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IB DP ESS — Topic 2: Ecology | SL / HL | Paper 1 & Paper 2A
🎯 Learning Intentions By the end of this note, you will be able to:
- Define food chain, food web, trophic level, gross primary productivity (GPP), net primary productivity (NPP), and ecological efficiency using IB-aligned ESS terminology.
- Explain how energy flows through trophic levels and why energy is lost at each transfer, with reference to the laws of thermodynamics.
- Describe the difference between producers, primary consumers, secondary consumers, tertiary consumers, and decomposers within an ecosystem.
- Calculate energy transfer efficiency between trophic levels using the 10% rule and interpret the significance of this for ecosystem structure.
- Construct a food web from given data and identify the trophic level of each organism.
- Analyse the implications of energy loss at trophic levels for human food choices, agricultural systems, and sustainability.
- Evaluate the limitations of food chain and food web models in representing real ecosystem complexity.
- Discuss how disruption to food webs — through species loss, invasive species, or overfishing — affects ecosystem stability at local and global scales.
Real-World Context & Relevance
In 1995, grey wolves were reintroduced to Yellowstone National Park in the United States after a 70-year absence. Within a decade, elk populations had declined and changed their grazing behaviour, allowing riverbank vegetation to recover. Beavers returned, streams were re-channelled, and songbird populations increased. This cascade of effects — triggered by the reintroduction of a single apex predator — is now one of the most cited examples of a trophic cascade in ecology. It demonstrates, powerfully, that the way energy and matter flow through feeding relationships is not a simple linear process but a dynamic, interconnected web of dependencies.
On a global scale, the structure of food webs and the efficiency of energy transfer have profound implications for human food security. The Food and Agriculture Organization of the United Nations (FAO) estimates that approximately 77% of agricultural land is used to raise livestock, yet livestock provide only about 18% of global caloric intake (FAO, "The State of Food and Agriculture 2022"). This stark disparity is a direct consequence of energy loss at trophic levels — a fundamental ecological principle that shapes everything from the size of predator populations to the sustainability of human diets.
🌍 Why This Matters Understanding how energy moves through food chains and is lost at each trophic level is essential for evaluating the sustainability of food systems, managing fisheries, conserving biodiversity, and making informed decisions about land use. The efficiency — or inefficiency — of energy transfer is one of ecology's most powerful explanatory principles.
Core Concepts & Definitions
Every organism in an ecosystem needs energy to survive, grow, and reproduce. But where does that energy come from, and how does it move from one organism to another? The answer lies in feeding relationships — the connections between organisms that eat and are eaten. Before we can understand how energy flows, we need a precise vocabulary for describing these relationships and the positions organisms occupy within them.
📖 Key Definition — Food Chain A food chain is a linear sequence of organisms through which energy and nutrients are transferred, beginning with a producer and ending with a top consumer. Each