The Wild Bean Population: Estimating Population Size Using the Mark and Recapture Method

Transcription

1 Name Date The Wild Bean Population: Estimating Population Size Using the Mark and Recapture Method Introduction: In order to effectively study living organisms, scientists often need to know the size of a given population. A population is a group of organisms of the same species that live in a particular area. Wildlife biologists may worry that a certain organism is outgrowing the environment s capacity for sustaining that organism. At other times, the biologist may be worried that the organism is becoming too rare and that steps are needed in order to protect it. It is not reasonable to think that every individual in the population can be counted, and it is often difficult to get an accurate estimation of population size since organisms tend to hide, move around, etc. Population biologists have developed several methods for sampling a population. In this lab, you will use the sampling technique known as the mark and recapture method to estimate the size of a population of wild pinto beans! This method involves capturing a number of individuals from a population, marking or tagging them, and then releasing them back into the wild. At a later time, a second capturing is conducted. Some of the organisms in the second recapture were previously marked while others in the second recapture will have no mark. If you know the following information: (1) The number of individuals initially marked, (2) the total number of individuals recaptured in the second group, and (3) the number of marked individuals in the second recapture, it is possible to make an estimation as to the total population size. Purpose: 1. To learn the mark and recapture technique for estimating the size of a population. 2. To calculate the size of a population from given data. 3. To make predictions about the size of a population under various conditions. Materials: Brown lunch sack White Beans Beaker or Cup Red Beans Calculator Procedure: Estimating Population Size 1. You have been provided with a brown lunch bag containing white beans. Reach into the bag and remove a handful of beans. Do not look into the sack and do not attempt to grab all of the beans. 2. Count the number of wild beans you have captured and record this number in the data table below as Total Number in First Capture. Set these beans aside. Do not return them to the bag. 3. In the mark and recapture technique, these beans would be marked with an X and returned to the brown lunch sack. Since I need to be able to use these beans over and over, we will mark them as described in the next step. 4. Count out a number of red kidney beans that is equal to the number of white beans you removed from the sack. Place these red kidney beans in the brown sack. Consider these red beans to be the marked beans. 1

2 5. Gently mix the beans in the bag. 6. Once again, reach into the bag and capture a second handful of wild beans. 7. Count the total number of beans in the recapture and record this number in the data table under Total Number in Recapture. 8. Now, looking at the beans in your recapture, count the number of red beans and record this in the data table under Number of Marked Beans in Recapture. 9. Let s get a second set of data for comparison purposes. Return all the white beans to the lunch sack, even those you had set aside earlier. Remove all red beans from the lunch sack and return all the red beans to the cup. 10. Repeat steps 1-8 to get a second set of data. Record these numbers under Trial 2 of your data table. 11. Complete the calculation for determining your estimate of the population size. The equation and workspace can be found beneath the data table. 12. Finally, count the total number of white beans that were initially in your lunch sack. DO NOT include the red beans you added to the bag in step 4. Record this number in the data table under Actual Total Population. 13. Calculate the percent error between the actual population size and your estimated population size. The equation and workspace can be found beneath the data table. 14. Complete the Final Analysis Questions. 15. Please separate the red beans from the white beans, leaving all white beans in the lunch sack and all red beans in the cup. Mark-Recapture Data Table: Total Number in First Capture (Marked and returned) Total Number in Recapture Trial 1 Trial 2 Number of Marked Beans in Recapture (Red beans) Calculated Estimate of Population Size Actual Total Population Percent Error Amy Brown Science 2

3 Calculations: 1. Use the following equation to make an estimate of the population size. Population Size = (Total Number of Beans in Recapture) (Total Number in First Capture) (Number of Marked Beans in Recapture) Estimate the size of the population for both Trial 1 and Trial 2 and show your work in the space below. Record your answers in the data table above. Trial 1 Calculation: Trial 2 Calculation: 2. Calculate your percent error between the actual population size and your calculated population size. Percent Error = Actual Population Calculated Population x 100% Actual Population Show your work in the space below. Record your answers in the data table above. Trial 1: Trial 2: Analysis Questions: 1. A population is a group of organisms of the same species that live in a particular area. a) Why might a scientist need to know the size of a population? b) Why must scientists develop methods such as these to estimate population size? 3

4 2. Do you feel that the mark and recapture method of estimating the size of a population is an accurate and reliable procedure for determining the size of a population? Explain your answer. 3. Was your estimate of the wild bean population size close to the actual population size? Explain. Identify possible sources of error in the procedure you used. 4. When this method is used for determining the population size in a real population in nature, it is important that every individual in the population has the possibility of being captured, marked and released. a) Did each type of bean have equal chance of being grabbed out of the lunch bag? b) Which type of bean were you more likely to select and why? 5. By completing two different trials, you estimated the population size twice. a) Describe the differences or similarities in population size of the two trials. b) When the mark and recapture technique is used by scientists, why would it be a mistake to base the population size on just one mark and recapture? 6. You wish to determine the number of grasshoppers in a large cornfield. a) Describe how you would use this technique to estimate the population size. List specific steps to your procedure. 4

5 b) Identify possible sources of error with the technique you have outlined. 7. Practice Problem: A group of scientists collected data on a koala population once a year for five years to determine if the population was growing or in decline. Based on the following information, calculate the estimated population for each year. Show your work and your answer in the last column. Year Number of Koalas Captured and Marked Total Number of Koalas in Recapture Number of Koalas Recaptured with Marks Estimated Koala Population a) What trend is seen over the 5-year period? b) List two possible reasons for this trend. 8. Explain the bias in using animals that can be captured and recaptured as the basis for determining the size of a population. 5

6 9. In a particular population, it was determined that an animal captured the first time was more likely to be recaptured the second time than animals that had never been captured. What effect might this have on the estimated population size? 10. In the mark and capture method of sampling a population, it is assumed that the mark or tag will not wear off or be removed. If some animals lose their mark during the study, how would this affect the estimation of the population size? 11. Suppose the time between the capture and the recapture is too long and some of the marked animals die. Suppose also that enough new animals are born so that the population size remains constant. Will the death of the marked animals tend to make the estimate of the population size too large or too small? Explain. 12. For the mark and recapture method to be a valid method for estimating population size, scientists must be convinced that three basic assumptions are satisfied during the sampling. Based on questions 9, 10, and 11, what are these three basic assumptions? ü ü ü Amy Brown Science 6