Names__________________________________
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# Estimating Population Size

Objective: You will be expected to estimate the size of a sample population using the mark-recapture technique. Be able to apply the technique to new population problems and compare the mark and recapture technique to other methods of population estimating.

1. If you were in charge of a team given the responsibility to determine the number of sunfish in Horseshoe Lake, discuss with your partner how would you accomplish this task and describe in detail below.

## Technique 1: Sampling

A technique called sampling is sometimes used to estimate population size. In this procedure, the organisms in a few small areas are counted and projected to the entire area. For instance, if a biologist counts 10 squirrels living in a 200 square foot area, she could predict that there are 100 squirrels living in a 2000 square foot area.

2. A biologist collected 1 gallon of pond water and counted 50 paramecium. Based on the sampling technique, how many paramecium could be found in the pond if the pond were 1,000 gallons?

3. What are some problems with this technique? What could affect its accuracy?

DATA

 Trial Number Number Captured Number Recaptured with mark 1 2 3 4 5 6 7 8 9 10 Total:

## Technique 2 - Mark and Recapture

In this procedure, biologists use traps to capture the animals alive and mark them in some way. The animals are returned unharmed to their environment. Over a long time period, the animals from the population are continued to be trapped and data is taken on how many are captured with tags. A mathematical formula is then used to estimate population size.

Procedure:

• Capture 10 “animals” by removing them randomly from the bag.
• Place a mark on them using tape or string
• Return the 10 marked “animals” to the container
• With your eyes closed, grab a handful of the population. This is the recapture step. Record the number of “animals” recaptured in total and the number that have a mark on them on the data table
• Return the “animals” to the bag and repeat. Do 10 recaptures.
• When the ten recaptures are completed, enter the totals on the data table

4. Calculations = Find your Population Estimate

Estimated Size ___________

5. Use the code-name on your bag to check with the teacher about how many “animals” are really in your population.
Name on Bag ___________________________ Actual Size _________

## Analysis

6. Compare the actual size to the estimated size. Did you overestimate or underestimate?

 Trial Number Number Captured Number Recaptured with mark 11 12 13 14 15 16 17 18 19 20 Total: (original data + new data) (original data + new data)

7. Continue the experiment by filling out the data table.
Recalculate your estimate using the formula. (Show below)

a. Is the second estimate closer than the first one? ______
b. To get the most accurate results, you would generally
do [ more / less ] trials . (circle)

8. Given the following data, what would be the estimated size of a butterfly population in Wilson Park.

A biologist originally marked 40 butterflies in Wilson Park. Over a month long period ­ butterfly traps caught 200 butterflies. Of those 200, 80 were found to have tags. Based on this information, what is the estimated population size of the butterflies in Wilson Park? SHOW WORK to get credit.