Discover Population Balance Through Hands-on Experiments

Discover Population Balance Through Hands-on Experiments

At this point you should have used the population model enough to feel comfortable with it. In this section we will introduce and do experiments with the gray squirrels.
The population model assumes that squirrels only eat acorns and depend on them for survival. Similarly, it assumes that the hawks depend on a minimal amount of squirrels for their own survival. Of course, nature is more complicated than this with complex food webs and lots of interdependencies between the different populations. In the model there are three reasons for the death of squirrels: lack of food, old age, and being eaten by hawks. Hawks only have two causes of death: not enough squirrels to eat and old age.
From the experiments in the previous page we learned what the optimal conditions for the oak trees, and therefore for acorn production are. In all future experiments you should use those conditions to make sure that there are plenty of acorns which form the base of the food chain pyramid.
-- Long term size of the squirrel population
We now have a forest with plenty of trees and we want to introduce gray squirrels. The question that we pose is, does it make a difference how many squirrels we introduce? What is the long-term difference in the size of the squirrel population if we introduce only two squirrels (a male and a female) as compared to introducing say, 100, squirrels? Another question is, does it make a difference if the squirrels are introduced early vs. late?
What do you think will happen? Should there be a difference? Think about it for a while and come up with your own hypothesis.
Now setup an experiment where you only vary the number of squirrels that you introduce to the area, run it, and observe the results. Follow that up with an experiment where you vary the time (or year) when you introduce to the area the exact same number of squirrels. Introduce them very early on (of course when there is food available), and then introduced them much later on.

COMMENT: If you look at the chart you can see how both the number of acorns and the number of squirrels grow and stabilize in very similar ways. Does this make sense? Yes it does. You can see very clearly how the supply of food (the acorns) is directly tied to the number of squirrels (the consumer). If the number of acorns would go up and down over the years, so would the number of squirrels. One final comment. Even though in all the different experiments the final number of squirrels stabilized at the same level, what was different among the experiments? Think about that.
-- Number of acorns needed per squirrel

How many acorns per year does a single squirrel need to survive? In addition, how many squirrels can a single oak tree support?
Come up with your hypotheses for both questions.
Set up an experiment to prove your hypotheses. You can probably also come up with answers based on the results of the previous experiments.

Are you surprised by the answer? Think about it. It takes about 25 years before an oak tree starts producing acorns. During all that time there has to be plenty of nutrients available and many gallons of water. You would have to wait much longer before the tree becomes full grown and yearly production of acorns grows to a level where it can sustain a couple of little squirrels. It is so easy to break this delicate balance and therefore impact the squirrel population!
-- Squirrels life expectancy
We would now like to figure out how many years a squirrel normally lives.
This is more a guess than anything else, but come up with a guess of the number of years.
Define and run an experiment to figure this out.

COMMENT: To obtain a more scientific measure of squirrels' life expectancy, you would need to study large populations of squirrels in the wild for many years. Some squirrels die younger and some older. After observing many squirrels go through their life cycle, you would analyze the data and obtain some type of average. The number used in the model is based on such studies and analysis. You can now see a nice advantage of a model such as this. In a mater of minutes you can figure out the answer.
-- Squirrels Birth Rate
Let's now try to figure out the squirrels' birth rate in the model, which means the number of baby squirrels that each female squirrel will give birth to every year.
You can look at the graphs or tables in the previous experiments to come up with a hypothesis.
As in all previous cases, come up with an experiment that will allow you to figure out the birth rate.

COMMENT: The process used to figure out the birth rate is a simplified one. A more scientific one with a bit more involved mathematics can be checked in the Facts about Population Growth Mathematical Models page. Make sure to check it because it is a more formal process with an interesting and general result.
NOTE: In reality the birth rate of squirrels is not always what the model uses. Make sure to check the Facts about Squirrels page to find out the more common birth rate.

Get Acquainted with Model

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The Impact of Predators
Last Updated: Monday, 13-Nov-2000 05:22:04 GMT

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	We now have a forest with plenty of trees and we want to introduce gray squirrels. The question that we pose is, does it make a difference how many squirrels we introduce? What is the long-term difference in the size of the squirrel population if we introduce only two squirrels (a male and a female) as compared to introducing say, 100, squirrels? Another question is, does it make a difference if the squirrels are introduced early vs. late?
	What do you think will happen? Should there be a difference? Think about it for a while and come up with your own hypothesis.
	Now setup an experiment where you only vary the number of squirrels that you introduce to the area, run it, and observe the results. Follow that up with an experiment where you vary the time (or year) when you introduce to the area the exact same number of squirrels. Introduce them very early on (of course when there is food available), and then introduced them much later on.

	How many acorns per year does a single squirrel need to survive? In addition, how many squirrels can a single oak tree support?
	Come up with your hypotheses for both questions.
	Set up an experiment to prove your hypotheses. You can probably also come up with answers based on the results of the previous experiments.
	Are you surprised by the answer? Think about it. It takes about 25 years before an oak tree starts producing acorns. During all that time there has to be plenty of nutrients available and many gallons of water. You would have to wait much longer before the tree becomes full grown and yearly production of acorns grows to a level where it can sustain a couple of little squirrels. It is so easy to break this delicate balance and therefore impact the squirrel population!

	We would now like to figure out how many years a squirrel normally lives.
	This is more a guess than anything else, but come up with a guess of the number of years.
	Define and run an experiment to figure this out.

	Let's now try to figure out the squirrels' birth rate in the model, which means the number of baby squirrels that each female squirrel will give birth to every year.
	You can look at the graphs or tables in the previous experiments to come up with a hypothesis.
	As in all previous cases, come up with an experiment that will allow you to figure out the birth rate.