Research Question and Hypothesis
Once students have some experience working with a basic terraqua column (see the Terraqua Columns Lesson), they have an opportunity to design and conduct their own investigations with their mini-ecosystems. There are hundreds of variables students can manipulate with a minimum of materials – temperature, light, pollution, type of water, type of soil, etc. As a class, students brainstorm variables that might affect the plants, soil, and/or water in a terraqua column. In teams, students propose a project, and once approved, set about testing their ideas and observing the effects of their manipulations on their mini-ecosystem. If your school participates in a local science fair, this is a fantastic activity to introduce students to experimental design, variables, and control groups.
How is an ecosystem affected by changes to just one factor within the habitat?
If one part of an ecosystem is changed, then it will affect all other species within the ecosystem.
5. Organisms in ecosystems exchange energy and nutrients among themselves and with the environment. As a basis for understanding this concept:
a. Students know energy entering ecosystems as sunlight is transferred by producers into chemical energy through photosynthesis and then from organism to organ ism through food webs.
b. Students know matter is transferred over time from one organism to others in the food web and between organisms and the physical environment.
c. Students know populations of organisms can be categorized by the functions they serve in an ecosystem.
Students are most engaged in learning when they can steer the direction of their own learning. In this lesson, students are able to design an original experiment, learn about experimental variables and controls, and present their results to the class. They use their experience with terraqua columns, water quality testing and soil analysis to guide their inquiry in a direction of their choosing.
To get a sense of what students might come up with, these are a few of the experiments my students conducted:
- add soda to the water
- add vinegar to the water
- compact the soil by stomping on it after planting the seeds
- compare a regular column made from 2 liter soda bottles to a tiny column made from 500 ml water bottles
- use a wick made from thick cotton twine rather than a strip of towel
- throw garbage from a school lunch on top of the soil before watering
- use water from the San Francisco Bay versus tap water
- use water from the creek versus tap water
- use garden soil versus school yard soil
- add fertilizer
- add compost
- put one in direct sunlight and the other in shade
- put one outside and one inside the classroom
- add ice to the water each day
A key outcome of this project is a basic understanding of the scientific process and of variables in experiments. While your students may not need to know everything that is outlined below, it is essential that the teacher is solidly grounded in the scientific method and experimental design issues. Pick and choose from the content below to decide how much your students need to know.
The Scientific Method
The scientific method is simply the process scientists use to learn about the world. Generally, the steps are
- Pose a question.
- Research as much as possible about that question.
- Based on that research, come up with a hypothesis (an educated guess).
- Design and conduct an experiment.
- Collect observations (your results).
- Draw conclusions that hopefully answer your question and compare your results to your hypothesis.
For example, in this project, a team might start with the question: “If I spilled a soda, how would that pollution affect the plants, water, and soil in an ecosystem?” They would find out what they know about pollution, urban runoff, and the chemical composition of soda. Finally, they might hypothesize: “Pollution will make the plants grow more slowly or not at all, make the water have a low pH because of the acid in soda, and make the soil have a low pH as well.” They would design an experiment: in one terraqua column, pour a soda into the water chamber and in the other, use plain water. They would measure the height of any plants that grew as well as the pH of the water and soil. Finally, they would look at their results over the month and draw some conclusions.
The scientist changes the independent variable. In a good experiment there is only one independent variable. As the scientist changes the independent variable, he or she observes what happens. In our example, the independent variable is whether or not the scientist added soda to the water compartment. Each student will choose their own independent variable for their column.
The dependent variable is what you measure and want to know about in your results. The dependent variable changes in response to the independent variable. For example, the dependent variable is the height of the plants, the pH of the soil and the pH of the water. The dependent variable will depend on the independent variable chosen by the student.
Constants and Controls
Experiments also have controlled variables. Controlled variables are quantities that a scientist wants to remain constant, and he must observe them just as carefully as the dependent variables. For example, if we want to measure the difference in the height of plants in soda versus water, it is essential that we use the same kind of seeds in both terraqua columns. If we use radish seeds in one column and carrot seeds in the other, we can't be sure if the carrot seeds with soda grew more slowly because of the soda or because carrot seeds normally grow more slowly than radish seeds. Similarly, we want to make sure that they get the same amount of light, that they have the same type of soil to grow in, and that they stay the same temperature.
