Populations

Learning Target

• Construct an argument supported by evidence that changes to physical or biological components of an ecosystem affect populations.

Success Criteria

• Students learn the different ways to calculate the size of populations.
• 
  

Questions to Ponder

• What are some ways that scientists calculate the size of a population of organisms?
• How would you go about estimating the number of fish in a pond?

Counting Populations

There are Four Different Ways to Estimate Populations of Organisms

• Direct Observation
• Indirect Observations
• Mark and Recapture
• Samplings

The Ups and Downs of Population Growth

A population is a group of organisms of the same species that live in a certain area. Ecologists regularly monitor the number of organisms in many populations, but why do they do this? Why do we care if the number of organisms in an area is growing or shrinking? Well, populations that are growing and shrinking can be indicators of possible problems occurring in the organisms’ environment, and gives ecologists a “heads up” if something is going wrong. However, it is not enough to simply know if the number of organisms in an area is going up or going down; ecologists need to know why the number of organisms is changing. Therefore, one of the main questions ecologists ask themselves is this: Why is a population’s size going up or going down?

There are many factors that can cause a population’s size to change. But first, you must understand the basic reasons behind why a population grows or shrinks. Any population, whether it be humans, chipmunks, the mold growing on bread, or the bacteria living in your intestines, will grow if more organisms are being created, or born, than are dying. If a population has more organisms dying than are being born, then the population will shrink. The number of births in a population is called the birth rate. The number of organisms that are dying in a population is called the death rate. Thus, if the birth rate is greater than the death rate, a population will grow. If the death rate is greater than the birth rate, then the population will decrease in size.

Adapted from http://www.crazyteacherlady.com/uploads/5/1/4/8/5148626/objective_2_activities_pop_growth.pdf

Population Simulations

• A population of organisms can change over time. Complete the following activities to better understand these changes

1. Click and drag a rabbit into the rabbit warren to change the birth or immigration rate.
2. To change the death rate click and drag one of the foxes onto the rabbits above ground.
3. To change the emigration rate click and drag a rabbit from the warren into the Emigration Rate box.
4. Click on the Graph button to graph these rabbit population changes. You will be able to change the different rates once per year for ten years.
5. Click this link to answer a few questions about the simulation. Each student should answer the questions.

1. Add the following terms with your own definitions to your notes on Ecology
   • population
   • birth rate
   • death rate
   • immigration
   • emigration

Procedure:

1. Copy the data table below.
2. Get a cup half filled with white beans for your group. The cup represents a lake and the beans a bluegill population.
3. Without disturbing (touching) the bluegills in any way, estimate the number of bluegills in your lake and record your guess in your notebook (not in the data table.)
4. Use a spoon to collect some beans to represent the portion of the bluegills netted and tagged.
5. Using a marker or crayon to color your fish, count and tag each bluegill that was captured and then record the number on the data table under total fish tagged. This is the only time you will mark the fish (beans.)
6. Now, release your bluegills by pouring them back into the lake. Simulate their daily movements and interaction with the rest of the fish in the lake by shaking the cup (holding a hand over the cup so the beans don't go flying.)
7. Use a spoon to scoop a sample of bluegill from your lake. Count the total number in the spoon - write this number down under Total Fish in Trapnet. Now, count the number in the spoon that are marked. This will be Tagged Fish in Trapnet. Remember - you are not marking any more beans! Record the numbers on the data table.
8. The total population of bluegills in the lake can be estimated mathematically using the following formula:
   • population = Total fish tagged X Total fish in trapnet ÷ Tagged fish in trapnet
   • Substitute your data in the formula, and using a calculator, perform a population calculation and record it on your data table.
9. Return your bluegills to the lake, mix the population as before and then take a second sample. Record the new data on your data table and perform a second population calculation.
10. Find the average of the two numbers.
11. Compare your two population calculations with each other and with the initial estimate. Discuss possible reasons for any differences. Then, determine the accuracy of your estimate and calculations by returning your second sample to the lake and actually counting your fish (beans).
12. Be prepared to answer the following questions:
    • Which was closest to the actual population, your original estimate or your calculations? Why?
    • What conditions (not present in this activity) in an actual lake might affect the bluegill population?
13. Add the Following Term to your Science Notebook:
    • Mark and Recapture

Population Sampling

How Many Grasshoppers?

Use the directions below to do a sampling activity using beans to represent the number of grasshoppers in a field.

Materials:

• 1 piece of printer paper
• 1 pen or pencil
• 1 ruler
• 1 handful (approx. 5 mL) of beans (Each bean represents one grasshopper) in the field.
• 1 calculator

Procedures

1. Copy the data table below (Population Sampling Chart.)
2. Using the pen or pencil and ruler, divide the printer paper into four columns across and five rows down so that you have 20 equal squares on the paper.
3. Scatter the grasshoppers (beans) as evenly as possible over the entire paper. Make sure some beans get in each box on the page.
4. Randomly choose one square on the page. Count the number of beans in one square and record below.
5. Count the number of beans in a second (different) square and record it on the chart.
6. Count the number of beans in a third square and record.
7. Find the mean (average) number of beans for the three squares. (Find the total for your three squares and divide the total by three.)
8. Multiply the mean (average) you got by the number of squares on the paper (20). This will give you an estimate of the number of beans on the whole paper.
9. Count the actual number of beans on the page.

Back to Teams...

1. Describe a situation (besides counting grasshoppers) where an ecologist would want to use sampling to estimate population. Why would they want to use sampling?
2. Describe a situation where an ecologist may not want to use sampling to estimate population. When would they not want to use sampling?

Be sure to include the following terms in your Ecology notes

• direct observation
• indirect observation
• sampling
• mark and recapture