A closed terrarium is a miniature ecosystem sealed within a transparent container, functioning as a self-sustaining world. This glass enclosure creates a controlled environment where life forms thrive without external watering or air exchange. The system depends entirely on the continuous recycling of water, gases, and nutrients to maintain its balance. It operates as a model of the Earth’s biosphere, demonstrating how biological and physical processes interact to support life.
Internal Water Cycle
The primary feature of a closed terrarium is its ability to water itself through a continuous internal hydrological cycle. This process begins when light energy warms the container environment. Water evaporates from the moist soil and is released by plants through transpiration. This combined release, known as evapotranspiration, saturates the air within the sealed space.
As the warm, moisture-laden air rises, it meets the cooler surface of the glass container. This temperature difference causes the water vapor to change back into a liquid state, forming tiny droplets on the inner walls in a process called condensation. The container’s seal traps this moisture, preventing its escape and maintaining the high humidity required for the cycle.
The condensed droplets accumulate on the glass until gravity pulls them downward. They trickle back into the soil, simulating rainfall or precipitation. The soil absorbs this water, making it available for the plants’ roots, which restarts the cycle. This constant loop ensures plants receive necessary hydration without outside intervention.
Gas Exchange and Energy Flow
The ecosystem is driven by the flow of energy, managed through a balanced exchange of atmospheric gases. Light serves as the sole external energy source, powering photosynthesis in the plants. During the day, plants absorb carbon dioxide (\(\text{CO}_2\)) and combine it with water to produce glucose, releasing oxygen (\(\text{O}_2\)) as a byproduct.
This oxygen is used by all living organisms, including plants and soil microorganisms, during cellular respiration. Respiration is the converse process: organisms break down glucose for energy, consuming oxygen and releasing carbon dioxide and water vapor. This exchange is noticeable at night when photosynthesis ceases, and plants primarily respire, temporarily increasing \(\text{CO}_2\) levels.
The terrarium’s survival hinges on the equilibrium between these two processes. Photosynthesis must produce enough oxygen to sustain life, and respiration must return carbon dioxide to the air for plants to continue producing food. The sealed environment ensures the total amount of gases remains constant, allowing the carbon and oxygen cycle to persist.
Nutrient Recycling Through Decomposition
The system maintains fertility without external fertilizers through continuous nutrient recycling. When leaves drop, roots die, or organisms expire, the resulting organic matter contains locked-up nutrients essential for plant growth. If this matter accumulated, the ecosystem would eventually run out of available resources.
Microorganisms, primarily bacteria and fungi in the soil, prevent resource depletion. These decomposers chemically break down complex organic compounds into simpler, usable forms. This decomposition releases vital elements, such as carbon, nitrogen, and phosphorus, back into the soil substrate.
The released nutrients are reabsorbed by the plants’ roots and incorporated into new tissues. Nitrogen, for example, is converted by specialized bacteria into forms like ammonium and nitrate that plants can readily utilize. This continuous loop repurposes the limited nutrient supply, sustaining the plants and maintaining soil fertility.
External Factors for System Equilibrium
While the internal cycles are self-sustaining, their speed and efficiency are controlled by the external environment, primarily light and temperature. Light intensity dictates the rate of photosynthesis, which sets the pace for the ecosystem’s productivity. Placing a terrarium in bright, indirect light provides the necessary energy without causing excessive heat buildup.
Too much direct sunlight can rapidly overheat the container, leading to a greenhouse effect that stresses and kills the plants. Elevated temperatures accelerate metabolic processes, including respiration and decomposition, which quickly deplete oxygen and nutrient reserves. The ideal temperature range falls between 65 and 80 degrees Fahrenheit (18 to 27 degrees Celsius) for most common terrarium plants.
Maintaining a stable, moderate temperature prevents the water cycle from becoming erratic. Excessive heat causes rapid evaporation and heavy condensation, which can lead to mold growth and waterlogged soil. Conversely, low temperatures slow the cycles to a near halt. Controlling these external factors is essential to keeping the internal water, gas, and nutrient cycles running smoothly and in balance.