In a controlled experiment the scientist has considered all 3 types of variables. She tests a hypothesis by changing the independent variable and noting the effect on the dependent variables, all the while making sure that controlled variables stay the same. Good experiments make it so that the only difference between the control group and the experimental group is the independent variable.
Experimental and Control Groups
In many experiments, such as these terraqua column experiments, the scientist makes a comparison between different groups. The group that does not receive any treatments - the one where the independent variable is not changed - is called the control group. For example, a plant with no fertilizer. The group or groups that do receive a treatment - the ones where the independent variable is changed - is called the experimental group. It is essential to have a control group in these types of experiments or you will not be able to determine if a plant that got a treatment is any different.
When all of these details are taken care of, and when a difference in the dependent variable exists, then the experimenter can say it was the independent variable that caused the difference. There have been plenty of bad experiments in the real world. So, a smart scientist (and a good students) will look to see exactly how the experiment was designed and conducted in order to determine if the conclusion drawn from the data is really true.
Each team needs:
- 2 fully constructed terraqua columns (reuse the ones from the Terraqua Columns lesson or see Bottle Biology building instructions)
- 1 foot wick (1-2 cm wide strip of old cotton towel)
- other materials brought from home, varies for each group depending on the experiment the group wants to try
- copies of the “TAC Experiment Checklist”
Materials for the class to share:
- Soil, either store-bought potting soil or soil from outside
- Hand trowel
- 1 package radish seeds OR Wisconsin Fast Plant seeds
- Graduated cylinder
- pH test strips
- dissolved oxygen test kit
- pool or aquarium thermometers
- optional: microscope and glass slides
You may want to have on hand for students to use in testing:
- creek/pond water
- an herbicide like Round-Up
- a weak acid like vinegar
If you wish each team to create a poster to display their results, you should have (or have the students get):
- for each team - 1 piece of posterboard, tri-fold display board, large sheet of cardboard (Costco and other warehouse distributors have lots of cardboard), or even 11x17 sheets of construction paper
- glue sticks
- Make copies of the “TAC Experiment Checklist”, either to hand out to each student or to display as an overhead.
Setting up the experiments
- Have your students empty out their old terraqua columns and rise the containers
- Prepare new wicks (1-2 cm wide strip of old cotton towel)
- Set out soil, hand trowels, seeds and graduated cylinders
- At this point, you should have an idea of what each group intends to do. Make sure that each group has the materials they need OR provide the materials for them.
Making observations and recording results
- Each day you plan to make observations, have pH paper, rulers, dissolved oxygen test kits, and thermometers available
- Optional: set out microscope(s) and glass slides
Presenting the results
- Optional: set out poster making supplies – posterboard, markers, scissors, and glue sticks
Brainstorming, researching and proposing an experiment
- Have your students take out a sheet of paper and fold it in half vertically.
- Title one half “Things that might help the mini-ecosystem in my terraqua column”. Title the other half “Things that might hurt the mini-ecosystem in my terraqua column”. Divide the board into two columns as well.
