Population Estimation Lab

Transcription

1 Background Population Estimation Lab The ability of scientists to accurately estimate the total population of a target organism is fundamental to any ecological study. Species and resource management is closely tied to population size counts performed in the field. Because counting every individual in a population can be impractical and very difficult, field biologists have developed a variety of techniques and mathematical formulas for estimating populations. Several of these methods require counting a portion of the population while others involve successive captures of individuals. Part 1 Mark and Recapture Method One common population sampling technique is the mark-recapture. This technique involves capturing a number of individuals in such a way as to identify them if recaptured, and then releasing them back into the population. A subsequent capture is performed, and the ratio of marked to unmarked individuals is noted. Typically, several recaptures are performed. Ultimately the total population of the target species is estimated on the basis of this ratio. Assumptions must be made for all types of population estimates, and it is a valuable exercise to think about some of the assumptions required to ensure the validity of a given sampling method, as well as some of the circumstances that might cause these assumptions not to be met (e.g., are individuals from outside the sampling area likely to move into the area during the sampling period?). The following conditions were assumed to be true in deriving the mathematical formula used to determine the mark-recapture population estimation. If the following conditions do not apply to a particular population, the mark-recapture method will not provide a reliable population estimate. Assumptions: The ratio of the actual population to the number of captured individuals is the same as the ratio of the number of released, marked individuals to the number of recaptured, marked individuals. Each individual in the population has an equal and independent chance of being captured. Except for the effects of trapping, the population is not increasing or decreasing in size during the sampling period. Biologists conducting a population census will probably not have proof that these conditions are met. Field biologists study situations carefully and learn all that they can about the target organisms to ensure that the assumptions are reliably met. In situation in which the assumptions are in question, other methods must be found for estimating populations, or less trust must be placed on the accuracy of the estimate. During this section of the lab, you and your partners will use the mark-recapture method to estimate a population. Whereas a fisheries biologist might use this method to estimate the population of largemouth bass in a pond, you will be using the method to estimate the number of beads in a cup.

2 A simple formula for estimating a population using the mark-recapture technique is: N = m s t Where: N = Estimated Population Total m = number of individuals marked from 1 st capture s = number of individuals captured each round t = number counted that were marked during the recapture Procedure Part 1 1. Count your green beads from the red cup you should have 100 beads. If you have too many, please place them into the bag in the front of the classroom. If you are missing green beads, please take a few. If you have any BLACK beads, replace them with green beads. 2. The green beads represent the total population. Using your fingers, capture 40 individuals (green beads) from the red cup. 3. Using the black marker, mark these 40 captured individuals by completely coloring them in black. These beads will serve as the marked individuals. 4. Release the 40 marked individuals back into the population (the red cup). 5. Cover and shake the cup vigorously to redistribute the marked individuals throughout the population. 6. Without looking into the cup, remove 20 beads, one at a time. 7. Count the number of marked individuals (black beads) captured and record this number in your data table as m. 8. Replace the beads captured into the cup with the remaining population. 9. Shake up the cup to distribute the marked individuals among the population. 10. Without looking into the cup, remove 20 beads, one at a time. Count the beads and record this number in your data table as s Of these beads, count the number of marked beads, and record this number as t Using these values and the m value, calculate N and record this as N 11. Return all of the captured beans to the population shake up the cup and repeat steps 9 and 10. You are to do this nine more times. 12. When your group has completed ten captures, average all of the population estimates (N) together and enter this value in the data table.

3 Visual estimation: Part 1 - Mark and Recapture Method: m = Trial s t N Average N = Actual N = Percent error = 1. How would it be possible to decrease the percent error using this technique? 2. List three additional populations for which this technique would work well. 3. List three populations for which this technique would not work well.

4 Part 2 Quadrant Method The mark and recapture method may not always be an appropriate means to estimate a population size. Sometimes it may work better to simply examine a relatively small area in which the population exists and applying the findings to the entire area. There are three variations of this method. First, several same-sized areas can be arbitrarily chosen and marked off in which the population can be found. After counting the number of individuals in each area, an average can be found. By multiplying the average number of individuals to the size of the area the population exists, a population can be estimated. A better use for this method is to estimate the number individuals/m 2. A second version is to mark a grid covering the area the population inhabits. By counting the individuals in certain squares of the grid and comparing those numbers to the entire area, a population can be estimated. A plant ecologist may use one of the quadrant methods to estimate the population of a particular species of tree in a forest. During this section of the lab, your group will be using the second variation of the quadrant method to compete a second estimation of the population of beads. Procedure - Part 2 1. Dump the beads onto the poster paper with the grid on it and randomly spread the beads out. They do not need to be spread evenly. Be sure the beads are not stacked. Do not move or touch beads until the completion of this method of estimation. 2. Based on estimation, classify each square in the grid as having a high, medium, or low density of beads. It might be a good idea to use an eraser to mark 3. Once each square has been classified, pick a square that best represents each classification Count all the individuals in each of the three squares and record the number in the data table. 4. Count the number of square of each type of density and record the number in the data table. Once both methods of population estimation are complete, hand count ALL the beads in your population and record this number in BOTH data tables. To calculate the percent error between the estimate and the actual number, use the following formula. (Actual number - the average N / by the actual number) x 100

5 Part 2 - Quadrant Method: Density Representative # # of Squares Total High X = Medium X = Low X = Estimated Total Actual Total Percent Error 1. How would it be possible to decrease the percent error using this technique? 2. List three additional populations for which this technique would work well. 3. List three populations for which this technique would not work well. General Which method was more accurate? Why? Why might it be useful to know the population of a species? Name and describe at least two applications for estimating a population.