The Terrarium in the Closet
A class builds a terrarium in a clear plastic bottle: 2 cm of moist soil, three small green plants, four crickets, a few dead leaves, and a tight cap. It sits on a windowsill. For five days everything looks healthy. Then a student moves it into a dark closet 'so the crickets can sleep.' After a week the plants are yellow and drooping, the crickets are slow and hiding, the inside of the bottle is coated with water droplets, and white fuzz is spreading over the dead leaves. The cap has stayed on the whole time and nothing has been added or removed.
1. Explain the plants' decline using the terrarium as a system of living things that depend on each other.
Read the worked answer
Green plants make their own food using light. In the closet there is no light, so the plants stop making food and live off what they stored, which runs out. They also stop releasing oxygen. The crickets eat the plants, so as the plants weaken, the crickets' food supply shrinks, which fits their sluggish behavior. The chain runs in one direction: no light leads to weaker plants, which leads to less food for the animals. Adding more crickets or more soil would not fix this, because the missing input is light, not food or space.
2. A student claims the water droplets are 'new water created by the plants.' Use the closed cap to evaluate this claim.
Read the worked answer
The claim is wrong. The bottle is capped, so no water enters or leaves. The droplets are the same water that started in the soil and in the plants, cycling: plants take it up and release it through their leaves, it condenses on the cooler plastic, and it runs back down into the soil. The total amount of water stays about the same; only its location and state change. If the cap were truly sealed and the setup stayed stable, the water would keep cycling rather than increasing.
3. The mold is spreading on the dead leaves. Is that a problem to remove, or part of how the system works? Explain.
Read the worked answer
It is part of how the system works. Mold and other decomposers break down dead leaves and return nutrients to the soil, which the plants can then use. Removing the mold is not the fix. The real problem is the missing light, and possibly too much moisture with the cap sealed tight. The reasonable change is to move the terrarium back to the windowsill and loosen the cap slightly for air flow, then observe whether the plants green up and the crickets become active again.