Lab Objectives

Determine population sizes of organisms living in a certain area.
Calculate the population of a species using the Sampling Method and Lincoln-Petersen Equation.
Write a lab report using the specific rubric and instructions you have been given.

Introduction

The population is defined as the numerical measure for a specific organism found within an area, region, volume or location. This lab exercise will investigate how to determine population sizes of organisms living within a certain area or habitat. Finding the exact number of organisms living in a given area can be almost impossible, especially when you start to consider the difficulty of finding a specific animal or plant at any given location, large or small. For example, what if I asked you to count every single gray squirrel that lived in Decatur County, Georgia! Can you imagine how hard that would be to do and can you even imagine where you would begin in that task? Even if you attempted such an assignment, do you think your count of squirrels in Decatur County would be accurate or even close to correct? Fortunately there are methods we can use that help to determine what a population size is for an organism without having to count every single individual. We will investigate and explore how this is done in the particular lab exercise for Environmental Biology by using mathematical formulas to develop estimation methods.

Sampling is a method that is commonly used to help estimate the population of an organism. In sampling, a small area or location is examined very carefully and the organisms there are counted individually. Once the number of organisms in this small area is counted, then it is possible to estimate the number of organisms found in a much larger area using a formula that is based on ratios:

# of organisms in small area = # of organisms in a larger area
small area larger area

Example #1

the amazing striped pelicanPretend you work for a state agency and your job is to help figure out how many striped pelicans can be found living in a coastal region located in the state on 100 acres of land by the sea. If you are using a sampling technique as previously described with the above ratio, then you would have a sample area of 5 acres plotted out of the 100 acres you are interested in examining. You (and people that work with you) went to different areas to count and record the number of amazing striped pelicans using these supplies: maps of the area, notebooks to help you keep an accurate count of the stripped pelicans, and binoculars to help you find the critters.

The amazing striped pelican

You and your team did an excellent job of searching and combing through your area. You were able to find and count 18 pelicans. This is what your sampling formula for those 18 pelicans would look like:

18 pelicans (organisms) = X organisms
5 acres 100 acres

18/5 = X/100

Algebraically, ratio problems are the easiest to understand, which is good for us! We can now finish our sampling method through a few more steps. We will move the denominator across the equal sign and it will become the multiplier. Both denominators in our ratio will be moved across the equal sign so we can solve our equation.

18/5 = X/100

18/5 = 5 * X/100

100 *18 = 5 *X/100

1800 = 5X

In order to solve for X, we move the 5 multiplier back across the equal sign making it a divider:

1800/5 = 5X

You and your team of striped pelican searchers estimated that 100 acres of land by the sea hold 360 stripped pelicans: 1800/5 = 360

Now, you try a similar example. This time you are examining a different plot of 100 acres and still looking for the striped pelican. You sample 5 acres and count and record 15 striped pelicans. Do the calculations for the sample formula based on ratios:

# of organisms in small area = # of organisms in a larger area
small area larger area

What did you get for you answer? You should have found a sample of 300 striped pelicans for 100 acres based on your count of 15 pelicans on 5 acres. If you didn’t get 300, please try the formula again and check your algebra. You can do this!

Obviously our methods used to catch striped pelicans might not work for all animals, especially very hard to catch ones or even harder to find ones. Sampling techniques are best for plants (because they don’t move!) and animals that are slow moving and/or easier to find. Therefore, there are other techniques and methods available for us to estimate a population size of a specific organism. One such method is the mark and recapture technique. This concept states that it is not possible to count every single individual organism in the area you are sampling from. So what you would do for the mark and recapture method is trap several organisms at your specified location and mark them in some way. One common way to mark animals is by using tags or sometimes even collars are used that have radio-tracking devices on them. After the organisms are tagged, they are released back into their environment. After an allotted amount of time, the organisms are trapped a second time (hence the recapture). The recaptured organisms are marked and the unmarked ones are counted. Using this method of mark and recapture, an equation called the Lincoln-Petersen Equation can help determine an estimation of the population size:

(# of organisms captured the 1st time) X (# of organisms captured the 2nd time)=Population Size
(# of marked organisms captured the 2nd time)

Example #2

polka dotted lizardHere is an example of the Lincoln-Petersen Equation. Let’s say that a group of researchers wants to know how many purple polka dotted lizards are found living in cliffs by the sea. The lizards are hard to find though because they can hide well and are very fast. However, the researchers visited a 50 meter area on the cliffs by the sea over a 5 hour period of time on 2 separate visits. The researchers placed a plastic blue band around the lizard’s foot when one was captured on the first visit. It was then released after being banded. They were able to catch and band 67 lizards on the first visit. On the second visit, the captured 12 previously banded lizards and caught and banded an additional 52 lizards.

Polka dotted lizard

To determine the population size, the researchers used the following equation:

Lizards caught on the 1st visit (67) x Lizards caught on the 2nd visit (52) = Lizard population (290.3)
Banded lizards caught on the 2nd visit (12)

Now let’s use what we have learned and work on our procedures.

Lab Supplies

A container of some type (jar, bag, etc.)
A bag of light-colored dry beans (such as lima beans or pinto beans)
Permanent marker
Procedures

In order to experiment and determine the population size of a specific type of organism, we will need to put our equations to work! We will use the dry beans and pretend that they represent an organism in which we are determining the population of.

Place two large handfuls of the dry beans (your organisms) into a container.
Randomly select 25 organisms (beans) from this population. This will represent your sample.
Take your permanent marker and place a mark of some type on each bean.
Return the beans back to their population by placing them in the container.
Gently mix the population of beans so the sampled ones are evenly mixed in.
Randomly select another 25 organisms (beans) from your population again.
Record the number of recaptured organisms.
Use the Lincoln-Petersen equation to estimate your bean population and record your final estimate.
Repeat the experiment a second time with a fresh population.
Percent Error

Percent error is used to indicate how close a measured value is to the true or accepted value and is determined using the following equation:

(measured value – true value) x 100 = Percent Error
true value

The percent error is always reported as a positive number, so if you get a negative number with your equation, don’t worry. Just drop the negative sign. You will need to count your actual population size (the actual number of beans your sample came from that was in the container you used is the number for true value in your percent error equation and measured value is the number of organisms/beans you captured) to calculate percent error. What is the percent error for your bean population size based on the estimates you determined?

Lab Report

In addition to following the specific criteria of your lab report grading rubric, also consider and address the following questions in your report:

Describe the experimental design you carried out for estimating your bean populations.
Provide the calculations you did and results you came up with.
How accurate were your estimates of the population size of beans you used?
What type of difference did you find between the percent error in your first and second trial of bean population sizes?
What do you think the strengths of this method are? And the weaknesses?
What could be done to help improve this method, if anything?