Modeling Human Impact Part 1: Designing the Experiment

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Objective

Students will be able to design a controlled, long term experiment to be conducted by their entire class.

Big Idea

In this lesson, students design and implement a long term study of the effects a specific variable (e.g., fertilizer) has on vegetable growth and populations of aquatic micro-organisms.

Introduction

The ecocolumn is one of the most popular and common lessons in an environmental science class… almost like dissecting a frog in biology.  As I’m teaching this class for the first time, it seemed a good place to start when exploring the type of scientific problems environmental scientists study.  On a practical note, almost all the materials are easy to acquire for relatively low cost.  As far as how this project fits into my curriculum, it presents a hands-on opportunity to tie up the first two units (nature of science and the nature of environmental science) in a way that’s not merely a demonstration, but really asks the students to engage in scientific inquiry.  

Specifically, this project asks students to create a simplified model to investigate a single variable's effect on an ecosystem.  The continued examination throughout the rest of the first semester will tie in nicely with future units on issues such as soil quality, water quality, population growth, species diversity, limiting factors, etc.  

As it's such a common lab activity for environmental science classes to engage in,  I realize it's not necessarily breaking new ground to have students build an ecocolumn.  I would maintain, however, that as opposed to a "let's build this and watch what happens" approach, the project I describe here involves a more deliberate experiment designed with clear controls and isolated variables and thus makes a great follow-up to a unit on scientific principles.  

Connection to Standard: This project will give students the opportunity to both design and follow a complex multistep experimental procedure.  Additionally, they will practice writing over an extended time frame to document their sustained research.  

 

 Required Materials:

Please note that there is considerable set-up required for this lesson in terms of materials.  Most of the materials are easily acquirable at a hardware or garden store, but a few may need to be ordered from a science supply company depending on availability in your area.  Since the ecocolumns themselves are built from 2-liter soda bottles, I ask students to start bringing in empty bottles at least a week in advance.  It’s important that students bring in clear, uncolored bottles and is best if they use bottles from the same company due to slight differences in size and shape.  See the bottle biology webpage for more considerations. 

I have included a list of materials that I use to build my version of the “terra-aqua column” below.

 

  1. Three 2L bottles per group (it's good to have extra)
  2. Hammer, large nail (for putting holes in the bottle cap), and a short 2x4 (to place under the bottle cap when hammering the nail)
  3. Push Pins or small nails (for poking drainage holes in terrestrial chamber)
  4. Box cutters (for starting cuts in the bottle)
  5. One or more types of fertilizer 
  6. Gravel
  7. Potting soil
  8. Seeds (I use radishes and/or turnips)
  9. Silt or sand from a pond
  10. Filamentous algae (obtained from a pond)
  11. Chopsticks (for pushing seeds into the soil)
  12. Silicone Sealant or packing tape (for connecting bottles)
  13. Paper Punch (for making a hole to insert pipette for water testing)
  14. Hair Dryer (for removing labels)
  15. Dry Erase Marker (for making cutting lines)
  16. Scissors
  17. Strainer or mesh screen (for separating algae from water)
  18. A digital scale
  19. Measuring cups or graduated cylinders
  20. Watering cans and/or spray bottles

 

Additionally, there are a few items that can be introduced as variables which we discuss in the brainstorming section of this lesson.   

Please note that a significant amount of the data collection during this project requires a microscope (for observing and identifying microorganisms in the aquatic chamber).  I recommend having at least one microscope per group.  If you don’t have microscopes, the project can still be carried out by simply observing qualitative characteristics of the aquatic chamber such as clarity, color, composition, etc.  Again, this is discussed in more detail in part 3 of this project description.  

Warm Up

10 minutes

I begin the lesson with a quick review of the agricultural and industrial revolutions that we covered in the "What is Environmental Science?" lesson by asking the following questions:

  1. How did these revolutions affect the global human population?
  2. Why do we need to grow crops?  
  3. What are some resources we use to produce those crops?
  4. How are other species and whole ecosystems affected by humans using resources for agriculture?

I ask students if the justification for such large scale use of natural resources is anthropocentric or ecocentric.  I then ask them why an ecocentric person might be concerned about the effects of agriculture and how this concern for the environment as a whole is ultimately concern for humans as well.

Following this quick opening discussion, I ask students to take out their old homework assignment that included a question about an experiment comparing fertilizers as a way to review the experimental design vocabulary discussed in the first unit.  

At this point, I’m looking for students to recall that a controlled experiment needs:

  1. an experimental group that contains an independent variable (the variable to be tested)

  2. a control group that does not contain the independent variable (for comparison with the experimental group).  

  3. All other factors in both groups to be the same or constant.     


Once we finish this basic review, we move on to the brainstorming section.

Brain Storming

35 minutes

To begin our brainstorming to design the experiment, I let students know that we will be using a model to study the interactions between a terrestrial and aquatic ecosystem.  

On the board behind me, I have a picture of the basic set-up of the 2 chamber ecocolumn we’ll be building as well as the scientific question we'll be investigating in this project:

"How does _______________ affect plant growth and aquatic microorganisms?"   

