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Population Ecology Lab

The Wild Bean Population:
Estimating Population Size Using the “Mark and Recapture” Method

I have always found it difficult to find quality labs to use with my students when teaching my unit on ecology.  My school is on a busy city street, we have no access to a pond or woods, and only very limited access to grass in the school yard!  Therefore, any labs we do in ecology have to be labs that can be carried out within the classroom or within the laboratory.  This year, for the first time, I tried this lab that I called "The Wild Bean Population."

Anyone can do this lab since it uses very simple materials!  All you need is dry white navy beans, dry red pinto beans, and a brown lunch sack.

In order to effectively study living organisms, scientists often need to know the size of a given population. A population is a group of organisms of the same species that live in the same general area. It is not reasonable to think that every individual in the population can be counted, and it is often difficult to get an accurate estimation of population size since organisms tend to hide, move around, etc. 

Population biologists have developed several methods for sampling a population.  In this lab, you will use the sampling technique known as “the mark and recapture method” to estimate the size of a population of wild beans!   This method involves capturing a number of individuals from a population, marking or tagging them, and then releasing them back into the wild.   

In the photo to the right, the white beans represent the population of wild beans that lives within the brown paper sack.  Students grab a handful of beans and remove them from the sack.  This represents the initial capture of organisms.  The white beans are counted, marked and returned to the sack.  The easiest way to do this would be to have the students mark each bean with a Sharpie before returning it to the sack.  I teach 5 biology classes, and did not want to have to throw away that many marked beans at the end of each class.   My solution was to have the students replace the white beans from the initial capture with red beans.  The red beans represent organisms that were initially captured and returned to their environment.

At a later time, a second capturing is conducted.  Some of the organisms in the second recapture were previously marked while others in the second recapture will have no mark.  If you know the following information:  (1) The number of individuals initially marked, (2) the total number of individuals recaptured in the second group, and (3) the number of marked individuals in the second recapture, it is possible to make an estimation as to the total population size.  

This lab satisfied three purposes:
1.   To learn the “mark and recapture” technique for estimating the size of a population.

2.   To calculate the size of a population from given data.

3.   To make predictions about the size of a population under various conditions.

I generally do not like "simulation-type" labs such as this one, but I was very pleased with the results and the concepts that my students learned from this lab. The students were able to carry out mathematical calculations to determine the population size, and calculate their percent error.  The final analysis questions are thought provoking and require critical thinking skills.  All in all, it was a great lab, and I will definitely be using it again next year.

We Stop for Turtles!

Today after school, my daughter and I were driving home.  As we turned onto a very busy street, we noticed a VERY LARGE turtle topple off the curb and into the street.  The poor guy landed upside down!  I need a bumper sticker that says "I brake for turtles!"  We stopped and picked up the turtle and put him in our car.  We released him into our pond, and I hope he lives happily ever after.

Our First Experience With the Student-Designed Lab

Did you read my blog post from two days ago? In case you did not, here is a quick update:  I am trying to change the focus of my high school biology classes by incorporating more student-designed lab activities.  The previous post was about getting my students ready for their first attempt.

Well, today we have completed our first lab in which the student designed the lab from top to bottom.  I am happy to report that I am quite pleased with the results!!

For our first attempt, I gave the students a very, very, very simple idea to work with.  After all, the point of this lab is to learn how to design and carry out an experiment.  I asked my students to design an experiment that would test the effectiveness of different quantities of water on the germination of seeds.  Simple , right?  It turned out to be such a great idea!  There are limited options for the students and no advanced knowledge about the topic was needed.  The students only had these materials to work with:  Petri dishes, filter paper, radish seeds and a graduated cylinder.  Initially I did not include the filter paper, but we quickly realized that a small quantity of water would "pool " together in the plastic Petri dish, and we needed the water to spread out across the entire surface.  Students were instructed that if they lined the dish with a piece of filter paper, the water would evenly spread across the surface.

Radishes were a great choice!  They germinated within 24 hours, and I allowed my students to observe their germination rate every day for three days.  I had my students complete a lab report in which they had to design and complete the following:

  • State a hypothesis that is testable.
  • Write out detailed steps to their procedure.
  • Determine the independent and the dependent variables.
  • Include a description of their control and how it served as a control.
  • Include a description of their experimental groups.
  • Identify factors that must remain constant throughout the experiment.
  • Design a data table.
  • Graph their germination rates.
  • Form a conclusion based on the data gathered.
Below are a few pictures that I took during the lab activity:

The first dish on the left served as a control since these seeds received no water.  The other three dishes received varying amounts of water.

Some students elected to keep their dishes in front of the window....

.....while others elected to keep their dishes under a plant growth light that remained on 24 hours a day.  Other students placed their dishes in complete darkness.  

Light level was not a variable being tested.  Whichever light level the student picked, had to remain constant throughout the experiment.  The variable being tested was the amount of water placed in each dish.  

The only problem with this experiment was that some students had difficult reaching a conclusion.  They found that there was not that much difference between germination rates of radish seeds when given varying quantities of water.  

All in all, our first attempt at the "student-designed lab" when extremely well.  Most of the anxiety I have about allowing students to design the experiment is now gone, and I am very much looking forward to our next attempt!

Have fun teaching~

Let the Student Design their Own Experiment

Can we teach our science students how to be successful conducting open-ended labs?

As teachers, we can teach the steps of the scientific method until we are blue in the face.  Many times, all we succeed in doing is having the students memorize the 6 steps to the scientific method and then repeat them on a test.  Too often science teachers conduct laboratory activities in which students follow a list of steps and record an observation at the end.  I need to be one of the first in line to say "Guilty!".  

Today's science educators are pressured to have successful end of course scores.  Sadly, a student can often perform well on these standardized tests without every doing any "real" laboratory work.  One of my goals this year is to change the focus of my class.  I am going to devote the time it takes to allow my students to write and carry out open ended lab experiments.

I have been back in school with my students for one week now.  I have covered the scientific method and gone into great detail on how to design an experiment.  We have mastered (I think!) how to define the experimental group and the control group.  My students can identify the independent and dependent variables.  They understand that only one variable can be changed at a time, and that all other variables must remain constant.  They can write a hypothesis and a conclusion.  We are just about ready to put all we have learned into practice.

FREEBIE ALERT!  If you will allow me to pat myself on my own back, I have written a PowerPoint that my students responded to extremely well.  It covers the scientific method with slides that are bright and colorful and visually attractive.  You can download this for free from my store, Amy Brown Science on  It comes with a set of notes for the teacher and a notes outline for the student.  The student fills in the notes as the lesson is being taught.  I feel that this PowerPoint goes beyond just listing the steps to the scientific method.  Students are given practice problems in which they have to apply what they have learned.  Here is the link to my free PowerPoint and notes:  Scientific Method PowerPoint with Notes for Teacher and Student.

You might also want to consider:
Scientific Method Homework
Applying the Scientific Method and Scientific Writing

Coming up this week.... I am taking my students to the lab to conduct their first open ended experiment.  It is going to involve the germination of seeds.  It will be a simple idea and will use only simple supplies.  Perhaps it may end up being more of a "guided" inquiry, but we have to start somewhere, right?  Later this week, I will post about how this goes and have some pictures as well.

Have fun teaching!