A terrarium is a miniature garden enclosed within a glass container, designed to mimic a natural ecosystem on a small scale. Systems are categorized as open or closed, with closed terrariums being sealed containers housing moisture-loving plants like mosses and ferns. While they operate through continuous internal biological cycles, they still rely on external energy. A successful closed terrarium functions as a balanced, low-maintenance biosphere, recycling its own resources to thrive.
The Biological Cycles That Power a Closed Terrarium
The internal function of a closed terrarium is driven by three interconnected cycles that recycle resources. The water cycle is the most visible, beginning when water evaporates from the soil and is released by plants through transpiration. This warm, moist air meets the cooler glass walls, causing condensation into droplets. These droplets run down the sides or drip onto the substrate, rehydrating the soil in a process similar to precipitation.
Gas exchange is governed by the balance between photosynthesis and respiration, essential to the carbon cycle. During the day, plants use light energy for photosynthesis, consuming carbon dioxide (CO2) and releasing oxygen (O2). At night, respiration occurs in all living organisms, consuming oxygen and releasing CO2 back into the atmosphere. This reciprocal exchange maintains a stable atmospheric composition necessary for sustained life.
The third mechanism is nutrient cycling, carried out primarily by micro-organisms and detritivores. When organic matter like dead leaves or roots dies, fungi and bacteria decompose this material. This decomposition releases essential nutrients like nitrogen, phosphorus, and potassium back into the soil. This process ensures the finite nutrients remain available to fuel new plant growth.
Necessary External Inputs for Sustained Function
Despite internal resource recycling, a closed terrarium requires a consistent input of external energy to function. Light is the primary external requirement, powering photosynthesis, which is the foundation of the ecosystem. Plants convert light energy into chemical energy (sugars), sustaining their growth and the micro-organisms that feed on decaying matter. Without adequate light, photosynthesis stops, halting oxygen production and energy creation, leading to system collapse.
The system also depends on a relatively stable temperature, as extreme fluctuations can rapidly destabilize the internal balance. The glass container acts as a miniature greenhouse, and direct sunlight can cause the temperature to rise dramatically. Excess heat can harm the plants by denaturing enzymes and causing excessive evaporation, leading to soil over-saturation. Conversely, temperatures that are too low slow metabolic processes in the plants and microbes, putting the ecosystem into a dormant state.
Factors Leading to Internal System Failure
The most common reasons for terrarium failure stem from improper initial setup and an imbalance of internal elements. Adding too much water is a frequent mistake, as the sealed environment prevents excess moisture from escaping. Over-saturation leads to anaerobic conditions—the depletion of oxygen in the soil—which suffocates plant roots and encourages harmful bacteria and mold.
The selection of incompatible plant species can also lead to failure, as closed terrariums require plants that thrive in high humidity and low airflow. Mixing plants with vastly different moisture needs, such as tropical ferns with desert succulents, guarantees that one or both will perish. Plants that grow too quickly or too large will outcompete their neighbors for limited resources and overcrowd the container, disrupting the equilibrium.
Pests and pathogens introduced during setup can rapidly destabilize the enclosed ecosystem, as the sealed environment offers no escape or natural predators. Mold spores or fungal infections, if present on the original material, can quickly proliferate in the high-humidity conditions. While the nutrient cycle is effective, successful terrariums face a theoretical limit where nutrients may become permanently bound up in complex organic molecules or mineral deposits over decades, making them unavailable for new growth.