- Give students 3-5 minutes to come up with ideas for each side of their paper. For now, allow students to interpret “help” and “hurt” in any way they want. If students are having difficulty thinking of ideas, ask questions like:
When 5 minutes have passed, ask students to share their ideas with you. Write their ideas up on the board as they are suggested. One effective way to do this is to go around the room and have each student share one idea before opening it up to anyone to share a second or third idea. It is likely that different students interpreted the question differently – some might think about re-engineering the terraqua column while others might think about ways to help the plants grow better. This is GREAT! You can draw attention to these different interpretations as they appear. Tell students that they will now have a chance to change one thing about their terraqua column and compare the changed column to one without the change. Using the ideas on the board, go through a couple examples of things that they might want to change. Emphasize the importance of having two columns, one to make a change in and one without the change to compare the other one to. For instance, consider an imaginary student Bill who wants to know how adding fertilizer might affect the plants. He builds a terraqua column and adds fertilizer. The plants grow to be 20 centimeters tall. Can Bill conclude that the fertilizer made the plants grow taller? No! Taller than what? Without a column that was the same in every way except that it didn’t get fertilizer, Bill doesn’t have anything to compare the 20 centimeter plants against. If you want to introduce the associated vocabulary (experimental group, control group) then do so here.Emphasize the importance of changing only one thing. Lead students through several example experiments where several variables were altered simultaneously. For instance, think about Bill and his fertilizer experiment. Bill also thinks that garden soil is probably better for plants than school yard soil. So, in one column he plants seeds in garden soil and uses fertilizer. In the other column he plants seeds in school year soil and no fertilizer. In the end, the plants in garden soil and fertilizer grew taller. Can Bill conclude that the fertilizer made the plants grow taller? No! Maybe the garden soil made them grow taller. You can’t tell which variable caused the plants to grow taller if you change more than one variable at a time! If you want to introduce the associated vocabulary (independent variable, dependent variable, and controlled variable) then do so here. Divide the kids into their teams. Tell them to design an experiment with their terraqua columns. In their lab notebooks, have them write out their idea in words and also draw a picture of the two columns illustrating what they plan to do. Circulate around the room to help groups who are having trouble settling on a single thing to change.Once most groups have an idea of what they want to do, ask students to predict the differences between the plants, water and soil in the two columns. These predictions should be as specific as possible. “The plants will grow better” is not enough. Aim for details such as “the plants in the column with fertilizer will sprout earlier, will be taller, and will have more leaves than the plants without fertilizer.” The end goal is for students to choose 3-4 things they can measure over the next month. Again, circulate around the room to help groups make predictions and choose 3-4 dependent variables to measure.Before students turn in their experiment designs, make sure they all have the following 4 things written down:
- What do people do that is good for the environment in general?
- What do people do that is bad for the environment in general?
When they are finished, have them turn in the experiment design to you for approval before they can actually do it. Depending on your students, you may need to go through several rounds of revisions before they have an acceptable experiment designed.
- The idea behind the experiment in words
- A labeled picture of the two columns they plan to build
- Predictions of the differences between the plants, water and soil in the two columns
- A list of 3-4 measurements they plan to make 2-3 times a week for the next month
Setting up the experiments
- Make sure that all teams have a well designed experiment and have all the materials they need.
- Give students at least 20 minutes to build their columns and make their initial observations.
Making observations and recording results
- 2-3 times a week for the next month, students need an opportunity to make observations of their columns. Each time, they should make the same set of 3-4 measurements in both columns. You may wish to conduct periodic checks of their lab notebooks to ensure that the date of each observation and all the data is being recorded.
Organizing and presenting the results
- At this point, students should have 8-12 observations for each of the dependent variables the students chose. There should be a lot of data. The first task is for each group to go through their lab notebooks to organize the data and look for patterns. The best way is generally to create a table like the one below for each type of measurement that was made:
|pH of the water || || |
| Date||Column with fertilizer ||Column without fertilizer |
| || || |
| || || |
| || || |
| || || |
| || || |
I showed students 2 examples of how to create tables of their data (picking the types of measurements most common within the class – height of any plants and water pH) then let them get started.
- Next, students should look for patterns in each table. Often, graphing the data (if it is numerical) helps. The younger students may have difficulty graphing and may need more instruction about how to set up the axes of the graph and how to plot the data.
- Finally, have students summarize any other observations that weren’t on the original list of planned dependent variables. For example, “On 11/18/04 the column with fertilizer tipped over and all the soil and water spilled out.” Or “On 11/30/04 fuzzy white mold appeared on the surface of the soil in the column without fertilizer.”
- Have students draw conclusions. The questions I asked my students were:
- Go back and read the idea behind your experiment. Then look at the patterns you observed. What did you discover? Use your measurements and observations to describe how the terraqua columns were affected by the change you made.
- Go back to your predictions. Use your measurements and observations to explain why your hypothesis was right, wrong, or why you cannot tell.
- What you would do differently if you were to do this experiment again?
Naturally, you can create different questions or reword them to suit your own classroom.Finally, have students share their experiments with the class. This may be done orally or through a “poster session”. If you wish to have your students create posters, it is helpful to create one of your own with fake data so that they have an idea of what is expected. Once the posters are made, they can present their posters to the class.
Sample data and graphs
Still conducting experiements. We will post results once complete.
Photos & Movies