Name______Date______Class______
Lab Activity: 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 Centennial Lake, discuss with your partner how would you accomplish this task and describe in detail below. ______
Introduction:
Ecologist often spend time out in the field studying the various biotic and abiotic components that make up the ecosystems that they are interested in researching. Such factors may include things like water quality, soil type, and various species of plants and animals. One important element of an ecosystem that scientists may be interested in is animal populations. They may want to know how many individuals of a particular species reside in a specific habitat.
Researchers may find such information important to understanding an ecosystem’s food web, or the scientists may want to use population data to determine whether or not a threatened species is declining. Observed population fluctuations may be normal or they may indicate that the ecosystem is out of balance.
Unfortunately there is no easy way to count all the members of an animal population in an ecosystem. Animals move from place to place; they hide, they hibernate, and they camouflage themselves. Surveying a large tract of land trying to count all the members of a species would be extremely time consuming and expensive. In other words, conducting a census, which is the counting of every single individual in a population, is simply impractical if not impossible.
Knowing the size of a population of animals is important in making environmental decisions that would affect the population, but estimating the size of wild populations is extremely difficult. In the case of ocean dwellers, such as whales, the task is especially challenging. Estimates of the number of minke whales, for example, have differed by as much as a factor of 10. Deciding whether to allow hunting of minke whales, based on population estimates that are too high, could lead to extinction of the species. On the other hand, basing a decision on an estimate that is too low could unnecessarily ban hunting of minkes by people that depend on whales for food.
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 20,000 gallons?
3. What are some problems with this technique? What could affect its accuracy?
Technique 2 - Mark and Recapture
Another method often used to estimate population size is the "mark and capture" technique, in which scientists capture some animals from the population, mark them, and release them. At a later time, the scientists again capture animals from the same population and observe how many of them are marked. The method assumes that the ratio of the actual population to the sample size is the same as the ratio of the number of marked animals to the number marked in the recapture sample.
*Procedure:
1. You will receive a bag that represents your population of animals (beans).
2. Empty the bag and inspect the beans carefully. You will have to flip them all over and count the number of beans that have a mark. These are marked “animals.” Record total number marked animals here ______.
3. Return all the “animals” to the bag and GENTLY shake.
4. With your eyes closed, select 15 “animals” from the bag one at a time. This is the recapture step. Record the number of “animals” recaptured that have a mark on the data table.
5. Return the “animals” to the bag, GENTLY shake and repeat. Do 10 recaptures.
Data Table
Trial Number / Number Captured / Number Recaptured with mark1 / 15
2 / 15
3 / 15
4 / 15
5 / 15
6 / 15
7 / 15
8 / 15
9 / 15
10 / 15
Total: / 150
Calculations
In order to estimate your population size, follow this formula
Estimate of Total Population = (total number captured) x (number marked**)
(total number recaptured with mark)
**number marked is from your initial count in procedure #2 above
4. What is the estimation of your population? (Show your calculations below)
EstimatedSize ______
5. Now dump the bag out and count how many animals are actually in your population.
Actual Population Size ______
Analysis
6. Compare the actual size to the estimated size. Did you overestimate or underestimate?
7. Repeat the experiment, this time add another 10 data fields to the ten trials you already have.Recalculate your estimate using the formula. (Show below)
What does this say about the number of trials that should be conducted in a real mark & recapture? / Trial Number / Number Captured / Number Recaptured with mark
11 / 15
12 / 15
13 / 15
14 / 15
15 / 15
16 / 15
17 / 15
18 / 15
19 / 15
20 / 15
Total: / 300 / (add original data + new data)
8. Given the following data, what would be the estimated size of a butterfly population in Wilson Park.
A biologist originally marked 100 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?
9. In what situations would sampling work best for estimating population size, in what situations would mark & recapture work best. You’ll probably have to think about this one. Justify your answer.
10. Explain the problems that scientists might encounter in using this method in the field that you would not have encountered.