Energy rules in ecosystems
Energy movement in ecosystems follows scientific rules called the laws of thermodynamics.
These laws explain why energy becomes less available as it moves through food chains.
- First law: energy cannot be created or destroyed — it can only change form
- Second law: every energy transfer is inefficient and some energy is lost
Energy is never destroyed, but it often becomes unusable by living organisms.
Energy flow in ecosystems
Big idea: Ecosystems need a constant input of energy. As energy moves through organisms, it is transformed and gradually lost as heat.
An ecosystem needs both energy and matter to function. Energy does not cycle — it flows through the system.
Energy usually enters ecosystems as sunlight and leaves as heat.
Ecosystems as open systems
Ecosystems are open systems. Energy enters from outside, is transformed inside the system, and eventually leaves.
- Energy enters mainly as sunlight
- Producers convert light energy into chemical energy
- Energy moves through food chains and food webs
- Energy leaves the system as heat
First law of thermodynamics (conservation)
The first law of thermodynamics applies to ecosystems.
When energy moves through an ecosystem, it changes form — for example, from light energy to chemical energy in plants, and then to kinetic or thermal energy in animals.
Energy is not lost — it is transformed into forms that are less useful to organisms.
Second law of thermodynamics (inefficiency)
The second law of thermodynamics Every time energy is transformed, some becomes heat.
Living organisms use energy for movement, growth, and maintenance. During these processes, energy is released as heat and cannot be reused by other organisms.
- Energy available decreases at each trophic level
- Less energy is passed on to the next organism
- Food chains are usually short because of energy loss
Energy loss explains why ecosystems can support fewer organisms at higher trophic levels.
Why this matters for sustainability
Because energy transfer is inefficient, ecosystems are more efficient when humans rely on lower trophic levels for food.
- Less energy is wasted when eating plant-based foods
- Shorter food chains use energy more efficiently
- This supports more sustainable food systems
Big exam takeaways
- Energy flows through ecosystems, it does not cycle
- First law: energy is transformed, not created or destroyed
- Second law: energy transfer is inefficient
- Heat loss limits food chain length
- Lower trophic levels are more energy efficient