What insights can a jar full of mud and water give us about life on planet Earth? Can an ecosystem within a jar teach us about the Earth's biosphere? Consider inviting your students to set up windowsill or GrowLab investigations to explore these questions.
The biosphere is Earth's surface region, including the land, water, and atmosphere, that supports all life on the planet. It's a closed system, with materials such as hydrogen, oxygen, nitrogen, and other elements constantly recycling. The energy flow, however is open -- with energy from the sun constantly entering and dissipating through the atmosphere over time.
A humble jar of pondwater can act as a 'microcosm' of larger systems, similar in its makeup and function. It will contain producers (mainly algae and higher plants) that use light energy to manufacture food for themselves and for all other living things. It will also contain consumers (tiny animals) and decomposers (bacteria and fungi), that "live" off of energy captured by producers. Carbon, oxygen, and other important elements cycle through the minibiosphere, as in the larger world, through processes of photosynthesis, respiration, and decomposition. Windowsill sunlight or GrowLab lights can provide light energy.
Minibiospheres can provide an engaging focus for longterm observations, an understanding that systems cycle and change over time, and a glimpse into the tremendous diversity of life in even a small jarful of pondwater.
The grace of this type of project is its appropriateness to a range of grade levels. Lower levels may simply observe and describe changes over time and wonder at the diversity of emerging life. Older students can do more quantitative measuring and identifying of life forms. These minibiospheres also provide an opportunity to set up experiments to test the effects of simulated environmental changes on the system.
Consider breaking into small groups, with each group of students setting up several minibiosphere jars. If you'll be experimenting with various experimental conditions (described below), one jar can be the control, and the others the experimental jars. Denise Martin, biologist and teacher education instructor in Burlington, Vermont, suggests the following minibiosphere setup, which uses readily available materials.
1. For each minibiosphere, obtain a clean pint or quart jar (e.g., mason jar) with a screw-top lid.
2. Because too much organic matter in the jars can cause a gas buildup and potential explosion, use a nail to poke a hole in each lid and seal the hole with melted wax. This will act as a pressure-release valve.
3. On your own or with your class, collect water from a pond or lake. Fill each jar 2/3 - 3/4 full with the pondwater and 1/2" to 1 1/2" of mud from the bottom. You may want make a point of collecting some visible primary producers (algae and plants) and consumers (snails and insects). Don't include large animals or plants since it's such a small habitat.
Make your pondwater collections at any time of the year. Denise Martin reports that her students were awestruck as tiny green plants and baby snails "hatched" from seemingly lifeless mud and water collected during the winter.
4. To observe what hapens when more producers are introduced, you can purchase and introduce a plant called elodea from an aquarium store. Although this isn't necessary, and it may be more exciting to see what emerges from the pondwater, you may choose to add some to just a jar or two for comparison.
5. Before sealing the jars, have students make careful observations of the contents. Use hand lenses and microscopes, if available, to explore the pondwater. Suggest that students also observe and use crayons or pencils to record the color of the water -- or the amount of light passing through. The density of the green color is an indicator of the quantity of producers present, which will change over time.
Although, ideally, these systems should remain closed once they've been set up, Denise relays that student interest will be higher if they can open them briefly on a regular basis and remove a small quantity of water for closer observation. Have students observe changes in the abundance of different organisms, color changes, odor, etc.
Simulated Environmental Changes
Your class can simply observe and record changes in their jars over time, or may introduce an environmental change in some jars, to compare with control jars. Some suggestions follow. Have students use their imaginations to come up with their own ideas!
We recommend continuing observations for at least six weeks, although many will reach some state of equilibrium and remain healthy for months or years. While this type of activity has no right or wrong answers, we hope that it sparks more questions and stimulates further research and explorations. Students should develop an increased appreciation for the role of plants in transforming light energy into the food energy that sustains all life on Ear th.