Estimating Population Size - Teacher Guide

Original Document: Estimating Population Size

Teacher Notes: You will need to prepare the populations ahead of time. You can put marbles, beads, macaroni, or other small objects in bags. The numbers will work best if the bags contain between 30 and 100 objects. I have bought toys from the dollar store for use in this project, like little plastic insects or snakes to give it more of a "biology" feel. Masking tape can serve to mark the objects, but make sure students remove the tape for the next class, extra marks will skew the data.

Another option is to use white beans you buy at the supermarket, students can use permanent markers or pens to mark the beans and then count manually at the end of the experiment. Each group could receive about a beakerful of beans to work with.

A discussion prior to this activity should address why biologists have hard time estimating populations of animals in the wild: they move around, space is too big, you cannot capture them allfish

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.

Answers vary, you could count the fish in a portion of the lake and then multiply by the size of the lake

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? 50 x 1000 = 50,000

3. What are some problems with this technique? What could affect its accuracy? different areas might have more dense populations, if you took a sample from the bottom of the lake it may be different from a sample at the top of the lake

Technique 2 - Mark and Recapture

Over the years I have collected toys to serve as sample populations, things like legos, toy soldiers, and other fake animals you can buy from the dollar store make this activity more engaging for students.

4. Calculations = Find your Population Estimate

Estimated Size - this will vary based on their data, students often forget that "10" is the number they initially marked. Some need help with equation solving and plugging numbers.

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 _________ Keep this on a card for students to check, they can also count the bag themselves but I find this decreases the authenticity of the simulation. If you ask them to count, then they will say "why didn't I just do this in the first place"


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

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)

Generally more trials result in better results, but the scope of this simulation may actually cause the results to be worse. You may want to discuss with students why population studies usually continue for many years by the department of natural resources.

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.

Use the formula above

200 x 40 / 80 = 100