Introduction to Energy Transfer in Ecosystems
All living things need energy to survive — to grow, move, reproduce, and carry out life processes. But where does this energy come from, and how does it move through an ecosystem?
The answer starts with the Sun. Almost all energy in living systems originates from sunlight. Through a chain of feeding relationships, this energy flows from organism to organism — but never perfectly. At every step, some energy is lost.
Understanding how energy moves through ecosystems helps us answer big questions:
- Why are there fewer lions than zebras on a savanna?
- Why do we need to eat so much food to gain just a little body mass?
- How do human activities affect the stability of entire ecosystems?
Think of energy transfer like passing a water-filled bucket through a line of people. Each person spills a little before passing it on — so by the end of the line, there's far less water than when you started. Energy moves through ecosystems in the same way.
MYP Conceptual Lens: This unit explores the key concept of Systems — ecosystems are networks of interacting parts — and the related concept of Interaction, focusing on how feeding relationships connect all living things. The global context is Fairness and development, asking how natural resources are distributed and used sustainably.
Producers, Consumers, and Decomposers
Every organism in an ecosystem plays a role in how energy moves. We classify organisms by how they obtain their energy:
An organism that makes its own food using an external energy source (usually sunlight). Also called autotrophs. Examples include plants, algae, and cyanobacteria.
An organism that obtains energy by eating other organisms. Also called heterotrophs. Consumers are further classified by what they eat.
An organism that breaks down dead organic matter, recycling nutrients back into the ecosystem. Examples include fungi and bacteria.
Consumers are grouped into trophic levels based on what they eat:
| Level | Name | Description | Example |
|---|---|---|---|
| 1st | Producer | Makes own food via photosynthesis | Grass |
| 2nd | Primary Consumer | Eats producers | Grasshopper |
| 3rd | Secondary Consumer | Eats primary consumers | Frog |
| 4th | Tertiary Consumer | Eats secondary consumers | Snake |
| Highest | Apex Predator | Top predator in this chain; not eaten by anything else | Eagle |
Note on apex predators: "Apex predator" is a descriptor for whatever organism sits at the top of a given food chain — not a fixed 5th trophic level. Depending on the ecosystem, an apex predator might occupy the 4th, 5th, or even 6th trophic level. Food chains can have varying numbers of levels (typically 3–5). The table above shows one example structure.
Decomposers don't fit neatly into one trophic level — they break down organisms from all levels. Without decomposers, nutrients would stay locked in dead bodies and ecosystems would collapse.
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