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Population Study

One of the three essential questions ecologists ask is “How many are there?” in reference to the number of individuals in a population of organisms. Answering this question is not only important for research applications, but also has practical applications. F        or example if you were planning to harvest fish from a lake it would be important to have a reasonable estimate of the population, to create a sustainable fishery. Similarly, if you needed to determine the impact of a predator population on the population of its prey, you would need know the sizes of both populations.

It is not usually feasible to determine the size of a population by actually counting each individual. Instead, ecologists use a variety of sampling techniques to accurately estimate the size of a population. These techniques include random sampling, and mark-recapture methods.

In this lab exercise you will use both random sampling and mark-recapture methodologies to estimate the size of simulated population. You will compare the accuracy of both techniques and determine situation in which these techniques might be employed.

Random Sampling

In a random sampling techniques, ecologists randomly locate several plots within their study area and accurately count the number of individuals in these plots. Taking the average number of individuals per plot they can extrapolate to estimate the population size of in the entire area. This method is most accurate when the habitat is homogeneous and many sample plots are taken. Sometimes, instead of counting the individual organisms, ecologists will estimate using some index of population size, such as the number of nests, tracks, or even fecal droppings.


Mark-recapture methods for estimating the size of a population rely on capturing a random sample of the organism being studied. The captured individuals are counted, marked in some way (Fish are often marked by clipping small parts of their fins, mammals often are marked with ear tags, and birds are marked with leg bands), and released back into the environment. After a period of time a second sample is taken these individuals are counted and any marked individuals are noted in the results. From this information scientists can estimate the number of individuals in a population. Greater accuracy can be gained by repeated mark-recapture protocols. Here multiple samples are taken and the number of individuals marked grows each cycle. Example formula:

    R (marked recaptures) M (marked initially)

———————————   = ————————————

    T (total in second sample) N (total pop. size)

Your Task

You and your group are tasked with estimating the student population at University High School. In order to complete this task you must come up with two sampling techniques. One sampling technique will use some form of Random Sampling, while the other must be a mark and recapture technique. You will need to do some research to find out the appropriate formulas to use to estimate population size based on the  sampling method employed. Once you have a method you will then carry out your sampling of the population during the school day. All sampling and counting must take place outside the classrooms (ie. during lunch, nutrition, passing time, before or after school) . You may not ask for help from an administrator, or use the staff roster (online or off). However, you may use satellite imagery to help.

In class we will be able to crunch the numbers and determine the accuracy of the methods. You will then write a formal lab report in which you will present your findings and discuss the two sampling techniques. You will want to include any data, applicable formulas, and calculations you performed to estimate the population size. You will also need to discuss the accuracy of your methods and any adjustments to the procedures in a future study.