a lot for different reasons. You live in one city right now, but may choose to go to college or work in a different city.

The city you move to may begin to experience very fast growth if large amounts of people are moving there. The population growth will increase faster if people there are having multiple babies and thriving by living long lives. Their growth may be exponential and so their population growth curve, which is a line graph depicting the growth in a population, may begin to look like a J.

The various shapes of a population growth curve
Graph showing population growth curves

As growth continues, certain factors will cause changes in population growth causing a decrease in growth. Their population growth curve will begin to look more like an S, rather than a J as these factors begin to limit how many people can live there.

Density-dependent Factors

As the new city's population increases, certain factors will have more of an impact. Factors that impact the population more as the population increases are density-dependent factors. Everyone in your city has the exact same needs. These include a home, clean water, food, and jobs. The more people there are in your city, the more these resources will become limited.

Those of you who are there earliest will have what you need, but as more people come, you will have to start competing for the same resources. You moved there for your new job, so you will not have much of a problem getting a place to live, paying for clean water and buying food. Someone who moves there without a job or that gets a low paying job will not have the money needed to pay for these resources. As a result, they would be outcompeted for the resources of life which would force them to move away thus changing the population.

When every home is occupied and the amount of food and clean water available each day is being consumed, then the city has reached its carrying capacity, the total amount of a population that a given area can support. Carrying capacity is determined by how much of each resource is available versus how much each individual needs to survive. When the amount available becomes equal to the amount used, then the carrying capacity is reached. Going over the carrying capacity, will begin to increase the amount of individuals dying from lack of needed resources which will cause the population growth curve to level off.

Another density-dependent factor is disease and/or parasitism, where one organism harms another while living off of them and getting nutrients. Whether we are talking about people, animals or plants, the closer individual organisms live to each other, the easier diseases arise and spread. This is because disease-causing microorganisms do not have to travel very far between individuals. This allows microorganisms to be transmitted to more individuals in a smaller amount of time than it would if individuals lived further apart and rarely came in contact with each other.

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