GT BIOLOGY 2008 SR.ECO.7D
ESTIMATING POPULATION SIZE AND DENSITY
INTRODUCTION Some populations can be counted easily via a census. This is where each individual is counted separately. However, in many populations, counting each individual is not possible. This might be due to the size of the population, range of habitat, or mobility of the species. In such cases, ecologists use a variety of sampling methods. For instance, a designated area of study might be sectioned into grids or plots. Numbers of organisms counted in selected grids (quadrats) are extrapolated to estimate the total population size. Mark-and-recapture is another method used to estimate population size in large geographic areas. Traps are set in the study area. Trapped organisms are tagged and released. After a period of time, traps are set again, and calculations are made based on the number of marked organisms that are recaptured.
Once a population estimate has been obtained, ecologists can then calculate population density. This is a measure of the number of individuals of the same species living in a designated unit of space. It is influenced by relationships among organisms, movement of individuals in and out of the habitat, resources, and abiotic environmental factors (such as climate). Fluctuations in population density are important to monitor as they can indicate changes in the environment.
OBJECTIVE When students have completed this lesson, they will be able to estimate the size of a population using random sampling and mark-recapture in order to calculate population density.
MATERIALS Small beans, popcorn kernels, or peas Paper bag Permanent marker or wax pencil Random number generator (calculator or computer) Random sampling quadrat grid worksheet Ruler
PROCEDURE Part I: Random Sampling – Quadrat Technique For immobile animals or plants, estimating density is fairly easy. Individuals are counted in a number of replicated small areas, known as quadrats. The average density in these quadrats is used to calculate the estimated population size.
To illustrate this, do the following: 1. Use a random number generator, or other technique identified by your teacher, to locate the 10 quadrats you will sample on the grid. 2. Shade in or outline these quadrats on the random sampling quadrat grid worksheet. 3. Randomly scatter several handfuls of small beans across the random sampling quadrat grid worksheet. Once the beans are scattered, you may not move them. 4. Count the number of beans that are in each of the 10 quadrats you identified randomly. Record your totals in Data Table 1.
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5. Multiply the number in each quadrat by the total number of quadrats. Record this information in Data Table 1. 6. Calculate the average estimated population size. Record this information in Data Table 2. 7. Conduct a census by counting all the individuals to determine the actual population size. Record this information in Data Tables 2 and 3 . 8. Calculate the % difference between the population estimate by random sample and the actual population size obtained by census. Record this information in Data Table 2. 9. To determine the actual population density, determine the area of the random sampling quadrat grid. Record the area in cm2 in Data Table 3. 10. Divide the number of individuals by the area and record the population density in Data Table 3.
Part II: Mark-Recapture Technique In mark-recapture, a sample of organisms, usually mobile animals, is captured from the population whose density we wish to estimate and an identifying mark is applied to them. In practice, these marks can be of many types including radio collars in large animals, leg bands in birds, fin clipping in fish, etc. The marked animals are released back into the original population, and after a period of time a second sample is captured. The size of the population is related to the fraction of individuals in the second sample which carry marks, and is calculated as follows:
N = C*M / R
Where: M = number of individuals marked in the first sample C = total number of individuals captured in the second sample R = number of individuals in the second sample which are marked
To illustrate this, do the following: 1. Grab several handfuls of beans (at least 100) and place them into a paper bag. 2. Mark 10 of the beans with a permanent marker or wax pencil so that you can clearly identify them as being “marked”. This is your initial marked sample, so M = 100. 3. Shake the bag and withdraw 10 beans. This is your second sample, so C = 10. 4. Count the number of beans in the sample that are marked (=R) and record your answer in Data Table 4. Replace your beans in the bag. 5. Estimate the size of your bean population, N, by dividing M*C by the number of marked beans in your sample. 6. Repeat steps 3 through 5 nine more times and average the population estimates you obtained in each trial to get an overall population size estimate. 7. Repeat procedure #1-6 with the exception of taking 20 beans from the bag with each sample instead of 10. Thus, C for each population size estimate her will be 20 instead of 10. 8. Repeat procedure #1-6, after taking 10 unmarked beans from the bag and adding marks to them. In this set of estimates, M = 20 and C = 20. 9. Average your three final population estimates, to obtain one estimated population size. Record this information in Data Table 5. 10. After you are finished, conduct a census by counting the beans in your bag to determine the actual population size. Record this information in Data Table 5.
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11. Calculate the % difference between the population estimate by mark-recapture and the actual population size obtained by census. Record this information in Data Table 5.
DATA
TABLE 1: RANDOM SAMPLE QUADRAT COUNTS Population size Sample # beans in quadrat estimate 1
2
3
4
5
6
7
8
9
10
TABLE 2: RANDOM SAMPLE DATA SUMMARY Population Size Random Sample
Census
% Difference
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TABLE 3: POPULATION DENSITY Census Count
Area (cm2) Population Density (# individuals / cm2)
TABLE 4: MARK-RECAPTURE ESTIMATES FOR BEAN “POPULATION” C = 10 C = 20 C = 20 M = 10 M = 10 M = 20 Trial # marked Pop. size # marked Pop. size # marked Pop. size beans in estimate beans in estimate beans in estimate sample (R) (=100/R) sample (R) (=200/R) sample (R) (=400/R) 1
2
3
4
5
6
7
8
9
10 Average Estimated Pop. Size
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TABLE 5: MARK-RECAPTURE DATA SUMMARY Population Size Mark-Recapture
Census
% Difference
ANALYSIS 1. In the random sample study, describe the dispersion pattern of your population. Refer to the diagram below. TYPES OF POPULATION DISPERSION
UNIFORM RANDOM CLUMPED
2. In nature, would you expect the dispersion pattern you see in a population to change with quadrat size? Why or why not? Give an example of how the dispersion pattern seen in a population might change with scale.
3. In the random sample study, what do you have to assume to be true in order to believe your estimates of population size? What might happen in a real population of animals that would affect your results?
4. In the mark-recapture study, which combination of C and M gave you the best estimate? Which gave you the worst? Suggest a reasonable explanation to explain your data.
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5. In the mark-recapture study, what do you have to assume to be true in order to believe your estimates of population size? What might happen in a real population of animals that would affect your results?
6. If you were trying to estimate the population density of a real species why might you have to sacrifice some accuracy in your estimation?
7. Give an example of an animal population in nature that would be well-suited for a random sampling study. Explain your reasoning.
8. Give an example of an animal population in nature that would be well-suited for a mark- recapture study. Explain your reasoning.
9. Why do ecologists often repeat these studies on the same populations year after year?
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RANDOM SAMPLING QUADRAT
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