Ecologists use quantitative measurements to study, predict, and describe populations. However, since it is impractical, inefficient and very time-consuming to count each individuals within a population, ecologists use different methods to estimate a population size. One of these techniques is called mark-recapture: a method in which animals are captured, marked with a tag, collar, or band, released, then recaptured at a later time to determine an estimate of population size. In this lab, the technique, mark-recapture, was conducted through an experiment with large number of pastas in a plastic bag representing the population of the same species living in a specific area.
Side Note: Since I was unable to participate in this lab, thanks to Katherine Shin, all the photos and data posted on this blog is originally taken by Katherine and her group.
- Materials: 1 bag of uncooked pasta, a Sharpie marker (an inerasable, permanent marker)
- Methods:
(Trial #1)
1. From your pasta population, capture a sample population that will be prepared for marking and record this number of pastas (M).
In trial #1, 20 pastas were captured. Therefore, M=20.
2. Using an inerasable marker, mark the captured population of pastas in a recognizable way.
In trial #1, the 20 pastas captured were marked using the letter "A".
3. Release the previously captured population of pastas into the general population of pastas, and make sure that they are randomly mixed with the general population of pastas.
The 20 captured pastas were released back into the general population of pastas.
The pastas were mixed randomly with the general population of pastas by shaking the plastic bag several times.
4. After mixing, with eyes closed, take a second random sample of pastas and record this number of pastas (n). If there are any previously marked pastas found in the second sample, record this number of pastas (m).
With eyes closed, the total number of pastas captured were 46. Therefore, n=46
After the search of previously marked pastas, only one was found. Therefore, m=1.
5. Calculate the estimated population size of the general population (N) using the values for M, m, and n.
M/N=m/n
20/N=1/46
N=920
6. Perform two more trials of the previous steps and record the data.
Trial #2
M=65, n=72, m=8
M/N=m/n
65/N=8/72
N=585
Trial #3
M=64, n=69, m=8
M/N=m/n
64/N=8/69
N=552
Average=920+585+552/3=685.6=686
The actual population of the pastas was 558.
% Error=Theoretical-Experimental/Theoretical x 100%
=(686-558/686) x 100%
=0.229 x 100%
=23%
Analysis:
1. Comparing the overall estimate size and the true size of the pasta population, it is reasonable to say that the estimation was close to the actual population. The estimation size for the second and third trial was especially very close to the actual population of the pasta since the estimations were 585, 552, and the true size was 558. However, the first trial was not as close as the second and third trial since the estimation was 920 when the actual population was 558.
The reason for this inaccurate estimation in the first trial can be said due to such small number of pastas captured out of the actual population which led to only 1 recaptured pasta.
2. a) One of the problems that might affect the accuracy of the estimation size in this lab is when such a low number of pastas are captured compared to the actual population. For example, in the first trial, because the captured population was so small compared to the actual population, the accuracy of the estimation size was decreased.
Another factor that may have decreased the accuracy of the estimation size is a poor mixing of pastas in the bag. This would result with the previously marked pastas becoming too concentrated on one side or certain parts of the plastic bag.
b) In this lab, working with pastas instead of active and mobile animals, a lot of problems which could have occurred in nature were prevented. For example, one of the problems which ecologists may face when working with animals in a natural environment is their migration. This can result in a great loss of marked animals and therefore cause an inaccuracy in the estimation size.
Another problem which ecologists may encounter is the uncontrollable natural disasters. A natural disaster can suddenly wipe out a large population of animals or cause them to migrate to a different region. This, once again, will affect the number of marked animals.
In addition, a problem can rise even due to the immigration of animals from other places to the studying area. The different population size from when the study began due to an immigration, will decrease the accuracy of the estimation size.
c)To improve the accuracy of this experiment, it should be repeated numerous times to examine that there is a consistency in the results of the experiment.
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