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Population Migration

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.

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’re interested in looking at, is the population growth.

Population growth is the change in the amount of individuals of that species over time. You can do a lot of calculations. But basically the way it works out is essentially, how many births are there, minus the number of death. Plus how many people move in, i.e. immigrate, that’s moving into an area, or moving into a population, minus the numbers who are emigrating, that is 'e' as in exiting. That total gives you the rate of growth.

There is a couple of trends that you often see. There is one kind of growth called exponential growth. This is what is seen in basically every 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 the graph of human population growth and we can see that right now, we’re in exponential growth.

You’ll see a couple different areas. This area here is called the lag phase. 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 sizeable enough amount, you can start entering what’s called the exponential growth phase. That’s where your population starts to rocket up. That’s 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’s called logistic growth. That’s when the population starts to rise and you would 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. You start entering what’s 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 in German or whatever, for carrying capacity. Well other organisms typically do best when they’re in the rapid exponential growth phase and they’re often called R selection organisms. R for reproduction.

What are some of the factors that can influence population growth? There are those that are known as density independent, where the population density doesn’t impact or affect these factors. 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 is 20 squirrels an area or 200 squirrels an area, if a storm has lighting bolts sitting there and zapping squirrels, it doesn’t matter how many there were they’re 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 sneeze 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 in a forest, that they’re all eating all of the acorns, they start winding up competing with each other 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, look at all the squirrels up in the trees, and they’re going to start going after than. Whereas if there’s only one or two little squirrels leaping around, the predators will never notice them. There you go.