I then let them know the following basics of the experiment they will be designing:

  • The "blank" in the above question will be an independent variable of their choosing.
  • The upper chamber will contain gravel and soil and seeds.
  • The lower chamber will contain gravel and water containing filamentous algae.  
  • They will be required to collect data (quantitative and qualitative observations) at every class meeting and, at least once per week, also make microscopic observations of the aquatic environment.  


After covering these basic constraints, I invite students to come up to the table in the front of the room and observe the materials they will use to construct the ecocolumn.  In addition to the basic materials to build the ecocolumns, I also have additional items that may be included as the independent variables in the different groups’ ecocolumns. (I set all these up ahead of time without explaining to preserve a little mystery about why we have such varied items on a table)

These additional items could include:

  • worms
  • salt
  • motor oil
  • pillbugs (roly polys)
  • different kinds of fertilizer
  • distilled water
  • dead leaves

The possibilities really are endless, so feel free to add any other variables you think your students may find interesting.

 

Brainstorm #1: The Experimental Group

I then ask students to brainstorm:

  1. the effects that some of the variables might have on their ecocolumns
  2. at least two variables they could be interested in including in their ecocolumn
  3. the reasons they would be interested in studying those variables.

While the groups are discussing which variables they’d be interested in studying, I walk from group to group to check in and make sure they understand what I’m asking them to do and whether they’re considering what kind of data they’d need to collect (including what kinds of comparisons they’d need to make with a control group) to confirm whether or not their predictions of the variables’ effects were accurate.

If students are having a real hard time generating variables to "fill in the blank" of the scientific question (How will _____________ affect plant growth and aquatic microorganisms?), I may give a few examples such as, "more fertilizer", "watering a plant with Gatorade", "adding dead leaves to the soil", etc, to get them thinking.

Once students have discussed the variables with their groups for about 5-10 minutes, I gather the class’ attention and ask each group to mention one of the variables they discussed and what they thought its effects might be.  In this way, students from all groups can hear what others are thinking, which may inspire new questions that hadn’t occurred to them in their small group discussion.  

 

Brainstorm #2: The Control Group

After we have discussed potential independent variables, I remind the students that their experiments can really only yield information if their data is compared to a control group.  I then reveal that I will be making the “control group” ecocolumn, but that it’s up to the class to determine how I will make it.  I also remind them that the control group and the experimental group can only differ by the inclusion or exclusion of the independent variable, and in this way, by determining how I should build my ecocolumn, we will set the constants for the project as a class.  

Similar to the brainstorming regarding the variable, I will walk from group to group to check in on their progress and, after 5-10 minutes, have groups share out their suggestions with the entire class.      

Although I prefer to let students generate the constants to answer the questions below, I will mention a few of them if no one seems to be mentioning an important factor.

These questions fall into two main groups:

  1. Set up/construction/maintenance

  2. data collection


  1. Set up/construction/maintenance

    • How much water in the aquatic chamber?

    • How much soil in the terrestrial chamber?

    • How much gravel?

    • How much algae?

    • How do we water the soil? (pour?, trickle?, spray?)

      • How much (amount) do we water?

      • How often?

    • Which seeds do we plant? (radishes?, turnips?, both?)

    • How many seeds do we plant?

      • Where do we plant them? (spacing, depth)

    • If we’re fertilizing, how much? do we add more?

    • How many holes for drainage?


  1. Data Collection

    • What data do we collect? (height?, mass?, pH? appearance?  odor? leaf quality? species diversity? population sizes? water quality? etc)

    • How do we collect the data? (What units? instruments? procedures? etc)

    • How often and on what schedule do we collect the data? (i.e., do we collect all types of data every day or only at certain times?)


I fully expect students to come up with additional questions that will refine the experimental design even further, but even if they stick to the questions I’ve outlined above, I think it’s important to go through this process with them to give them a sense of ownership over the experiment that they couldn’t have if I simply handed out a worksheet with all the questions and requirements predetermined for them.  


If necessary, we may end up in disagreement on what factors to make constant (e.g., some groups may advocate daily watering while other groups want the constant to be weekly watering).  If such situations arise when we can’t come to a consensus, I hold a vote and a simple majority determines the constant.  In that case, I would then encourage any group with the minority opinion to consider the “rejected constant” to be the variable they study.  


At the end of this process, I will write the constants on the board and ask each group to come to a final decision on which variable they will be studying in their experimental ecocolumn.  While I prefer that each student group examines a different variable, if student groups prefer to investigate the same variable, I won’t disallow it as differences between “identical” ecosystems may yield interesting data.

Wrap Up

15 minutes

To wrap up, I ask each group to write the following information:

  1. The independent variable they will be testing

  2. Their hypothesis regarding how their variable will affect the ecosystems within their ecocolumn

  3. A list of materials they will use in their ecocolumn

  4. A sketch of their ecocolumn showing the measurements of the components

  5. An explanation of how they will introduce their indepentent variable to the ecocolumn (i.e., will the variable be introduced just once? multiple times? in what amount? etc).

  6. An “observation plan” explaining

    1. the schedule they will follow to conduct their research

    2. the kinds of data they will collect and the methods they will use to collect the data

I will eventually ask students to type this information into a more formal project report, but writing it down by hand is ok for this lesson.  If your students have easy access to computers, it may make sense to just ask them to have this typed from the outset.