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.
Food Chains vs Food Webs
- Food Chain: A simple linear pathway (e.g., grass → rabbit → fox).
- Food Web: A complex network showing how species interact with multiple food sources.
Food webs better reflect reality and are often tested in Paper 1 diagrams, where students may need to interpret or construct webs from given data.
Energy Pyramids
ESS students should know three pyramid types:
- Pyramid of numbers: Count of organisms.
- Pyramid of biomass: Total mass of living material.
- Pyramid of energy: Energy flow per unit area per time (most accurate).
Exam questions often ask students to compare pyramids, discuss energy transfer efficiency, or link pyramids to ecosystem stability.
Human Impacts on Food Webs
Humans disrupt energy flows and food webs in several ways:
- Overfishing: Removes top predators, destabilizing marine systems.
- Deforestation: Reduces producer biomass, collapsing food chains.
- Pollution: Biomagnification of toxins (like DDT or mercury) in top consumers.
- Climate change: Alters species distributions and energy flow dynamics.
These disruptions are prime examples of systems thinking—a small change in one part of the system can ripple through the whole web.
Frequently Asked Questions (FAQs)
1. Do I need to memorize exact energy transfer percentages?
You should know the 10% rule as a general guide, but exams test application rather than strict memorization. Be ready to calculate efficiency if data is provided.
2. How are food webs tested in ESS exams?
Expect diagram interpretation, where you may have to identify producers, consumers, and trophic levels. Extended responses may ask you to evaluate human impacts on a given web.
3. How does energy flow link to sustainability?
Understanding energy flows helps explain why plant-based diets are more efficient than meat-heavy diets, or why biodiversity conservation supports ecosystem stability. ESS emphasizes these connections between ecology and human choices.
Conclusion
In IB ESS 2026, energy flows and food webs are not just theory—they’re essential tools for understanding ecosystems, sustainability, and human impacts. By mastering trophic levels, pyramids, and system interactions, students can confidently tackle both data-response and extended essay questions.
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