Ecosystems as open systems and the source of energySpec C4.2.1, C4.2.2
In short
Ecosystems are open systems: both energy and matter can enter and exit them. In a closed system only energy can pass in and out. Sunlight is the principal source of energy that sustains most ecosystems, captured by photosynthesis. Exceptions include ecosystems in caves and below the depth of light penetration in oceans, which are sustained by chemical energy.
| Open system | Closed system | |
|---|---|---|
| Energy | Can enter and exit | Can enter and exit |
| Matter | Can enter and exit | Cannot enter or exit |
| Example | A natural ecosystem: a lake gains leaves and nutrients in streams, loses water and fish | A sealed bottle garden: light enters and heat leaves, but no matter is exchanged |
Natural ecosystems are open systems. Matter crosses their boundaries all the time: animals migrate, seeds and pollen blow in, nutrients wash out in rivers, gases are exchanged with the atmosphere.
Sunlight as the principal energy source
Most ecosystems are sustained by sunlight. Producers capture light energy in photosynthesis and convert it to chemical energy in carbon compounds, which then passes to every other organism.
Exceptions are ecosystems with no light. Caves: Movile Cave in Romania, sealed off for millions of years, supports a community based on chemoautotrophic bacteria that oxidise hydrogen sulfide. Deep ocean below the level of light penetration: around hydrothermal vents, chemoautotrophic bacteria oxidise hydrogen sulfide from the vents and support tube worms, clams and crabs. Other deep-sea communities depend on dead organic matter sinking from the sunlit surface, so they still rely on sunlight indirectly.
Nature of science: 'sunlight sustains most ecosystems' is a generalisation, like a scientific law. It describes a pattern and allows predictions but does not explain it, and a useful generalisation states its exceptions.
Written and checked against the IB Biology SL specification · Updated October 2026