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Population Growth

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Population growth is loosely defined as the change in the amount of individuals of a specials in an area over time. To find the growth rate of a population, we take the number of individuals moving into an area and subtract the number of individuals moving out of an area by taking the birth rate, adding the immigration rate and subtracting the death rate and emigration rate. Two types of population growth are exponential and logistic growth.

When scientists are studying a population that is a group of individuals of a particular species that are in a specific area, one of the things that they are interested in looking at is the population growth. Population growth is the change in the amount of individuals of that species over time. Now you can do a lot of calculations but basically the way it works out is essentially how many births are there happening minus the number of deaths plus how many people move in i.e. Immigrate with an "I" that's moving into an area or moving into a population minus the numbers who are emigrant there with an "e" as an exiting. So that total gives you the rate of growth.
Now there's a couple of trends that you often see, there is on kind of growth called exponential growth and this is what seen when basically we have a population that has virtually unlimited access to resources. Now obviously this is not a long term thing but you will often see this happening right now, this for example is a graph of human population growth and we can see that right now we are in exponential growth. You will see a couple of different areas, this area here is called the Lag phase and this is when you have a small population you simply can't grow very fast because you don't have a lot of people to have babies but then when you get a sizable enough amount you can start enduring what is called the Exponential growth phase and that is where your population starts to rock it up and that is because you have so many individuals it's easy for them to find mates and have lots of children.
Now another kind of growth that is more likely is what is called Logistic growth and that's when the population starts to rise and you will go through the lag phase and exponential growth phase but ultimately various factors in the environment start limiting the number of organisms that can actually be sustained by the environment and you start entering what is called the Deceleration phase and ultimately you'll reach some stable equilibrium. Some number of maximum individuals that the environment can sustain for long periods of time, we call that the carrying capacity and there is some organisms that are typically designed to do well when they're at carrying capacity and those are often called K selection organisms K being the abbreviation I guess drumlin or whatever for carrying capacity, well other organisms typically do best when they are in the rapid exponential growth phase and they are often called R selection organisms R for reproduction.
Now what are some of the factors that can influence population growth? Well there are those that are known as Density Independent where the population density doesn't impact or effect these factors, so these are often abiotic factors for example if there is extreme weather, a storm it doesn't matter how many individuals of a particular species if there's 20 squirrels in the area or 200 squirrels in the area, if a storm has lightening volt sitting there and zapping squirrels it doesn't matter how many they are all are, they are all going to get impacted by this. Then there is Density Dependent factors where this is impacted by the number of organisms of that population that are in a particular area. These are very often biotic i.e. for example a disease if you don't have a lot of individuals in a particular area it's very hard for a disease to really get hold in that population whereas if you have a very dense population where everybody is kind of crowded together one person gets sick they sniss and cough on somebody else everybody is getting sick. So that would be an example of a density dependent factor.
You can also wind up having increased competition for food if you just have so many squirrels and they are forced that they are all eating all the acorns they start winding up competing with each for those acorns and they start reducing the number of offspring that can survive in the next generation. Also if you have more and more squirrels ultimately the predators are going to start noticing "hey look at all the squirrels up in the trees." And they are going to start going after them whereas if there's only one or two little ninja squirrels zipping around the predators will never notice them, so there you go.