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IB DP ESS — Topic 2: Ecology | SL & HL | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define photosynthesis, cellular respiration, gross primary productivity (GPP), net primary productivity (NPP), and net secondary productivity (NSP) using IB-aligned ESS terminology.
- Explain how the laws of thermodynamics govern energy flow through ecosystems, including why energy is lost at each trophic level.
- Describe the roles of producers, consumers, and decomposers in transferring energy through food chains and food webs.
- Calculate NPP from GPP and respiration values, and estimate energy transfer efficiency between trophic levels using the 10% rule.
- Analyse the implications of energy loss at each trophic level for human food choices, agricultural systems, and global food security.
- Evaluate the sustainability of different food production systems using energy efficiency as a criterion, considering scale, trade-offs, and multiple perspectives.
- Discuss how human activities affect primary productivity at local and global scales, with reference to named ecosystems and data.
Real-World Context & Relevance
In 2021, the Food and Agriculture Organization of the United Nations estimated that approximately 828 million people were chronically undernourished, even though global agricultural systems produce enough calories to feed the entire human population (FAO, "The State of Food Security and Nutrition in the World 2022"). One of the most fundamental reasons for this paradox lies not in politics or economics alone, but in basic physics: the way energy moves through living systems is inherently inefficient. When a cow converts grass into beef, roughly 90% of the energy stored in that grass is lost as heat, used for the cow's own metabolic processes, or locked in indigestible tissue. The energy that reaches a human eating that beef is a tiny fraction of what the sun originally delivered to the pasture.
This is not a failure of agriculture — it is a consequence of the second law of thermodynamics, one of the most universal principles in science. Understanding how energy enters ecosystems through photosynthesis, how it is stored and released through respiration, and how it degrades as it passes through food chains is essential to understanding why ecosystems are structured the way they are. It also explains why a plant-based diet can feed far more people per hectare than a meat-based one, why tropical rainforests are so biologically productive, and why the open ocean — despite covering 70% of Earth's surface — contributes relatively little to global food production. These are not abstract ecological facts: they are the physical foundation of food security, land use, and sustainability.
🌍 Why This Matters Energy flow through ecosystems determines the carrying capacity of the biosphere for all life, including humans. The thermodynamic constraints on energy transfer set hard limits on how many people a given area of land can feed, which agricultural systems are sustainable, and how ecosystem degradation translates directly into reduced productivity. Mastering this topic is essential for evaluating food security, land use, and climate change impacts on ecosystems.
Core Concepts & Definitions
Before we can understand why ecosystems have the structure they do — why there are always more plants than herbivores, and more herbivores than carnivores — we need to understand the physical rules that govern all energy transformations. These rules are