Investigation Lab: Ecosystem Interactions
Learn
In this investigation lab, you will explore how organisms in an ecosystem interact with each other and their environment. Scientists use controlled experiments and observations to understand these complex relationships.
Key Concepts
- Independent Variable: The factor you deliberately change in an experiment
- Dependent Variable: The factor you measure or observe in response to changes
- Control Group: A group that receives no treatment, used for comparison
- Constants: Factors kept the same throughout the experiment
Investigation Question
How does the presence of decomposers affect nutrient cycling in a closed ecosystem?
Background
Decomposers like bacteria and fungi break down dead organic matter, releasing nutrients back into the soil. This process is essential for healthy ecosystems because it recycles matter and makes nutrients available for producers.
Materials
- Two clear plastic containers with lids
- Potting soil (sterilized and unsterilized)
- Leaf litter samples
- Small plants (same species and size)
- Spray bottle with water
- Ruler and observation journal
Procedure
- Label containers A (with decomposers) and B (control - sterilized soil)
- Add equal amounts of appropriate soil to each container
- Place identical leaf litter samples in each container
- Plant one small plant in each container
- Add equal amounts of water to each
- Seal containers and place in same location with equal light
- Observe and record changes over 2-3 weeks
Examples
Example 1: Identifying Variables
Scenario: A student wants to test if earthworms improve plant growth.
- Independent Variable: Presence or absence of earthworms
- Dependent Variable: Plant height, leaf count, or biomass
- Constants: Soil type, water amount, light exposure, plant species, container size
Example 2: Reading a Data Table
| Week | Container A (with decomposers) | Container B (sterilized) |
|---|---|---|
| 1 | Plant height: 5 cm | Plant height: 5 cm |
| 2 | Plant height: 8 cm | Plant height: 6 cm |
| 3 | Plant height: 12 cm | Plant height: 7 cm |
Analysis: Container A shows greater plant growth, suggesting decomposers helped release nutrients that promoted growth.
Example 3: Drawing Conclusions
Based on the data above, we can conclude that decomposers play an important role in nutrient cycling. The plants with access to decomposer activity grew 71% taller (12 cm vs 7 cm) over three weeks.
Practice
Answer the following questions based on the investigation concepts.
1. In an experiment testing how light affects algae growth in a pond ecosystem, what is the independent variable?
A) Amount of algae
B) Type of pond water
C) Amount of light
D) Temperature of water
2. Why is it important to have a control group in an ecosystem experiment?
A) To make the experiment more complicated
B) To have something to compare your results against
C) To use up extra materials
D) To satisfy your teacher
3. A scientist studies two forest plots. Plot A has decomposers removed; Plot B is left natural. After one year, Plot A has thick leaf litter buildup. What does this demonstrate?
A) Leaves fall faster without decomposers
B) Decomposers break down dead organic matter
C) Plot B has more trees
D) Leaf litter prevents plant growth
4. Which of the following would be a constant in an experiment testing how soil pH affects earthworm activity?
A) Soil pH level
B) Number of earthworms
C) Amount of soil moisture
D) Earthworm movement
5. A student hypothesizes that more decomposers will lead to faster plant growth. Her data shows plants with decomposers grew 15 cm while plants without grew 8 cm. Do her results support her hypothesis?
A) No, because the difference is too small
B) Yes, because plants with decomposers grew more
C) No, because she needed more trials
D) Yes, but only if she repeats the experiment
6. What is the purpose of repeating an experiment multiple times?
A) To use more materials
B) To take more time
C) To increase reliability of results
D) To confuse the scientist
7. In the decomposer investigation, why should both containers be placed in the same location?
A) To save space
B) To ensure equal light and temperature exposure
C) To make observation easier
D) Because there is only one good spot
8. A researcher finds that ecosystems with more biodiversity recover faster from disturbances. This is an example of:
A) A hypothesis
B) An observation
C) A variable
D) A constant
9. If an experiment has too many variables changing at once, what problem might occur?
A) The experiment will be too easy
B) You cannot tell which variable caused the results
C) The experiment will take too long
D) You will get perfect results
10. Which question would be best investigated using the scientific method?
A) Is nature beautiful?
B) Should we protect forests?
C) How does rainfall affect plant growth in grasslands?
D) What is the best ecosystem?
Check Your Understanding
1. Explain why decomposers are essential for nutrient cycling in ecosystems.
2. Design a simple experiment to test how temperature affects decomposition rate. Identify your independent variable, dependent variable, and at least two constants.
3. A student conducts an experiment but forgets to include a control group. Why might their conclusions be unreliable?
Next Steps
- Review the scientific method and experimental design vocabulary
- Try designing your own ecosystem investigation at home or school
- Continue to the next lesson on interpreting ecological data and graphs