Introduction
Energy flows and food webs are core concepts in IB Environmental Systems and Societies (ESS). They explain how ecosystems function and how energy moves from the sun through producers, consumers, and decomposers.
In the 2026 syllabus, these ideas are central to systems thinking, as they show how ecological systems balance, change, and respond to human impacts. Understanding energy flows and food webs is essential for Paper 1 data analysis and Paper 2 essay-style questions.
Quick Start Checklist: Energy Flows in ESS
- Sunlight → Producers → Consumers → Decomposers.
- Only ~10% of energy transfers between trophic levels.
- Food chains show linear pathways; food webs show interconnectedness.
- Pyramids of energy illustrate efficiency losses.
- Human activity can disrupt food webs through overfishing, deforestation, or pollution.
Energy Flows in ESS
Energy in ecosystems flows in one direction:
- Producers (autotrophs) capture solar energy via photosynthesis.
- Primary consumers (herbivores) feed on producers.
- Secondary and tertiary consumers (carnivores) feed higher up.
- Decomposers recycle nutrients back into the system.
At each stage, most energy is lost as heat, meaning only about 10% is passed on. This is why food chains rarely exceed four or five trophic levels.
